What actually happens to the human body during fasting/starvation?

What actually happens to the human body during fasting/starvation?

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I recently came across a UN study which advocated that controlled fasting can be beneficial for the the human body and I am fascinated by stories of wilderness survival against the odds.

It piqued my interest to ask; what exactly happens to a human body during fasting and ultimately starvation?

I understand that there is a period of glycogen use/exhaustion and functioning becomes impaired but what medically happens to the human body (organs, muscles, protein, body fat, cognitive processes etc) and over what timeline (24 hours to death). I read that halitosis is an interesting side-effect etc.

Assume the human body has ample access to water and can avoid dehydration.

Actually, the body goes into the fasting state about eight hours or so after the last meal, usually when the guy finishes absorbing nutrients from the food.
Normally, body glucose, which is stored in your liver and muscles, is your main source of energy. When in a fasting state, your body uses up the glucose that you have stored to provide energy. If you are still fasting after this store is used up, your body will start to burn fat to create energy, therefore leading to weight loss. After fasting for a few days, there will be a higher level of endorphin in your blood which will cause you to be more alert and, weirdly, giving you a feeling of mental well being. Eventually, if you are starved for too long, your body will start to break down muscle protein in order to create energy (that is why, during WWII, most POW's held by the Japanese were very thin - they were starved and in order to survive, their bodies had to 'eat' their own muscle) Technically, starvation is when your body starts to eat your protein, i.e. Muscle.
During starvation, your body will thin out, because your fat has been burned and your muscle is being 'eaten'. This obviously leads to weakness. When your fat is being burned, you go through a detoxification process which is caused because any toxins stored in your body's fat are dissolved and removed from the body. Your kidneys are very efficient in being able to maintain the body's water and salts, like potassium and sodium. But these can be lost through perspiration. This eventually leads to dehydration which does many things to your body, like: not allowing for your body to regulate body temperature; your heart works harder (most of your water is in your blood (about %$60$) and when you lose the water, your heart has to 'catch' up and pump more to keep the blood flowing and cooling you down - now, your heart is working harder which could give you a greater risk of heat exhaustion); you can feel cranky, drowsy, confused, forgetful. These are a lot of things that happen when your body is deprived of what it needs in terms of food and liquid.

This is what happens, and this is very general, when your body is in its starvation mode. I am sure that there are many more things, but I could only give a brief answer.

Fasting Physiology – Part II

There are many misconceptions about fasting. It is useful to review the physiology of what happens to our body when we eat nothing.

Glucose and fat are the body’s main sources of energy. If glucose is not available, then the body will adjust by using fat, without any detrimental health effects. This is simply a natural part of life. Periods of low food availability have always been a part of human history. Mechanisms have evolved to adapt to this fact of Paleolithic life. The transition from the fed state to the fasted state occurs in several stages.

  1. Feeding – During meals, insulin levels are raised. This allows uptake of glucose into tissues such as the muscle or brain to be used directly for energy. Excess glucose is stored as glycogen in the liver.
  2. The post-absorptive phase – 6-24 hours after beginning fasting. Insulin levels start to fall. Breakdown of glycogen releases glucose for energy. Glycogen stores last for roughly 24 hours.
  3. Gluconeogenesis – 24 hours to 2 days – The liver manufactures new glucose from amino acids in a process called “gluconeogenesis”. Literally, this is translated as “making new glucose”. In non-diabetic persons, glucose levels fall but stay within the normal range.
  4. Ketosis – 2-3 days after beginning fasting – The low levels of insulin reached during fasting stimulate lipolysis, the breakdown of fat for energy. The storage form of fat, known as triglycerides, is broken into the glycerol backbone and three fatty acid chains. Glycerol is used for gluconeogenesis. Fatty acids may be used for directly for energy by many tissues in the body, but not the brain. Ketone bodies, capable of crossing the blood-brain barrier, are produced from fatty acids for use by the brain. After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta hydroxybutyrate and acetoacetate, which can increase over 70 fold during fasting.
  5. Protein conservation phase – >5 days – High levels of growth hormone maintain muscle mass and lean tissues. The energy for maintenance of basal metabolism is almost entirely met by the use of free fatty acids and ketones. Increased norepinephrine (adrenalin) levels prevent the decrease in metabolic rate.

The human body has well developed mechanisms for dealing with periods of low food availability. In essence, what we are describing here is the process of switching from burning glucose (short term) to burning fat (long term). Fat is simply the body’s stored food energy. In times of low food availability, stored food is naturally released to fill the void. So no, the body does not ‘burn muscle’ in an effort to feed itself until all the fat stores are used.

Hormonal Adaptation

Fasting is the most efficient and consistent strategy to decrease insulin levels. This was first noted decades ago, and widely accepted as true. It is quite simple and obvious. All foods raise insulin, so the most effective method of reducing insulin is to avoid all foods. Blood glucose levels remain normal, as the body begins to switch over to burning fat for energy. This effect is seen with fasting periods as short as 24-36 hours. Longer duration fasts reduce insulin even more dramatically. More recently, alternate daily fasting has been studied as an acceptable technique of reducing insulin.

