In scientific studies, high-intensity aerobic exercise has been shown to actually slow the rate of weight loss when done at the same time as a low-calorie diet. One scientific study showed that adding six hours a week of aerobic activity to a low-calorie diet caused such a drop in metabolism that it completely canceled out all the extra calories burned from exercise.

Note the problem is the combination of the two: lots of exercise, particularly high-intensity, and significant calorie restriction.

It seems a little crazy ... because you would think the longer you exercise and more you reduce calories, the more weight you will lose. And that's exactly what many people try to do.

It's very common for those same people to discover through trial and error that it just doesn't work. They either feel terrible or very often their weight loss stops.

What do we mean by "a lot" of exercise?

Most commonly it is aerobic exercise (because that's the type of exercise you can do for a long time). High-intensity aerobic exercise makes things worse, or at least brings on the problems sooner. If you're breathing hard, that's considered high intensity. So, depending on your level of fitness, even jogging and extended walking could be considered high intensity exercise (again, notice your breathing). You can do some high intensity without problem, but not a lot.

Note: Another potentially bad combination -- if you don't know what you're doing -- is combining the two fads of HIIT (high-intensity interval training) and low-carb dieting. Your body needs carbs for high-intensity workouts. Unless you're implementing Carb Refeeds, you're only headed for burnout and/or weight loss stalls.

What's happening exactly?

Stalled weight loss from over-exercise and low calories is a mystery, a phenomenon that doesn't make sense but happens anyway. It's partly your metabolism slowing down, partly rising cortisol (which causes water retention, which in turn masks weight loss), among other hormonal changes. Overall, it's just too much physical stress; your body can't handle a lot of exercise on few calories.

Solution

To solve the problem of stalled weight loss from over-exercise, you have to one of two things, or both:

Increase calories so you are eating enough to fuel your exercise -- at which point your weight loss will start up again (again... crazy but true)
Reduce your aerobic exercise dramatically (or eliminate it entirely) until your restrictive diet is over -- again, you should see your weight loss pick up again.