The “Exercise to Lose Weight” Myth

By Jason Fung, MD

  • Exercise has many health benefits, unfortunately, weight loss is not one of them
  • The number of calories used daily depends on the Basal Metabolic Rate (involuntarily controlled) and Exercise (voluntary)
  • Exercise is a very small part of total energy expenditure
  • Amount of exercise has almost no relationship to weight loss
By Jason Fung, M.D., Co-founder of The Fasting Method


When most people think about weight loss, they consider diet and exercise as equally important components of lifestyle change. The Energy Balance Equation says that:

Fat stored = Calories In – Calories Out

As previously discussed, this is always true, but almost always misinterpreted. A simplistic interpretation of this equation says that simply reducing ‘Calories In’, or eating a few less calories per day leads to weight loss. This almost never works and usually leads to a lowered metabolism. Rather than simply cutting a few calories, it is more important to understand the hormonal control of what we eat.

The same problem exists for the ‘Calories Out’ part of this energy balance equation. The ‘Calories Out’, or the total number of calories used daily is more accurately termed Total Energy Expenditure (TEE) which consists of basal metabolic rate (BMR) plus voluntary expenditure (exercise).

TEE = BMR + Exercise

Basal Metabolic Rate is the amount of energy that is expended for housekeeping tasks as breathing, maintaining body temperature, keeping the heart pumping, maintaining the vital organs etc. These functions are automatic and not under any type of significant voluntary control.

  • You can’t ‘decide’ to increase your body temperature.
  • You can’t ‘decide’ that your heart will pump more blood.
  • You can’t ‘decide’ to increase kidney function

Organs like the lung, liver, and kidney are all regulated by very intricate hormonal systems and cannot be controlled by conscious decisions.

Exercise, on the other hand, is entirely voluntary. The main organ system under conscious control is our skeletal muscle, and we can decide to move, or lay around in bed. But that is the only organ system, of many, that we really control. Our smooth muscle, for example, which includes the muscles of our heart and the intestinal system, cannot be controlled and run automatically whether we want it to or not. We also have no conscious control over any of our internal organs (liver, heart, lungs, kidneys, endocrine system, pancreas, biliary system, immune system, brain, spleen, skin, thyroid etc.), all of which require energy to function normally.

The total calories needed in a day (TEE) consists of two components

  • Involuntary component – BMR with many, many organs and systems, and one that is
  • Voluntary component – exercise controlling only the skeletal muscle

Since BMR is cumbersome to measure, we assume that BMR is stable over time, and therefore any changes to TEE depend upon changes in the amount of exercise, which is the energy used by the skeletal muscles.

The problem is that the BMR does not stay stable and may increase or decrease by up to 40% usually in response to changes in our diet. The other major problem is that since exercise only considers the energy used by a single organ system (skeletal muscles) compared to the energy used by all the other organs (liver, kidney, brain, heart, lungs, intestines, pancreas, gallbladder, body heat generation, immune system, reproductive systems, etc.) it is a very minor consideration overall.

Does more exercise lead to weight loss?

The idea that there might be health benefits in regular exercise did not exist up until about 1966, when the US Public Health Service began to advocate that increasing physical activity was one of the best ways to lose weight.  This led to the fitness boom with aerobics studios sprouting up like mushrooms after a rainstorm. Local running races and marathons, which used to only host a few hundred participants now have tens of thousands.

Books such as The Complete Book of Running by Jim Fixx became a bestseller in 1977.  The fact that he died at age 52 of a massive heart attack was only a minor setback to the cause.  Dr Kenneth Cooper’s book “The New Aerobics” was required reading in the 1980′s where I went to high school. We could reasonably expect that as exercise increases, obesity decreases, right?

In this paper “Prevalence of physical activity and obesity in US counties 2001-2011″, we see that the percentage of adults (over age 20) getting sufficient physical activity has been steadily improving from 2001-2011. there is a general increase in physical activity in the adult population of the USA.  Certain areas (in dark green and blue – Kentucky, Virginia, Florida and the Carolina Twins) are increasing exercise at herculean rates.  You go girl! If too little exercise caused weight gain, then we should see some indication that obesity is improving. The dismal truth, however, is that these areas are showing some of the worst increases in obesity.

The paper further looks at the relationship between obesity and getting enough exercise. You can see from the scatterplot pictured below (labelled Figure 6) that there is no clear relationship.

Whether physical activity increases or decreases, there is virtually no relationship to the prevalence of obesity. It just does not seem to matter.  Irrelevant.   Higher levels of physical activity did not reduce obesity. Obesity has been steadily rising over from 2001-2011, even as we sweated with the oldies. Oh, snap. In general, there has been a gradual increase in exercise over time, as well as a gradual increase in obesity.

It is not likely that increased exercise caused obesity. Just that those with more weight to lose likely tried to ‘burn it off’ with more exercise, and were generally unsuccessful.

What about children? Isn’t it even more important for childhood obesity? For some answers, let’s look at this article “Association between objectively measured sedentary behavior and body mass index in preschool children” published in 2013 (Int J Obes (Lond).  2013 Jul;37(7):961-5.)

Researchers measured overall physical activity in children aged 3-5 using accelerometry and compared it to their weight. Accelerometers are devices similar to those that track our steps, and are generally more reliable than simple self-reporting. Here’s their conclusion “sedentary behavior was not independently associated with BMI z-score in two independent samples of preschool children.” Translation into plain English?  There is no association between activity and obesity. Increased activity doesn’t result in weight loss.

The relationship between exercise and obesity is weak or non-existent. People in the 1970s did little to no exercise and there was little obesity. People in the 2020s do lots of exercise (comparatively) and there is a lot of obesity. The idea that exercising more will reduce obesity is not supported by any scientific research. Again, I want to stress that there are many reasons to do exercise – it improves circulation, muscle strength, overall tone, heart function etc. There are so many reasons why exercise improves health, but weight loss is just not one of those reasons.

For more, see The Obesity Code.

Learn more about Pique Fasting Tea.

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

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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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