Regular fasting, in addition to lowering insulin levels, has also been shown to improve insulin sensitivity significantly. This is the missing link in the weight loss puzzle. Most diets reduce highly insulin-secreting foods, but do not address the insulin resistance issue. Weight is initially lost, but insulin resistance keeps insulin levels and Body Set Weight high. Fasting is an efficient method of reducing insulin resistance.

Lowering insulin rids the body of excess salt and water. Insulin causes salt and water retention in the kidney. Atkins style diets often cause diuresis, the loss of excess water, leading to the contention that much of the initial weight loss is water. While true, diuresis is beneficial in reducing bloating, and feeling ‘lighter’. Some may also note a slightly lower blood pressure. Fasting has also been noted to have an early period of rapid weight loss. For the first five days, weight loss averages 0.9 kg/ day, far exceeding the caloric restriction and likely due to a diuresis of salt and water.

Growth Hormone

Growth hormone is known to increase the availability and utility of fats for fuel. It also helps to preserve muscle mass and bone density. Secretion is known to be pulsatile, making accurate measurement difficult. Growth hormone secretion decreases steadily with age. One of the most potent stimuli to growth hormone secretion is fasting. Over a five-day fasting period growth hormone secretion more than doubled. The net physiologic effect is to maintain muscle and bone tissue mass over the fasting period.


Concerns about malnutrition during fasting are misplaced. Insufficient calories are not a major worry, since fat stores are quite ample. The main concern is the development of micronutrient deficiency. However, even prolonged studies of fasting have found no evidence of malnutrition. Potassium levels may decrease slightly, but even two months of continuous fasting does not decrease levels below 3.0 mEq/L, even without the use of supplements. This duration of fasting is far longer than generally recommended. Magnesium, calcium and phosphorus levels during fasting are stable. Presumably, this is due to the large stores of these minerals in the bones. Ninety nine percent of the calcium and phosphorus in the body is stored in the bones. The use of a multi-vitamin supplement will provide the recommended daily allowance of micronutrients. A therapeutic fast of 382 days was maintained with only a multivitamin with no harmful effect on health. Actually, this man maintained that he had felt terrific during this entire period. The only concern may be a slight elevation in uric acid that has been described in fasting .

Adrenalin levels are increased so that we have plenty of energy to go get more food. For example, 48 hours of fasting produces a 3.6% increase in metabolic rate, not the dreaded metabolic ‘shut-down’. In response to a 4 day fast, resting energy expenditure increased up to 14%. Rather than slowing the metabolism, instead the body revvs it up. Presumably, this is done so that we have energy to go out and find more food.

This is really quite interesting. Fasting, but not low calorie diets results in numerous hormonal adaptations that all appear to be highly beneficial on many levels. In essence, fasting transitions the body from burning sugar to burning fat. Resting metabolism is NOT decreased but instead increased. We are, effectively, feeding our bodies through our own fat. We are ‘eating’ our own fat. This makes total sense. Fat, in essence is stored food. In fact, studies show that the epinephrine (adrenalin) induced fat burning does not depend upon lowering blood sugar.

Recall our previous discussion of How Insulin Works. Fat is food stored away in the long term, like money in the bank. Short term food is stored as glycogen, like money in the wallet. The problem we have, is how to access the money in the bank. As our wallet depletes, we become nervous and go out to fill it again. This prevents us from getting access to money in the bank.

Fat is stored away in the ‘bank’. As our glycogen ‘wallet’ depletes, we get hungry and want to eat. That makes us hungry, despite the fact that there is more than enough ‘food’ stored as fat in the ‘bank’. How do we get to that fat to burn it? Fasting provides an easy way in.

What Fasting Does to Metabolism

When you severely limit calories, your body senses this shortage of fuel and slows down its functioning to conserve energy. Instead of boosting your metabolism, you may experience a suppression of your resting metabolism equal to as much as 20 percent. Your resting metabolism is based on the energy your body uses to fuel basic functions, such as pumping blood and breathing. These activities don't stop, your body just becomes more efficient and burns fewer calories to do them than it would when adequately fed.

This is one of the reasons very low-calorie diets and fasts don't often bring about the results you'd expect. Your body is fighting what it perceives as starvation by slowing the rate at which it burns calories.

Fasting ramps up human metabolism, study shows

Fasting may help people lose weight, but new research suggests going without food may also boost human metabolic activity, generate antioxidants, and help reverse some effects of aging. Scientists at the Okinawa Institute of Science and Technology Graduate University (OIST) and Kyoto University identified 30 previously-unreported substances whose quantity increases during fasting and indicate a variety of health benefits.

"We have been researching aging and metabolism for many years and decided to search for unknown health effects in human fasting," said Dr. Takayuki Teruya, first author of the paper and a technician in the OIST G0 Cell Unit, led by Prof. Mitsuhiro Yanagida. "Contrary to the original expectation, it turned out that fasting induced metabolic activation rather actively."

