The Two Key Reasons Calorie Restricted Diets Fail

By Jason Fung, MD

Calorie counting fails for weight loss and usually leads to long-term weight gain. This pattern of yo-yo dieting is a well-recognized danger of calorie restricted diets.

  • Long-term calorie restriction diets result in two key adaptations that stymie further weight loss:
    • Decreased Metabolic Rate
    • Increased Hunger
  • Successful long term weight loss and weight maintenance requires focusing on fixing the underlying hormonal cause of obesity, rather than just calories.
By Jason Fung, M.D., Co-founder of The Fasting Method.


A key to the scientific understanding of calorie restriction for weight loss is that a number of the key assumptions underlying this theory are false. When we lose weight through calorie restriction, basal metabolic rate usually falls, too. It’s partially this resulting lowered basal metabolic rate (BMR) that ultimately dooms weight loss efforts. We hope that over time, the BMR will go back up towards ‘normal’ but unfortunately, studies show that it does not. BMR falls with calorie restricted diets and hunger increases, but worse, over time, these changes do not revert back to baseline but continue to worsen. This is proven by many of the scientific studies done over the years. Let’s take a look.

Basal Metabolic Rate (BMR)

Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight by Dr. Rosenbaum et al. 

In this study, subjects who experience a 10% weight loss were noted to have a reduced total energy expenditure (TEE) of about 300-400 calories/day. We hope maintaining that weight loss over time allows TEE to go back up to baseline. But it does not. Even by the end of the study, 1 year, the TEE did not go up but actually continued to go down further, an average of almost 500 calories/day.

In other words, this slowed metabolic rate (which reduces weight loss efforts) starts almost immediately after caloric reduction and persists for a long time – at least 1 year and going strong.


This study, published in the prestigious New England Journal of Medicine in 2011 clearly demonstrates the effect of weight loss through calorie restriction on hunger. This study is almost a decade old, so none of this information is exactly new. Using a liquid shake diet (51% carbohydrates), researchers reduced caloric intake to cause a 13.5 kg average weight loss, and then tried to maintain it over a year with a low-fat diet.

Long-Term Persistence of Hormonal Adaptations to Weight Loss N Engl J Med 2011; 365:1597-1604 October 27, 2011 Sumithran P 

Despite their best intentions, almost half of the weight was regained over the year. But what were the hormonal effects of this diet/ weight loss?

Hormonal Change

Researchers measured the hormone ghrelin – sometimes called the hunger hormone. The higher the ghrelin, the hungrier we are. Ghrelin levels moved higher during the weight loss phase, but the higher levels persisted even after a year. The hormone peptide YY, a satiety hormone released in response to protein and fat, was also measured. Essentially, Peptide YY makes us feel full. During the initial weight loss phase, peptide YY dropped and stayed low even after 62 weeks. The results for the other satiety hormones, amylin and cholecystokinin are similar. What does this all mean? It means when you lose weight by restricting calories, you feel more hungry and less full (satiated). Even after a year, the feeling of hunger never leaves. This isn’t some psychological voodoo from weight loss or loss of willpower effect. They were hungrier because their hunger hormones were higher and their satiety hormones were lower. There was a good physiologic reason for their hunger – the hormonal changes caused by calorie restriction.

Guaranteed to Fail

Let’s put all this together. Suppose you weigh 200 pounds and eat 2000 calories per day. Because your weight is stable, you must also burn 2000 calories per day (Total Energy Expenditure). One day, you decide to lose weight and start a calorie reduced diet, but don’t change the types of foods much, nor meal timing. You now eat 1600 calories per day, and to start, the weight comes down, say to 180 pounds. So far, so good.

In response to the weight loss and calorie restriction, we know from the many scientific studies done over the years the body will respond in a predictable manner. First, your body will reduce its TEE by 300-400 calories per day. So, instead of burning 2000 calories per day, you will only burn 1600. This means that you will feel colder, more tired, and fatigued. But weight loss will also slow down, because if you eat 1600 calories and burn 1600, there is no caloric deficit and therefore no weight loss.

The second predictable response is that your body increases hunger signaling by increasing ghrelin (the hunger hormone) and decreases satiety hormones (peptide YY, cholecystokinin). This makes you hungrier than before and increases your desire to eat. While you are consciously trying to eat less, your body is trying to make you eat more. This persists day after day, week after week, year after year.

The end result – weight regain. This, of course, is obvious to anybody who has ever been on a diet. This has NOTHING to do with a lack of willpower or any kind or moral failure. It is simply a hormonal fact of life. These are all real, measurable and well-known physical effects of calorie restriction.

So, here’s the bottom line. Focusing on caloric restriction alone for weight loss is virtually guaranteed to fail because of the two hormonal responses (decreased energy expenditure and increased hunger). This is why these diets have consistently failed despite thousands of doctors and dieticians giving this terrible advice over the last 50 years.

This is the vicious cycle of undereating.

  • Start by eating less
  • Lose some weight
  • Metabolism slows and hunger increases
  • Weight starts to go back up
  • Redouble our efforts by eating even less
  • A bit more weight comes off
  • Metabolism slows more and hunger increases
  • Weight starts to be regained
  • Repeat the cycle until intolerable

At some point, we simply cannot follow this diet any more. We increase our calories back up a little because we are feeling so tired and hungry. As we do go, the weight just goes back to our original weight. Friends, family, medical professionals now blame the victim by secretly thinking it is ‘our fault’ we could not lose the weight. They feel that we are a failure, with insufficient willpower. But willpower had nothing to do with the failure of calorie restricted diets. It was all just physiology, aka, our hormones.


For more, see The Obesity Code.

Learn more about Pique Fasting Tea.

For fasting education, support and community, go to The Fasting Method.


Jason Fung, MD

By Jason Fung, MD

Jason Fung, M.D., is a Toronto-based nephrologist (kidney specialist) and a world leading expert in intermittent fasting and low-carb diets.

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