A recent study sought to determine the relationship between physical activity, total daily energy expenditure, and basal metabolic rate. They sampled a large data set of people (n>1700) and found that as physical activity increased, BMR DECREASED such to partially compensate for the increase in activity.
This is in contrast to how many people think of exercise. Most people believe that if you ‘burn’ 2000 calories per day and start exercising and burn 300 calories from exercise, this means you will burn 2300 in total. This data set suggests that is NOT true. In fact, the data suggests that as you increase calories burned from activity, BMR compensates approximately 28%. That is, for every 100 calories you burn from activity, BMR decreases by 28 calories on average.
Interestingly, amongst obese people, there was a significantly greater compensatory reaction to increased activity with obese people compensating by nearly 50%! This suggests that certain people may be more pre-disposed to becoming obese based on their body’s ability to compensate for increased activity by decreasing BMR.
This study does NOT mean that exercise is useless for weight loss. After all, these compensations were only partial. If an obese person increased their activity by 400 calories per day, they are still getting ~200 calorie per day increased total daily energy expenditure. The average person is still getting 72% increase in calorie output contributing towards TDEE from physical activity, plus exercise has a ton of other benefits OUTSIDE of weight loss. It’s important to note however, that even though it’s still a net increase in TDEE when exercise is added, this cannot be at the cost of increased appetite. That is, if you are obese and increase your physical activity by 500 calories per day, netting ~250 kcal TDEE increase, if it simply makes you a lot hungrier and you end up consuming an extra 250 kcal per day or more, then the extra physical activity may not be worth it.
Overall, this data is still a ‘win’ for exercise and weight loss, but it should temper our expectations about how much weight loss exercise can actually produce.