The study, published January 29, 2019 in Scientific Reports, presents an analysis of whole human blood, plasma, and red blood cells drawn from four fasting individuals. The researchers monitored changing levels of metabolites -- substances formed during the chemical processes that grant organisms energy and allow them to grow. The results revealed 44 metabolites, including 30 that were previously unrecognized, that increased universally among subjects between 1.5- to 60-fold within just 58 hours of fasting.

In previous research, the G0 Cell Unit identified various metabolites whose quantities decline with age, including three known as leucine, isoleucine, and ophthalmic acid. In fasting individuals, these metabolites increase in level, suggesting a mechanism by which fasting could help increase longevity.

"These are very important metabolites for maintenance of muscle and antioxidant activity, respectively," said Teruya. "This result suggests the possibility of a rejuvenating effect by fasting, which was not known until now."

Metabolites Give Clues to Mechanism and Health Effects

The human body tends to utilize carbohydrates for quick energy -- when they're available. When starved of carbs, the body begins looting its alternate energy stores. The act of "energy substitution" leaves a trail of evidence, namely metabolites known as butyrates, carnitines, and branched-chain amino acids. These well-known markers of energy substitution have been shown to accumulate during fasting.

But fasting appears to elicit effects far beyond energy substitution. In their comprehensive analysis of human blood, the researchers noted both established fasting markers and many more. For example, they found a global increase in substances produced by the citric acid cycle, a process by which organisms release energy stored in the chemical bonds of carbohydrates, proteins and lipids. The marked increase suggests that, during fasting, the tiny powerhouses running every cell are thrown into overdrive.

Fasting also appeared to enhance the metabolism of purine and pyrimidine, chemical substances which play key roles in gene expression and protein synthesis. The finding suggests fasting may reprogram which proteins cells build at what time, thus altering their function. The change may promote homeostasis in cells, or serve to edit their gene expression in response to environmental influences.

When metabolized, purine and pyrimidine also boost the body's production of antioxidants. Several antioxidants, such as ergothioneine and carnosine, were found to increase significantly over the 58-hour study period. Antioxidants serve to protect cells from free radicals produced during metabolism. Products of a metabolic pathway called the "pentose phosphate pathway" also stay the harmful effects of oxidation, and were similarly seen to increase during fasting, but only in plasma.

Newfound Health Benefits of Fasting?

The authors suggest that these antioxidative effects may stand as the body's principal response to fasting, as starvation can foster a dangerously oxidative internal environment. Their exploratory study provides the first evidence of antioxidants as a fasting marker. In addition, the study introduces the novel notion that fasting might boost production of several age-related metabolites, abundant in young people, but depleted in old.

"Recent aging studies have shown that caloric restriction and fasting have a prolonging effect on lifespan in model animals. but the detailed mechanism has remained a mystery," said Teruya. "It might be possible to verify the anti-aging effect from various viewpoints by developing exercise programs or drugs capable of causing the metabolic reaction similar to fasting."

The findings expand on established ideas of what fasting could do for human health. The next step would be to replicate these results in a larger study, or investigate how the metabolic changes might be triggered by other means.

"People are interested in whether human beings can enjoy the effects of prevention of metabolic diseases and prolonging life span by fasting or caloric restriction, as with model animals," said Teruya. "Understanding the metabolic changes caused by fasting is expected to give us wisdom for maintaining health."

Fasting myths

There are many myths associated with fasting. These myths have been repeated so often that they are often perceived as infallible truths. Some of the these myths include:

  • Fasting puts you in ‘starvation’ mode
  • Fasting will overwhelm you with hunger
  • Fasting causes overeating when you resume feeding
  • Fasting will make you lose lots of muscle
  • Fasting causes hypoglycemia
  • The brain needs glucose to function
  • It’s just ‘crazy’

Even though they have been long ago disproven, these fasting myths still persist. If they were true, none of us would be alive today.

Does fasting burn muscle and worsen brain health?

Consider the consequences of burning muscle for energy. During long winters, there were many days where no food was available. After the first episode, you would be severely weakened. After several repeated episodes, you would be so weak that you would be unable to get hunt or gather food. Humans would never have survived as a species.

The better question would be why the human body would store energy as fat if it planned to burn protein instead. The answer, of course, is that is does not burn muscle as long as other fuel, such as fat, is available. It was only a myth.

There is another persistent myth that brain cells require glucose for proper functioning. This is incorrect. Human brains, unique amongst animals, can use ketones as a major fuel source during prolonged starvation, allowing the conservation of protein such as skeletal muscle.

Again, consider the consequences if glucose were absolutely necessary for survival. Humans would not survive as a species. After 24 hours, glucose becomes depleted and we become blubbering idiots as our brains shut down. Our intellect, our only advantage against wild animals, begins to disappear. Humans would have soon become extinct.

Fat is simply the body’s way of storing food energy for the long term, and glucose/glycogen is the short-term solution. When short-term stores are depleted, the body turns to its long-term stores without problems.

