Why Fast? Part 1 – Weight Loss Mark's Daily Apple
So, yes: it works. But does fasting work solely through caloric restriction, or is it doing something special?http://www.blogger.com/img/blank.gif
That’s the real question. There’s no question that fasting causes weight loss through caloric restriction. Obviously, when you don’t eat anything, your body turns to its own stored energy reserves, reserves that take up physical space and have mass. Depletion of those energy stores reduces mass and thus weight. Total and absolute caloric restriction. That’s elementary stuff and the studies from the 1960s show that.
To dig a bit deeper, let’s look at how weight loss occurs during a fast. I’ll stick to research involving humans only (sorry, rodent personal trainers).
Secretion of growth hormone, one of the premier fat burning hormones, increases during a fast. In a five-day fasting protocol, men experienced increased GH secretion on day one and day five (the only two days where GH was measured). A later study showed that during two-day fasting sessions, growth hormone secretions increased in both frequency and intensity in men. They experienced more frequent GH bursts and each burst secreted a higher mass of GH. A more recent study found that 24-hour fasts increased GH by 1300% in women and almost 2000% in men.
Fasting decreases fasting insulin levels. The presence of insulin inhibits lipolysis, the release of stored triglycerides (body fat). Without lipolysis actually releasing stored body fat, it’s rather difficult to, well, burn that body fat for energy. During a fast, fasting insulin decreases and lipolysis increases. This insulin-blunting aspect of fasting quite literally allows the fast to be successful, because without the ability to access stored body fat for energy, making it through a period of zero caloric intake will be nigh impossible.
Fasting improves insulin sensitivity. 20-hour fasts were enough to improve insulin sensitivity in men.
Fasting increases the catecholamines, both adrenaline (epinephrine) and noradrenaline (norepinephrine). Both catecholamines increase resting energy expenditure during a fast, and guess where your fasting body finds the energy to expend? From body fat. Catecholamines activate hormone sensitive lipase present in adipose tissue, spurring the release of said fat. This makes intuitive sense, doesn’t it? If you’re hungry in the wild, you need to hunt (or gather, or fish, or somehow procure food) and you need energy to do it. The catecholamines help provide some of that energy while burning fat in the process.
Hmm, notice anything? All those mechanisms dealt with fat burning specifically. While there may be some weirdo out there who’s interested in reducing bone mineral density and muscle mass while maintaining fat tissue, I would wager that what most people mean by “weight loss” is “fat mass loss.” From the stuff I just linked, it looks like fasting burns fat, rather than just weight. But what about Conventional Wisdom which claims that fasting increases muscle wasting – maybe because your body will totally recognize the lethal nature of all that arterycloggingsaturated animal fat and choose to break down muscle instead? Is it true?
Let’s go to the research:
In one study, normal weight subjects ate just once a day without reducing overall caloric intake. Weight didn’t change, which isn’t really surprising, but body composition did change – and for the better. Body fat decreased and lean weight increased (in addition to a bunch of other beneficial changes) without an overall reduction in calories.
A recent review of the relevant literature found that while fasting and caloric restriction are “equally as effective in decreasing body weight and fat mass,” fasting is “more effective for the retention of lean mass.”
Conventional Wisdom strikes out again.