We burn about the same number of calories daily no matter our activity level.
This is why diets fail and exercise doesn't help one lose weight.
For people at healthy weights, exercise has many benefits. But it does not significantly increase the number of calories one burns throughout the day.
Evolutionary anthropologist Herman Pontzer's book Burn: New Research Blows The Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy explains how, in detail.
The concept is called "constrained daily energy expenditure" and was found from studying people who live active vs. sedentary lifestyles. Both types of people burn about the same number of calories daily. Our bodies have adapted to burn a relatively fixed amount of energy daily. And if you exercise more, your body slows your metabolism to keep total energy expenditure relatively the same.
If a 200-pound man added 60 minutes of medium-intensity running four days per week while intaking the same number of calories, and did this for 30 days, he'd lose five pounds.
Energy metabolism, how many calories a person burns, varies between individuals based on factors like body size and muscle mass. It is in a very specific range that can't be changed much. The limit it can expand to is well under 2.5 times the Basal Metabolic Rate (BMR) calories per day which only professional athletes completing an Ironman could possibly achieve.
What exercise changes is how that same number of calories is burned.
Over 50% of calories are spent just on resting. These activities include using the brain (which burns the equivalent of 300 kilocalories daily, the equivalent of running a 5K) and powering the immune system, stress response, and reproductive system.
What happens if someone at a healthy weight is sedentary?
A body with excess energy, such as in a sedentary person, will spend that energy on less critical body functions which have a high energy cost, like the inflammatory and stress responses. Highly sedentary people do show higher levels of chronic inflammation and an increased stress response. I emphasize that these are people who are sedentary and not under-eating.
The consequences of exercising without intaking enough energy
People who are under-eating and/or underweight will not get the benefits from exercise that are often touted by the literature.
Active people who under-eat have compromised immune systems, taking longer to heal from infections and injuries. The cortisol increase people typically get in the morning, which helps wake them up, is no longer there, so they feel fatigued 24/7.
Since the energy budget one has daily doesn't change, someone who exercises excessively will not burn more calories Instead, their body will spend less energy/calories on running other processes, which in turn hurts the person's health.
What the starving body takes energy away from are areas such as:
This is why females lose their periods. All under-eating people can lose their hair, sex drive, and feel more anxious, depressed, and irritable.
Also, remember that exercise actually slows down the metabolism. The body will optimize for the increased activity by spending less energy on other processes which keep us alive long-term.
Building muscle does help one burn more energy, since muscles take energy to maintain, even when we do nothing. But it's not possible to build muscle while under-eating (as explained in another article on EndAnorexia). The body won't bother creating such expensive tissue when one isn't intaking enough to even sustain basic functions.
Giving overtrained athletes more calories doesn't fix their reproductive system
“Tellingly, giving overtrained athletes more food doesn’t solve the problem (unless there’s an underlying eating disorder—sadly, not uncommon among elite athletes). For example, a 2014 study by Karolina Lagowska and colleagues provided food supplements to thirty-one women endurance athletes (rowers, swimmers, and triathletes) who had irregular ovarian cycles and other symptoms of overtraining. After three months of being plied with extra calories, the women saw their daily energy expenditure increase by a modest amount: they were eating and burning about 10 percent more calories each day, the metabolic effect we’d expect given the body’s usual response to overeating. The women’s weight and body fat didn’t change—they weren’t storing the extra energy, they were using it. Some of those extra calories went to the reproductive system, increasing luteinizing hormone (which stimulates the ovary) by a modest amount. But it wasn’t enough to make a meaningful impact on ovarian function. Daily energy expenditure was still too constrained to take in enough calories to make a difference, and their prodigious exercise regimens were still taking up too much of the energy budget for the reproductive system to function normally.
“The intuitive treatment is to provide more calories, to try and increase daily energy expenditure. Constrained energy expenditure helps explain why that doesn’t work very well. With daily energy expenditure fixed, the only way to increase energy availability is to decrease training workload.”
Exercise has benefits, but only for those who are eating enough and at healthy weights
Exercise does improve health and happiness. It improves mood, helps regulate blood pressure, and helps prevent or mitigate chronic health conditions. Since exercise isn't a great calorie burner, one should choose exercises they truly enjoy, rather than exercises they feel will somehow burn the most calories.
The takeaway: If you're under-eating and underweight, don't exercise.
Exercise isn't giving you the benefits it would give to a healthy, adequately-fueled person. What exercise is instead doing is damaging your body and causing grave long-term consequences.