Fasting burns fat – not muscle

Studies of alternate daily fasting, for example, show that the concern over muscle loss is largely misplaced. Alternate daily fasting over 70 days decreased body weight by 6%, but fat mass decreased by 11.4%. Lean mass (including muscle and bone) did not change at all. Significant improvements were seen in LDL cholesterol and triglyceride levels. Growth hormone increases to maintain muscle mass. Studies of eating a single meal per day found significantly more fat loss despite the same caloric intake. Importantly, no evidence of muscle loss was found.

More recently, a randomized trial of fasting versus caloric restriction found no evidence that muscle is ‘burned’ during fasting. In this trial, the fasting group followed a protocol of 36 hour fasting every other day (alternate daily fasting or ADF).

According to some ‘experts’ fasting will burn approximately ⅓ of a pound of muscle per day. This equates to 1 pound of muscle per week, and over the course of 32-week study, the fasting group should lose 32 pounds of muscle. The ACTUAL amount of lean mass lost was 1.2 kg (2.6 pounds), but importantly, this was LESS than caloric restriction (1.6 kg). Also, some lean mass is lost during weight loss (skin, connective tissue) and the lean mass percentage INCREASED by 2.2% during fasting.

My clinical experience is the same. Having treated well over a thousand patients with fasting, the total number who have complained of persistent muscle weakness is a grand total of zero. Notice also, how the fasting burns more than TWICE the amount of dangerous truncal fat, also known as abdominal fat. This fat around the midsection is far more harmful to health than fat carried under the skin.

Starvation mode

Consider an analogy. A freezer stores food in the long term, and a refrigerator used for short-term storage. Suppose that three times a day, everyday, we go to the market to buy food. Some goes into the refrigerator, but the excess goes into the freezer. Soon one freezer is not enough, so we buy another, then another. Over a period of decades, we have ten freezers, and nowhere else to put them. Food in the freezer does not get eaten because three times a day, we still buy more food. There is simply no reason to release the food from the freezer. What would happen if, one day, we decide not to buy food? Would everything shut down in ‘starvation mode’? Nothing could be further from the truth. We would first empty the refrigerator. Then the food, so carefully stored in the freezer would be released.

So, in the body’s case, glucose is used for short-term energy and fat for long-term storage (the freezer). Fat is not burned when plenty of glucose is available. Over decades of abundant glucose, fat stores proliferate. What would happen if glucose were suddenly unavailable? Would everything shut down in ‘starvation mode’? Nothing could be further from the truth. Energy, so carefully stored as fat, would be released.

Starvation mode, as it is popularly known, is the mysterious boogieman always raised to scare us away from missing even a single meal. Over one year, approximately 1000 meals are consumed. Over a span of 60 years, this equals 60,000 meals. To think that skipping three meals of the 60,000 will somehow cause irreparable harm is simply absurd. Breakdown of muscle tissue happens at extremely low levels of body fat – approximately 4%. This is not something most people need to worry about. At this point, there is no further body fat to be mobilized for energy and lean tissue is consumed. The human body has evolved to survive episodic periods of starvation. Fat is stored energy and muscle is functional tissue. Fat is burned first. This is akin to storing a huge amount of firewood but deciding to burn your sofa instead. That simply doesn’t make sense. The body preserves muscle mass until the body fat becomes so low that it has no choice.

The other persistent myth of ‘starvation mode’ is that basal metabolism decreases severely and our bodies ‘shut down’. This too is highly disadvantageous to survival of the human species. If, after a single day of fasting, metabolism decreased, then we would have less energy to hunt or gather food. With less energy, we are less likely to get food. So, another day passes, and we are even weaker, making us even less likely to get food. This is a vicious cycle that the human species would not have survived. Again, it simply doesn’t make sense. There are, in fact, no species of animals, humans included that are evolved to require three meals a day, everyday. We have already seen in a previous post that resting energy expenditure (REE) goes UP, not down during fasting. Metabolism revs up it does not shut down.

Again, in the most recent study, caloric restriction lowered the Resting Metabolic Rate (RMR) by an average of 76 calories per day (statistically significant), whereas the fasting group only lowered RMR by 29 calories per day (NOT statistically significant). In other words, caloric restriction lowered metabolism but fasting did not.

It’s unclear to me where this myth originated. Daily caloric restriction leads to decreased metabolism so people assumed that this would simply be magnified as food intake dropped to zero. This is wrong. If you rely on food for energy, then decreasing food will lead to decreased energy intake, which will be matched by decreased energy expenditure. However, as food intake goes to zero, the body switches energy inputs from food to stored food (fat). This significantly increases the availability of ‘food’ and this is matched by an increase in energy expenditure.

The Minnesota starvation experiment

So what happened in the Minnesota Starvation Experiment? These participants were not fasting. They were eating a reduced-calorie diet. The hormonal adaptations to fasting were not allowed to happen. In response to a prolonged period of lowered food intake, the body makes the adjustment to lower TEE.

Everything changes when food intake goes to zero (fasting). The body obviously cannot take TEE down to zero. Instead, the body now switches to burning the fat stored on our bodies. After all, that is precisely, exactly what it was put there for. Our body fat is used for food when no food is available. It’s not put there for looks.

