The Key (but False) Assumptions of Calorie Counting

By Jason Fung, MD

Calorie counting doesn’t lead to weight loss.

  • Learn why calorie counting for weight loss doesn’t work. Don’t exclusively count calories to lose weight.
  • Many key assumptions of calorie counting are untrue.
  • ‘Calories In’ is NOT independent of ‘Calories Out’. That is, the number of calories you eat affects the number of calories you burn.
  • If eating fewer calories causes you to burn fewer calories, then you will not successfully lose body fat overall over the long term.
By Jason Fung, M.D., Co-founder of The Fasting Method


Calorie counting simply doesn’t work. But why? It seems logical and intuitive, but there are many key assumptions in the Calorie theory, explaining why it is such a fantastically poor method of weight loss, with virtually no chance of success.

Below are the assumptions and why they are known to be untrue.


The Energy Balance Equation

The commonly misunderstood Energy Balance equation is the following:

Body Fat = Calories In – Calories Out

This is always true, but its implication is usually misinterpreted. Body fat is simply a method of storing food energy, commonly measured in calories. Looking at this equation, people often deduce losing weight is simply a matter of eating less and moving more, since reducing ‘Calories In’ and increasing ‘Calories Out’ should easily result in loss of body fat. But it almost never works because of the many embedded, but hidden assumptions to the success of this Eat Less, Move More strategy.

Assumption #1: What you eat is independent of how many calories you burn


The first key assumption of the Eat Less, Move More strategy is that ‘Calories In’ and ‘Calories Out’ are independent variables. That is, if you eat fewer calories, the number of calories burned is not affected. This is crucial. If eating fewer calories causes your metabolic rate to slow, and therefore burn fewer calories, then body fat is unaffected. For example, if you eat 500 fewer calories in a day, and this causes your metabolic rate to slow by 500 calories per day, you will not lose body fat. Turns out this is exactly what happens. The bottom line? Calorie counting does not work for long term, sustained weight loss.

So, with all our calorie counting tables and apps and books, we can easily eat 500 fewer calories per day, but what happens to ‘Calories Out’? ‘Calories Out’, or how many calories you burn in a day is generally composed of two things – basal metabolic rate (the energy required for generating body heat, liver, kidney, brain etc. or BMR) and exercise. The effect of exercise on ‘Calories Out’ is relatively small unless you are exercising many hours a day.

Consider this: an average person may use 2000 calories a day for basic metabolism compared to an hour of moderate walking, which may burn about 150 calories. Measuring BMR and how it changes is extremely difficult without specialized lab equipment, and therefore we don’t think about it much, making the crucial erroneous assumption it’s constant. But that’s completely wrong. BMR changes a LOT.

The changes in BMR occurring with changes in diet have been well documented for many decades. For example, this 1971 study measured the effect of a calorie restricted diet on BMR, here called resting energy expenditure (REE). In this case, they used total oxygen consumption as a proxy for REE, due to the technical limitations of that era.

  • When experimental subjects were overfed by giving them an extra 1500 calories per day, their metabolic rate increased by 10-20%.
  • When they were deliberately underfed, their metabolic rate decreased by 10-20%.

Eat more, and BMR goes up. Eat less, and BMR goes down. In both cases, this effect will tend to maintain body fat levels. In biology, this powerful tendency to maintain the status quo through feedback loops is called homeostasis.

Since the 1970s, many experiments have replicated these results that changes in diet alter the BMR. A 1991 scientific review of the data noted, “The first statement which can be made with some certainty is that a decrease in energy expenditure is a universal response to energy restriction.”

“A universal response.” “Made with some certainty.” The metabolic rate slows in response to deliberate underfeeding in both lean and obese individuals, men and women, and in all races. The graph below, for example, summarizes 29 studies showing BMR drops by up to 25% in response to calorie restriction. Of further concern is the finding that the longer the dietary restriction, the more severe the drop in BMR. That is, longer term calorie restriction diets tend to cause more slowing of the metabolic rate.

In another 2001 study, directly measuring metabolic rate by indirect calorimetry, a calorie restricted diet reduced calories expended by 700 per day. Men started the study with a BMR of about 2000 calories/day, which dropped to 1690 calories/day by week 8. Where you may have expected a calorie deficit of 700 per day from eating less, you only got a deficit of only 390. This may seem like a reasonable trade-off as you will still lose weight, but this was only at week 8. The metabolic rate tends to continue decreasing over time, so eventually, weight loss completely stops – the dreaded plateau.

The bottom line? Eating less will cause metabolic rate to slow. Fewer ‘Calories In’ equals fewer ‘Calories Out’.




What about the opposite? Does changing ‘Calories Out’ affect ‘Calories In’? That is, if you exercise more, do you eat more? Absolutely. One of the dirty little secrets of the calorie counting crowd is that when you exercise more, you tend to eat more. A prospective cohort study from the Harvard school of Public Health found for every extra hour of exercise, adolescents, on average, ate an extra 292 calories.

How vigorous an activity is can be measured in METS (metabolic equivalents). The higher the METS, the more vigorous the activity. In this study, researchers measured both the ‘Calories In’ and ‘Calories Out’ of various activities to calculate the net effect of various  activities. Watching TV, for example, is associated with an excess of 102 calories/hr. But mild physical activity was almost as bad!

Moderately vigorous physical activity was only neutral in calorie terms, about on par with reading. When was the last time somebody told you that the net caloric effect of moderate exercise was about the same as sitting on your butt and reading a book? Sure, moderate exercise increases caloric expenditure. But that, on average, causes an increased caloric intake. Only a sustained intense exercise could create a calorie deficit.


Many other studies show the same effect. It’s also just common sense. They don’t call it ‘working up an appetite’ for nothing, you know, Einstein. Bottom line? Increasing ‘Calories Out’ increases ‘Calories In’.

Guaranteed to Fail

The idea you could lose body fat by simply Eating Less and Moving More, as superficially shown by the Energy Balance Equation was simple, easy to understand, and completely, fatally wrong. Actually, here’s what the science of the last few decades has shown:

  • Eating less slows the metabolic rate (BMR).
  • Moving more makes you eat more.

Both of these compensatory effects mean this ‘Calories In/ Calories Out’ (CICO) strategy of Eat Less, Move More is not only ineffective, but it is virtually guaranteed to fail.

Instead of focusing myopically on calories and calorie counting, it is more important to understand the determinants of what controls ‘Calories In’ (hunger) and ‘Calories Out’ (metabolic rate). These are controlled, not by calories, but by 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.

Share this article with a friend

More articles you might enjoy…