There is an ongoing debate on the specific effect — as well as magnitude of effect — of physical activity or exercise on weight loss and weight maintenance. This question is particularly relevant for patients who have experienced weight loss after a standard treatment regimen utilizing the new GLP-1 drugs. Doctors need to know how to help their patients avoid gaining back lost weight after the pharmacotherapy has been discontinued. Should they emphasize exercise or calorie restriction? Patients typically find both approaches difficult to maintain for the long term so physicians want to suggest the option that is most effective.
In a review of 32 randomized controlled trials that involved a total of 4,774 participants (Morze et al. 2021) with obesity, the average loss of weight of those who participated in a regular exercise routine, as compared to controls without exercise, was small; only about 2 kg. Nationwide surveys show that 43% of Americans are overweight by at least 10-15 kg. The authors concluded that the impact of exercise was modest in terms of clinically meaningful weight loss. The current evidence indicates that although exercise is important for cardiovascular and bone health, it is not an effective way to achieve significant weight loss (Ponzer, 2022). (If you want to learn about the effects of exercise on the brain, see here.)
Why can’t you lose weight by exercising?
Energy expenditure is not linear. More activity does not necessarily result in greater daily calorie expenditure or weight loss. This is why most people who exercise lose less weight than would be expected for any given effort expended. Why? Our bodies evolved complex compensatory mechanisms to balance the energy necessary to contract muscles with the energy available for other critical biological processes. In addition, muscles evolved to be efficient so that they did not waste too many valuable calories. Studies have shown that our body automatically compensates (with some variation from person to person, of course) during exercise and holds back at least a quarter of the calories we might expect to expend. Essentially, our bodies evolved mechanisms to protect us from wasting too much energy that might later be important for our survival. Our brains and reproductive organs require a significant percentage of our daily caloric intake. Unfortunately, these biochemical compensations and evolutionary priorities have become maladaptive for modern humans who consume far too many calories each day and consistently fail to burn off excess calories with movement.
Your biochemistry evolved to prevent you from wasting energy with exercise.
A recent investigation collected data from 1,754 adults who drank doubly labeled water (containing isotopes that allow researchers to determine the number of calories burned per day). The researchers also monitored their body compositions and basal rate of energy expenditure. This basal rate represents the number of calories someone burns by simply being alive. The researchers subtracted the basal energy consumed from total energy expenditure to obtain an approximation of the energy expenditure from exercise as well as other movements, such as standing, sitting, and walking. As expected, more movement burns more energy. However, the data revealed a surprising outcome. As each person exercised, they did not burn as many total calories as expected. Most subjects burned only about 72 percent as many additional calories as would be expected given their level of activity. The compensation was even greater in obese adults. Their bodies tended to reduce by 50 percent or more the actual number of calories burned by exercising.
So, given that you are working against evolution, how can you get rid of those extra pounds?
The answer was clearly demonstrated on a once-popular television show. The trainer for the NBC television show The Biggest Loser used to claim that more exercise was all that was necessary to lose weight. After many years of helping severely obese people lose weight, however, Bob Harper concluded that exercise is not the key; your diet matters the most. Most of his clients became obese. Visit any gym or spa, and you will see that people exercising carry a significant amount of body fat overlying their muscles even as those muscles get bigger and stronger.
Body fat releases lots of inflammatory proteins into the blood. Researchers recently investigated whether diet or exercise most effectively reduced the levels of inflammatory proteins in overweight or obese women. After 12 months, the scientists concluded that the greatest weight loss and most significant reduction in the level of inflammatory proteins came only from dieting, not exercising.
Bottom line: If you want to strengthen your cardiovascular system and enhance your bone health, find an exercise that you enjoy doing and stick with it! If you want to lose weight, the evidence overwhelmingly indicates that caloric restriction is the only valid, scientifically proven dietary intervention that will achieve your goal. It is also much cheaper—you will save money by eating much less food and paying for fewer tennis shoes, workout clothing, and gym memberships.