Switching fuels

Detailed physiologic measurements show that TEE is maintained or sometimes even increased over the duration of a fast. Alternate daily fasting over 22 days found no measurable decrease in TEE. There was no ‘starvation’ mode. There was no decreased metabolism. Fat oxidation increased 58% while carbohydrate oxidation decreased from 53%. This means that the body has started to switch over from burning sugar to burning fat with no overall drop in energy. Four days of fasting actually increase TEE by 12%. Norepinephrine levels (adrenalin) absolutely skyrocketed 117% to maintain energy. Fatty acids increased over 370% as the body switched to burning fat. Insulin measurements decreased 17%. Blood glucose levels dropped slightly but remained in the normal range.

All the incredibly beneficial adaptations to fasting are not allowed to happen in a low calorie diet.

In fact, look how quickly the merest touch of glucose reverses the hormonal changes of fasting. Only 7.5 grams of glucose (2 teaspoons of sugar or barely a sip of a soft drink) is enough to reverse the ketosis. Almost immediately after consuming glucose, the ketones beta hydroxybutyrate and acetoacetate drop to almost nothing, as does fatty acids. Insulin rises, as does glucose.

What does this mean? The body stops burning fat. It has now returned to burning the sugar that you are eating.

What about overeating?

Repeated concerns are raised that fasting may provoke overeating. Studies of caloric intake do show a slight increase at the next meal. After a one day fast, average caloric intake increases from 2436 to 2914. But over the entire 2-day period, there is still a net deficit of 1958 calories. The increased calories did not nearly make up for the lack of calories on the fasting day. Personal experience in our clinic shows that appetite tends to decrease with increased duration of fasting.

Does fasting deprive the body of nutrients? Most people have more than ample quantities of nutrients. That’s the whole point. To get rid of some of these nutrients – also known as fat.

If you are worried about micronutrients and minerals – you can always take a general multi-vitamin. A different regimen such as alternate daily fasting (ADF) can also alleviate concerns about nutrient deficiency.

The science is clear. The myths surrounding fasting were only falsehoods.

Stage 4 Fasting (36-48 hours): Growth Hormone and Recovery

At stage 4, you’re leaving intermittent fasting territory and entering a longer fast.

As you undertake multiple days of fasting, your growth hormone levels begin to shift, which provides you with a new set of benefits.

Muscle Growth and Repair

A study of healthy adults found that 48-hour fasting increased human growth hormone (HGH) secretion by up to 400%. It also increased the frequency of growth hormone bursts throughout the day[ * ].

HGH increases muscle mass[ * ] and stimulates faster muscle repair[ * ]. It may also speed up the healing process for wounds and more serious injuries[ * ]. HGH is so effective that taking it externally is banned in professional sports and is considered doping.

It seems counterintuitive, but occasionally going without food for two days may actually help you build muscle, not lose it.

What happens if you fast for a day?

What happens if you don’t eat for a day? The answer might seem relatively straightforward, but fasting for 24 hours has a complex ripple effect in the body.

Fasting is a longstanding part of many religious traditions, including the Jewish and Muslim observances of Yom Kippur and Ramadan. A form of fasting known as intermittent fasting has also gained popularity as a weight-loss tool.

Many studies have examined the benefits and risks of giving up food for a day, including how it affects weight loss.

In this article, we look at what happens to the body during fasting, as well as what a person can do to make fasting safer.

Share on Pinterest Studies suggest fasting may help with weight loss.

Whether a person is fasting or not, the body still needs energy. Its primary energy source is a sugar called glucose, which usually comes from carbohydrates, including grains, dairy products, fruits, certain vegetables, beans, and even sweets.

The liver and muscles store the glucose and release it into the bloodstream whenever the body needs it.

However, during fasting, this process changes. After about 8 hours of fasting, the liver will use the last of its glucose reserves. At this point, the body enters into a state called gluconeogenesis, marking the body’s transition into fasting mode.

Studies have shown that gluconeogenesis increases the number of calories the body burns. With no carbohydrates coming in, the body creates its own glucose using mainly fat.

Eventually, the body runs out of these energy sources as well. Fasting mode then becomes the more serious starvation mode.

At this point, a person’s metabolism slows down, and their body begins burning muscle tissue for energy.

Although it is a well-known term in dieting culture, true starvation mode only occurs after several consecutive days or even weeks without food.

So, for those breaking their fast after 24 hours, it is generally safe to go without eating for a day unless other health conditions are present.

It does appear that fasting can help with weight loss . However, studies make it clear that this is not the case for everyone.

Popular diet plans include 12-hour or 16-hour fasting periods, as well as the 24-hour fast. Some diets require people to drink only water during the fast, while others allowed any zero-calorie beverage.

Fasting is not necessarily better than any other weight-loss method, including reducing daily calorie intake by a small amount.

In a recent study , people with obesity who fasted intermittently for 12 months lost slightly more weight than those who dieted in a more traditional way, but the results were not statistically significant.

The limits of fasting appear to have less to do with its physical effects than how it fits into a given lifestyle.

