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Your body uses several sources of fuel for energy, and it uses them in a certain order of preference. So, the fuel used to sprint for 30 seconds is not the same fuel used in the second hour of a marathon. Believe it or not, your body is quite stingy about burning stored bodyfat for fuel. It would rather break down your muscle and lean tissue first. When people drastically reduce their calories, as in most crash diets, the body first turns to muscle breakdown to make up the difference. The same thing happens in most cases of caloric reduction where exercise is not performed in conjunction with dieting. In other words, if you diet without exercising, or if you diet too stringently, your body eats through its muscle tissue.

Why is this a bad thing? Well, besides the fact that you feel weak and crummy, muscle is a very metabolically active tissue. You need to actually expend energy to keep your muscle healthy. Think of the difference between muscle and fat as the difference between orchids and cacti. An orchid has to be fussed over and carefully maintained, while you can pretty much leave cacti to fend for itself. So, if you don’t provide your muscles with enough caloric nourishment, and if you don’t encourage them to grow with exercise, then they’ll be like an orchid in Death Valley. But that fat will stay there with only minimal encouragement. When it comes to maintaining a healthy metabolism, muscle is thus fundamental to keeping the body chugging along. If muscle is reduced, so is metabolic rate. The more muscle you have, the faster your metabolic rate. This is why bodybuilders’ daily food intake would make an elephant jealous: they need lots of food to feed their metabolically active muscle.

Most of us have heard about the starvation response, but few people know how it actually works.

For one thing, it’s not an instantaneous reaction. You don’t go into “starvation mode” if you have a late lunch. But there are indeed systems that kick in to prevent long-term, drastic calorie restriction. When the body perceives that it has not been fed enough to meet its needs for too long a period, as would be the case in a crash diet lasting more than a couple of days, it compensates in a few ways.

First, the hormones that control appetite, hunger, and satiety kick in. You feel hungrier as ghrelin levels go up, and you get more interested in eating as leptin levels drop. You start thinking about food more. Your tummy growls. It becomes much harder to resist trigger foods. It seems like the messages are coming from your conscious brain, because they’re in the form of thoughts such as “I could really go for waffles” or “Boy that Taco Bell commercial looks good”. But in reality, they’re probably coming from deep down in your reptilian hindbrain that’s screaming FAMINE FAMINE SAVE THE SPECIES MUST EAT NOWWWW!! Studies on prolonged starvation found that people who were even moderately calorically deprived for long periods of time became obsessed with food: they traded cookbooks and recipes, spent inordinate amounts of time planning elaborate meals, etc. They became emotionally unstable and did weird things like hoard unusual objects.  What can I say — hormones can make you kind of nutty.

And when you do get to eat, you eat more than normal. Scott Abel has described how bodybuilders and fitness competitors will actually eat to the point of intestinal rupture following a contest. A friend of mine who competed successfully in bodybuilding shared her experiences of dieting so restrictively. She described to me how once a week she would quietly go insane and set up a little “circuit” in her kitchen, which enabled her to move from “food station” to “food station” continuously in a sustained binge. The foods she chose were straight-up brown sugar, butter, and peanut butter. Hardly rational eating behaviour, especially for an athlete who’d normally prefer salads to burgers.

Second, the body lowers its basal metabolic rate (BMR), or the rate at which it burns calories throughout the day. Think of the BMR as the “idling speed” of your body. If your BMR drops, then your body burns calories more slowly, which means that once you go “off” the diet and begin eating normally, your body will more readily deposit bodyfat. This is the principle behind yo-yo dieting. But what most people don’t know about the starvation response is the connection between BMR and muscle tissue.