How are fasting and weight gain linked? Why do some people experience weight gain in the long run after going through a period of fasting?

How you can gain weight after a fast

In previous articles we read about the benefits of intermittent fasting and what intermittent fasting is about. Having understood its purposes and functions from these previous discussions, this article looks deeper into the causes of weight gain after a period of dieting and what can be done about it.

Total Energy Expenditure

When the body undergoes a period of nutritional ketosis, be it from fasting or a low carbohydrate diet, total energy expenditure is eventually reduced. This is a normal and expected outcome for a complex equilibrium system. In medical terms, the body always works to maintain homeostasis, and it is done by slowing down metabolism.

Symptoms before adaptation

So what happens before the liver begins breaking down fatty acids into ketones? There is a period of adaptation, which varies in severity with each individual. During this period, you will experience the following symptoms going into a fast:

  • The heart gets less glucose and has to slow down;
  • You get brain fog, until your liver begins producing ketones;
  • You become irritable;
  • You feel hungry;
  • Blood pressure drops as lower insulin levels induce the kidney to release water and salt;
  • You feel colder because less heat is generated with reduced energy expenditure;
  • Muscles are depleted of glycogen, and you feel physically exhausted.

What happens after adaptation?

As soon as the liver begins to produce ketone bodies in sufficient quantities by metabolizing fat, the body goes back into homeostasis:

  • The resting heart rate goes up;
  • Your temperature adjusts to normal levels;
  • Your irritability is gone;
  • Your become less hungry;
  • The liver begins to synthesize the glucose needed for the anabolic activities of muscles;
  • Your brain begins to operate at a higher level of efficiency than it did when it was fueled only by glucose;
  • Autophagy process breaks down old damaged tissue and promotes production of younger tissue.

The body becomes more efficient at conserving energy

The end result of this process is that the body adapts to the low energy intake by becoming more efficient at utilizing its own energy stores. The slower metabolism means less oxidative stress and reduced free radical production, which has its benefits for longevity.

The nervous system responds to the stress by entering a protective and regenerative mode (recall that fasting induces the production of neurotrophicfactors). So the neural cells are protected and autophagy is mainly happening in muscle tissue.

In extreme famines, muscles and organs shrink to adapt to a lower energy environment. This is the mechanism which homo sapiens evolved to survive periods of extreme stress, and it is a reversable process with its own benefits for longevity.

Intermittent Fasting and Weight Gain

Persistence of Metabolic Adaptation

So what happens when the famine is over? It turns out that muscle and organ sizes not only return to normal, but they overshoot in their growth cycle if the diet is excessive. What if the feasting the follows is permanent and the intermittent fasting is abandoned entirely?

The dirty secret of most diet plans

The rebound phenomenon is real, and this is why the failure rates are so high in most dietary plans. During a diet, the body adapts to the low energy intake. After returning to a “standard” diet, the body’s slow metabolism persists and causes a quick weight gain.

If the intermittent fasting pattern of eating is abandoned, then weight gain can exceed the initial point when the dietary regimen began. So what can be done about it?

Read on to find out.

Intermittent fasting can work

How to make intermittent fasting work

We read about the “calories in, calories out fallacy” in a previous article, and this article shows how total energy expenditure goes down by necessity in response to reduced calorie intake. The body will always work to maintain homeostasis. Therefore, it is a fact of biological necessity that metabolism will slow down when fasting.

Weight gain only happens because the diet chosen during the re-feeding cycle triggers hormonal changes that promote increased storage of fat. The system’s efficiency backfires if the primary macronutrient for re-feeding is carbohydrates, especially in its easily digestible refined form.

Increased insulin sensitivity is still good

One of the benefits of metabolic adaptation to intermittent fasting is that insulin sensitivity is increased. This is undoubtedly good news for promoting longevity. Increasing insulin sensitivity is key to reducing the risk of many diseases, and it is a major theme many articles on this site.

But increased insulin sensitivity also means more sensitivity to carbohydrates. The reintroduction of this macronutrient into the diet has to be tempered, or it will be absorbed rather quickly into fat cells.

Put another way: you will need fewer carbohydrates in your diet as a result of the optimizations you did to your body during the intermittent fasting period.

Do not reduce the overall number of calories

If you are doing this correctly, you should not feel hunger. The point of intermittent fasting is to change the frequency of eating while reducing overall insulin levels. The overall number of calories should not change. The composition of these calories, however, must change to reduce the insulin spikes during the re-feeding cycle.

Bone broth is a good way to make the fasting period more pleasant and eliminate the hunger. It works to maintain the essential nutrients while getting all the benefits of a fast.

Intermittent fasting is not starvation

Done correctly, intermittent fasting would not deprive your body of any essential nutrients. Important minerals, salts and water must be maintained during the fast. Re-feeding cycles should aim to increase dietary fat at the expense of carbohydrates. Refined carbohydrates must be eliminated entirely from the diet; otherwise, you will defeat the purpose of fasting.

Permanent adaptation is a lifestyle

Metabolic adaptation to a low carbohydrate diet happens rather quickly in comparison to the reverse adaptation to a high carbohydrate diet. Much of this adaptation to a high carbohydrate diet is genetic. For example: European populations are more adapted to higher carbohydrate content in their diet than Native American Indians.

Witness the Native American populations plagued with obesity and diabetes when they switched from a hunter-gatherer society to one that consumes corn and wheat as its primary food source. Descriptions of a fit and slender people by literature preceding the transition no longer hold true.

If you choose to do intermittent fasting, keep in mind that it is best to treat it as a lifestyle rather than a temporary diet. Genetic factors, family history, objectives, and even age group are a few things to consider if you choose this route to longevity.


Falchi M, et al. Low copy number of the salivary amylase gene predisposes to obesity. Nature Genetics 46, 492–497 (2014) doi:10.1038/ng.2939.

Schulz, Leslie O., and Lisa S. Chaudhari. “High-Risk Populations: The Pimas of Arizona and Mexico.” Current obesity reports 4.1 (2015): 92–98. PMC. Web. 1 Dec. 2017.

Taubes, Gary. 2009. The Diet Delusion [Kindle version].

Taubes, Gary, 2011. Why We Get Fat: And What to Do About It [Kindle version].