For example, the same study found that people who fasted were more likely to give up on weight-loss efforts than those who dieted in a more traditional way, such as counting calories. The researchers concluded that fasting might be harder to maintain over time.

Another possible concern is post-fast binging. Some fasting experts agree that it is easy to derail weight-loss successes by overeating after the fasting period.

Fasting days can also offer a false sense of security, leading people to disregard positive eating habits on non-fasting days.

How your body fights to keep you alive when you’re starving

The human body can go without oxygen for about five to ten minutes, and about three to eight days without water. But remarkably, people have been known to live upwards of 70 days without food. How is this possible?

The answer lies in a series of evolved physiological and metabolic defenses that work to keep you alive for as long as possible in the unfortunate event that you don't have access to food. Just because you're starving doesn't mean you've become helpless. Here's how your body fights to keep you alive and active.

By definition, starvation is a process. Our bodies are not like cars which immediately shut down when they're out of gas. When we experience prolonged low energy intake, and as long as water is available, our bodies enter into a successive series of metabolic modes. It's the body's way of recognizing that food is scarce, and that it needs to re-allocate resources in preparation for what could be an extended period. In essence, your body is buying you some valuable time to give you a fair chance of finding some food.

0-6 hours after eating

Soon after eating, our bodies start to break down glycogen (molecules that store energy) to produce glucose (an important carbohydrate that fuels cells). When we're eating normally, we use glucose as our primary fuel source all is well, we're happy, and in storage mode. Glucose gets packed into our liver and muscle, with the fatty acids getting stored around our body for (potential) future use.

In terms of energy allocation, our brains require 25% of the body's total stored energy (which is a lot if you think about it), with the rest going to fuel our muscle tissues and red blood cells.

We can go for about six hours in this glucose-burning mode, which is why we tend to get a bit cranky if we have to go without food for longer than that.

Now that said, not everyone metabolizes energy in this way. Some individuals are in a state of ketosis in which they have elevated levels of ketone bodies — compounds that are produced when fatty acids are broken down for energy instead of glucose. People in ketosis include those on a ketogenic diet (a high-fat, protein rich, low-carbohydrate diet), or those who have just completed a long physical training session. Technically speaking they're not starving — they're just in a different metabolic mode.

6-72 hours after eating

Now, whether you like it or not, you will enter into a state of ketosis should you go without food for six hours or more this represents the first significant metabolic phase shift as you enter into starvation. At this point, all your glycogen stores will have been exhausted, and your body has no choice but to start hitting the fatty acids for energy. During lipolysis, fatty acids are directly broken down to produce ketone bodies.

Now all this is fine and well except for one very important thing: your brain cannot use fatty acids directly as its fuel source. These fats are large and cannot cross the blood-brain barrier. So, for the first 24 to 48 hours without food, your brain will continue to use the remaining glucose stores as fuel, while the rest of the body goes into the ketosis cycle.

Trouble is, the glucose isn't enough the brain requires about 120 g of glucose per day (the same amount of sugar found in three cans of soda!). At this rate, the brain would starve and die in about three days — but clearly it does not. This is because your body has evolved a backup plan.

It's at this critical stage when the ketone bodies become all the more critical. Because they're short-chain derivatives of fatty acids, they function as tiny powerpacks that can cross the blood-brain barrier and in turn be used by the brain as an alternative metabolic fuel. Your brain will get about 30% of its energy from ketone bodies on day three, but by day four it will jump to 70%. Moreover, your brain's glucose requirement will drop from 120 g per day to 30 g once the body enters into this phase.

Fascinatingly, humans may be the only species who have brains that don't require the ongoing ingestion of glucose to function. Most animals are forced to break down skeletal muscles at a higher rate. The going theory is that, because we humans are so greatly dependent on our intelligence to survive, we have evolved the capacity to stay cognitively sharp while in the midst of prolonged starvation, thus allowing us to search for food.

72 hours and onward

That said, your brain is not out of the woods yet. It's still short of about 10 g of glucose per day. The brain has got to get its energy from somewhere, and that somewhere is your body's own proteins. At this stage in your starvation (or fast), all the cells in your body will start to break down protein that releases amino acids into the bloodstream. These amino acids are then converted into glucose by the liver, and your brain is happy again.

Fed, Fasting and Starved Metabolic States

What happens when you don’t eat enough? This post is going to talk about the biological consequences of fasting and starvation. Starvation and/or under-eating may occur for a variety of reasons: circumstance, prolonged extreme dieting, or fasting diets may put the body in a state of starvation. This post will detail what exactly happens in your body when you are starving.

Before we dive deep into the biological impacts of fasting, I thought this little epiphany I had while researching this post was worth sharing.

Even though modern medicine has evolved tremendously, and we have adapted to incredible technology, at the end of the day we are still creatures, and much of our underlying biology is designed to help us survive as our hunter-gathering ancestors did. Our bodies have astounding protective mechanisms and tightly regulated systems that work to keep us alive.

Personally, I find this stuff fascinating. Ever since I had to read this paper many many years ago, I found the metabolic shifts the body undergoes while eating, post-meal, and during extended periods of time without food riveting.

