Lots of people, the majority I would say, believe that exercise is a key determinant of obesity and weight loss. Food companies want you to believe this; there’s even a term for what food companies do: leanwashing. This occurs when the food companies whitewash the bad, obesogenic aspects of their products and tell you that all you need to do is exercise to lose weight. The current scientific consensus is that “overnutrition”, i.e. eating too much, is the most important cause of obesity. I believe that it’s more complicated than just more calories; hormones play a crucial role, insulin being the most important here.
But surely one would think that, even if food is a more important cause of obesity, that exercise helps people to lose weight. Unfortunately the answer is that usually it does not. The reason is pretty simple: exercise makes you hungry, and it’s almost trivially easy to eat as many calories as you burn. Another reason is that people may reduce other physical activity to compensate for their exercise.
A recent study put overweight women on an exercise program of 12 weeks worth of treadmill walking. Result: most of the women gained weight.
What about longer term, and with more strenuous exercise? A study published in the International Journal of Obesity looked at nearly 13,000 runners. These people were presumably well-motivated, since they kept up their running habits for at least several years. They likely ate well, at least according to the low-fat dogma of the age. The result:
Age-related weight gain occurs even among the most active individuals when exercise is constant. Theoretically, vigorous exercise must increase significantly with age to compensate for the expected gain in weight associated with aging.
So, dedicated runners gained weight, and the authors of the study think they would have to run a lot more to prevent that weight gain. But perhaps a better conclusion is that more running would have made them even fatter.
Running or other vigorous aerobic activity will also usually increase the level of the hormone cortisol, which has a catabolic effect on muscle tissue. So the more one runs, the less one has of muscle, which is a metabolically active tissue. Therefore one will burn fewer calories.
There do exist forms of exercise that may better help fat loss, such as high-intensity training, and weightlifting. The beauty of both of these is that they help put on muscle, which burns calories at a higher rate than fat tissue, helping fat loss. They also have effects like improving mitochondrial number and function. However, they do suffer from the same strictures that aerobic exercise does, and that is that they make you hungry. So while I believe that HIT and weight training are better than aerobic exercise for weight loss, the fact still remains that food is the most important determinant of body weight, at least in our modern society, with its abundant food.
You may have noticed that in your local gym, the fit people seem to be lifting weights and the fat people are on the treadmills, cycles, and so on. The fat people are laboring (literally) under the belief that they will burn off fat, when if they really wanted to lose weight they should be doing resistance training.
There are any number of reasons to exercise: it’s one of the most solid anti-aging practices there is, it fights depression, keeps bones and muscles strong, prevents heart disease and cancer, and just makes one feel better and more alive. But aerobic exercise just isn’t too good at helping one lose weight.
Most of the exercise industry – the gyms, the trainers, the product manufacturers – make their living on selling exercise as a weight loss aid. Given what most people feel about exercise, the whole industry would probably fall apart if people came to realize that exercise wasn’t helping them lose weight. So the exercise business, like the food business, has an interest in perpetuating the myth.