Why exercise may or may not help you to lose weight.

Exercise. I hate it. I'm about as active as a Brazilian 2-toed Sloth. The problem that I have with exercise is that it makes me feel really hungry and I end up eating more Calories than I burned by exercising. This is due to the effect of AMPk on my brain.

The thing about exercise in moderation is that it's good for health and fitness. The problem is that some people think that lots more is better. That ain't necessarily so. The 38 year old lady sitting near me at karaoke last night used to run a lot when she was at school. She now needs a replacement knee joint due to damaged cartilage and she had her knee wired up to a TENS machine.

Too much early morning exercise can also make you ill, by raising serum Cortisol levels. Excessive Cortisol is immunosuppressive. See Early morning exercise could make you ill. Excessive Cortisol also causes water retention, so people who over-exercise can gain (water) weight. Excessive Cortisol can also cause muscle loss, thin skin and osteoporosis.

Starving yourself and over-exercising just makes things even worse. From WHY is the combination of high intensity and/or long duration activity a mistake when calories are being severely restricted?

"Water retention: cortisol binds to the mineralocorticoid receptor (the receptor involved in water retention, well one of them). And although cortisol has 1/100th of the effect on water balance of the primary hormones (aldosterone and a couple of others), since there is like 8000 times as much of it, it can cause a major effect." and

"Excessive cortisol, especially chronic elevations cause other problems not the least of which is leptin resistance. Which only magnifies the drop in leptin from dieting. This could be another mechanism behind the greater drop in metabolic rate for the study I mentioned above."

So, what's the best thing to do for maximum fat loss with minimal muscle loss? A mixture of high-intensity exercise (resistance training with weights, or sprinting) and medium-intensity exercise (jogging) is better than just medium-intensity exercise. See Resistance Weight Training With Endurance Training Enhances Fat Loss, Impact of Exercise Intensity on Body Fatness and Skeletal Muscle Metabolism and HIIT & Run.

One theory to try and explain the improved fat loss is that Calories are burned after the high-intensity exercise is finished. However, the Excess Post-exercise Oxygen Consumption (EPOC) only amounts to about 35kcals so it isn't significant. What is significant is the appetite reduction produced by high-intensity exercise, the opposite effect from low and medium-intensity exercise.