The Law of Thermodynamics states that energy in versus energy out dictates our body composition. More or less, if we want to lose weight, we have to make sure we take in fewer calories than we burn off.
If we want to get bigger, we need to have more calories in when compared to the energy that we burn. This argument is flawed in so many ways, yet that is how people looking to get super lean still view body fat reduction.
To put it bluntly, decreasing calories sucks. Okay, I know everyone knows that; if it were easy, we'd all be lean and there would be no failed diets. Why are decreasing calories a no-win situation? When calories are restricted, our bodies' internal thermostat adjusts to the amount of carbohydrates and protein that we ingested. By not giving our engine any fuel, less calories and fewer fat are burned
Think about the metabolic slowdown for a second, it actually makes a lot of sense. Our body realizes a decrease in energy coming in and energy going out. So now our body has to get more mileage out of our stored energy (fat) and ensure that we can keep functioning normally.
It is at this point between functioning and not functioning that people begin to fail on their diets and consume whatever foods they feel like eating. Recent research has shown that lipoprotein lipase, a fat storing enzyme, increases greatly when calories are restricted. While this is happening production of the thyroid hormone (T3) rapidly decreases, which will maintain muscle mass, but also body fat.
The thyroid is the main regulator of overall metabolism. It sets our body temperature rate and adjusts other metabolic processes such as protein synthesis. The main culprit in failed weight loss is the metabolic slowdown caused by a decrease in active thyroid hormone. Thyroid hormones adjust to the level of muscle mass that the body possesses. When you lose muscle, the thyroid gland secrets less, it's because doing increased amount of aerobic activity is not the smartest thing.
So while performing aerobic work to expend more calories seems natural, it just contributes to the muscle mass loss due to caloric restriction. Muscle loss resulting from restrictive dieting results from 20-40% of total weight lost. If we want the best body around, we can not afford to lose any muscle, it is the most metabolic challenging thing that we have. When the dieter resumes normal eating, lipoprotein lipase activity remains elevated while our metabolic rate lasts depressed causing us to regain the fat back and maybe even add more.