exercise slows metabolism. Now for those of us who have believed (and preached) that exercise does exactly the opposite, this comes as a huge shock. Yet, upon closer examination, there are indeed some good reasons to doubt the effectiveness of exercise for weight loss. I'll list them now.
I meet very few overweight people who feel that exercise has ever helped them lose much or any weight. This includes people who have taken up very light exercise like walking but it also includes many people who have hired personal trainers and worked at great intensity for a long time. Indeed when one consults the scientific literature, there is very little evidence to support the notion that exercise is a terribly effective means of weight loss.
Very simply, it appears that exercise LOWERS metabolism and it does so to such a degree that it almost exactly compensates for the calories burned during exercise. In other words, if you burn 400 calories doing an exercise, your metabolism slows so that you burn about 400 calories LESS.
Why would this happen? The answer probably lies in the distant past when starvation and not obesity was a leading cause of human death. In that context, it would make sense to have a metabolism that slows after exercise. By causing starving people to burn fewer calories after, say, a day of hunting, this exercise-induced metabolic reduction should help conserve body weight and prolong life. While this worked very well for hunter-gatherers forty thousand years ago, it has backfired on us today in the Western world.
A number of studies suggest that for most of us exercise makes us more hungry. Again, in the context of life many thousands of years ago, this makes perfect sense.
Now THAT is another matter. Undoubtedly the body's ability to lower metabolism in response to exercise has a limit. It appears that this limit is exceeded at or slightly beyond three hours of intense activity. After three hours, all the extra calories burned are "real". The only problem is that most of us don't have three hours a day to commit to exercise.
Almost any amount of almost any type of exercise seems help a lot with keeping the weight off. This is one of the reasons why, for example, lifelong walkers may never become real athletes but nevertheless stay thin(er).