I am not sure if anyone else will find this interesting (lol really selling myself here…but for real if this is to much science lingo and too in depth, let me know), but I think understanding this process is also important if you want to better understand fasting diets, consequences or physiological damages caused by eating disorders, or what “keto” really means. Hope you enjoy!

*Disclaimer: As always, this is general information intended for healthy adults. Your needs may vary based on medical status, lifestyle, or life-stage. Please never replace generalized health information you’ve read online with individualized clinical care.

Forms of Energy Stores in the Body:

When your body has extra energy after a meal that isn’t immediately needed, it is stored. Your body tightly regulates the amount of glucose (energy/sugar) in the bloodstream and tucks the rest away into storage for a rainy day. This is your body’s way of defending against possible potential starvation scenarios.

Many of us are aware that fat (adipose triglyceride) tissue is one of the major forms of energy reserve in the human body. While adipose tissue serves other important roles, including insulation and padding for your organs, one of its main functions is to serve as an energy depot.

In addition to adipose tissue, glycogen (from carbohydrates) is stored in the liver and muscles. Protein is also stored in muscle. Although it can be broken down by the body and transformed into energy in dire circumstances, as it serves a variety of roles, the body makes great effort to spare it unless depleted of all other options.

Why You Need Food:

To maintain proper function, the brain, nervous system, and cells needs energy. That energy comes in the form of glucose. Typically, your body tightly regulate levels of blood glucose to be able to properly fuel your heart, lungs, brain, and muscles to fuel your daily activities.

The body does a great job tucking extra energy into storage for later, and harvesting it as needed. More details below.

The Well-Fed State:

In a well-fed state, glucose and amino acids are transported from the intestines into the bloodstream to be carried all over the body for energy. The liver has first dibs on using dietary glucose, where it can be converted to glycogen (for storage). Red blood cells (RBC) also use a lot of glucose in the fed state, because unlike other cells, they don’t have a mitochondria, which is the powerhouse of the cell used to create fuel.

In the fed state, insulin is secreted by the pancreas, and stimulates the storage of fuel and protein kinase cascades. To put it simply, insulin (read more about hunger-related hormones here) promotes glucose to enter the muscle and adipose (fat) tissue for storage. Insulin also promotes the uptake of amino acids into muscles, favoring a build up of muscle proteins.

Metabolism in the Post-Fed State:

Once you stop eating, your body can no longer harvests energy directly from ingested glucose but must start to rely on other sources of stored fuel. For the first few hours after a meal or overnight, glycogen in the liver is harvested as the major source of glucose.

Metabolism in the Early Fasting State:

Carbohydrate stores are typically depleted after about a day. After 18-48 hours of no food intake, the body starts the evolve its energy-harvesting strategy.

When glucose levels in the blood begin to drop, insulin decreases and a different hormone known as glucagon begins to increase. Glucagon signals a starved state to the body. As the brain and other tissues (including red blood cells) are dependent upon glucose for fuel, the body does everything it can to protect and fuel itself.

Glucagon stimulates the breakdown of glycogen (stored energy) from the liver, releasing the storage from the liver into the bloodstream as a form of fuel. Fatty acid stored in adipose tissue can also be oxidized for energy, and transported to the brain for energy.

Glucocorticosteroid hormones are also released. The combined presence of low insulin levels and glucocorticosteroid hormones send signals to the body to bring to hydrolyze proteins in muscle cells and to provide amino acids that can be used to create fuel.

Large amounts of nitrogen are typically lost in urine during the early fasting rate, as a result of the high rate of muscle protein breakdown and synthesis of glucose from muscle glucogenesis.

Metabolism in Prolonged Fasting:

If fasting or starvation is ongoing, eventually the body will deplete stores of glycogen. It goes through another metabolic shift.

The body’s goal at this point is to maintain vital physiological functions. It aims to save proteins as best it can.

Fat is broken down into compounds known as glycerol and fatty acids. Through a complex series of events and biological processes (I could go into detail, but I’m guessing no one reading this really wants me to since this is rather science-heavy already, but if you do, let me know) the fatty acids travel to the liver where they are broken into carbon units, which then form units called ketones.

Ketones may be used in place of glucose as a source of fuel for the brain and nervous system during periods of starvation. When ketones are feeding the brain, glycogenesis (the breakdown of muscle for fuel) slows down to try to spare muscle mass.

You may have heard of the ‘keto’ diet. The keto diet strives to achieve ketosis. As you can tell, ketosis is your body’s line of defense against starvation. I am planning a whole post about that, but thought the metabolism of starvation may be useful background information to have before the keto diet is detailed.

During this time, the kidney is also harvested for glucose. The kidney also releases NH3 to try to neutralize the organic acids from the ketone bodies.

After a couple weeks to a couple months (depending on the person), the body may run out of fat stores to harvest for energy, and will turn back to harvesting protein from muscle to make fuel. The metabolism slows down to protect the body from death.

Keep in mind that your heart is a muscle. Eventually, your body harvesting protein for fuel may cause organ failure, such as cardiac failure, resulting in death. Survival time depends on the amount of fat a person has before enduring starvation.

Even for those with large fat stores, fasting state’s can cause physiological damage or death due to extreme ketosis.

It’s also important to note that you don’t have to be eating literally nothing to go into a starvation state. You burn a lot of energy just by thinking, breathing, and engaging in other life-sustaining activities, even if you are completely sedentary. Under-eating for a prolonged period (due to circumstance or due to an eating disorder) can put your body into metabolic states like those of starvation.

That wraps up this post for today! Stay tuned for posts on fasting, under-eating, and keto diets coming soon.

Thank you for reading and as always, feel free to leave questions or comments below, or say hello on Instagram, Twitter, or YouTube!

Shock results

It was difficult to believe that something that made me feel so awful could possibly be doing me good, especially since my test results from before the fast showed that I was already metabolically healthy. I had low levels of “bad” cholesterol, healthy blood sugar and fat levels and very low amounts of visceral fat – the stuff that sticks to our organs and which can be a risk factor for cardiovascular disease and diabetes. The body scan showed a fair amount of body fat (30 per cent), but nearly all concentrated on my hips and thighs, a pattern that has been linked to a lower risk of heart disease and diabetes.

After five days of fasting, none of this had shifted. The body composition scan revealed that I had lost just over 1 kilogram in weight, 584 grams of which came from a loss of lean mass and only 168 grams from body fat. This was a bit of a shock – one selling point of the fasting mimicking diet is that it is supposed to target visceral fat while protecting lean mass. According to Longo, ketosis doesn’t target the visible wobbly bits, only fat around the organs. As I started off with little visceral fat, it instead targeted my lean mass, he says.

“From a global health perspective, I find it quite a negative outcome,” says Karpe. “Half of your change was muscle. The fat regions have not changed much at all. That’s not what you intended.”

Could it be that the lean mass loss was the result of autophagy? Mice put on Longo’s diet in middle age certainly seemed to have some kind of clear-out: their liver, heart and kidneys all shrank during the fast and they had a temporary cull in the numbers of some kinds of blood cells. All went back to normal within a few days of normal eating, heralded by an increase in markers of liver regeneration and tentative signs of muscle regeneration. The assumption is that the decrepit cells that were removed were replaced by newer, shinier versions.

The evidence for autophagy and regeneration in human trials, however, is purely circumstantial. And we don’t know whether any new cells are healthier than what was lost. Longo concedes that this is something his team is still working on.

When it came to blood markers of health and longevity, my results were similarly unimpressive (see “Graph”). The only marked difference was to the hormone IGF-1.

In the ProLon trials, volunteers saw a significant reduction in IGF-1, which Longo says was still there three months after going back to a normal diet.

“When people did the diet they had lower body weight, fat and cholesterol”

Whether this adds up to increased longevity, however, is less clear. Epidemiological studies have linked both low and high IGF-1 levels to early death, with high IGF-1 levels linked to increased cancer risk and low IGF-1 to cardiovascular disease.

Of course, my experiment of one isn’t very scientific, but it did get me wondering: for those of us who are healthy to begin with, does fasting truly offer a benefit beyond the fact you inevitably cut a few calories and lose a bit of weight?

How to Really “Cleanse” Your Body via Autophagy

While a detox or juice cleanse may help clean out your colon for the short-term, it’s not really “cleansing” your body—certainly not in the way autophagy does.

Autophagy is a natural cleansing process that allows cells to recycle their proteins and other essential parts to help make new, healthy cells, while ridding of toxic waste products and damaged parts.

It basically cleans out your body’s toxins, pathogens and damaged molecules at the cellular level, making way for strong, healthy cells. It’s the ideal “cleanse.”

Autophagy is essential to our survival, and is triggered when the body is stressed or starved. However, its activity level tends to decrease as we age.

That said, it’s possible to boost autophagy to take advantage of its crucial role in protecting the brain, heart, immune and metabolic systems, as well as decreasing inflammation and the risk of neurodegenerative diseases.

Fasting, including intermittent fasting (IF), is one of the key ways to kick-start autophagy. But IF may not be ideal for everyone, especially women, who may experience hormonal imbalances.

Another proven way to boost autophagy is through exercise, possibly more effectively via high-intensity interval training (HIIT).

Overall, we know that autophagy is a crucial part of our survival and closely connected to longevity, but how much of it and how much we should work to boost it—especially as we age—is still unknown. And it all likely differs by individual.

This means it’s probably best to look at the big picture. Rather than try to “hack” your body at the cellular level, focus more on a strong and sustainable diet and exercise plan that works for you. This will inevitably promote longevity anyway.

If anything, at least we know that the body is already working hard to cleanse itself, so there’s no need to struggle through another juice cleanse or expensive “detox” program ever again.

About Stephanie Garr (Certified Nutrition Consultant)

Stephanie is a certified nutrition consultant. She graduated from the University of Iowa with degrees in journalism and psychology in 2003, and later studied holistic nutrition at Bauman College in Berkeley, California.

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Joe Leech, Dietitian (MSc Nutrition)

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