Different parts of bodies demand different quantities of energy. Cells that burn a lot, such as muscle and liver, have more mitochondria. A human liver cell has about 800 mitochondria; the average is about 100. Fat burns less energy than muscle, which is why a woman will usually have a lower metabolic rate than a man of the same weight, because a greater proportion of a woman’s body is fat.
Metabolic rate can vary on timescales longer than fright or fever. A cell in a hibernating animal is like a mothballed factory: It keeps ticking over, but only just. Fewer protein molecules are made and broken down, and smaller amounts of substances pass in and out of the cell. Molecular security guards come in to keep key pieces of equipment safe. Energy consumption can drop by 90 percent.
The body can also regulate its weight by tuning energy consumption. Fat cells release a hormone called leptin, which raises energy expenditure, so burning off fat, part of a beautifully precise system which means that, although each human in the developed world eats about 1 tonne of food each year, our body weight over the same period changes by only a tiny fraction of this, if at all. Of course, our biology can only adjust to our diets up to a point. Humans evolved in environments where feeding oneself took strenuous effort and where periodic starvation was common. In affluent Western societies—and an increasing number of developing countries—our instincts and tastes lag behind our abundant food and sedentary lifestyles, leading to today’s well-publicized spike in obesity. We combat this trend by becoming acutely aware of our energy budgets, finding out how much fuel our diets contain, and how rapidly different forms of exercise burn it off. Our metabolic rate also declines as we get older—partly because mitochondrial performance drops off—which is why it is harder to stay thin in middle age. The team at South Bank University most commonly measures metabolic rate to help obese people plan their diets. The same measurement is used to ensure that hospital patients who have had major surgery, or been severely burned, get enough food.
In concert with hormones such as leptin and adrenaline, the other main controller of metabolic rate is the nervous system. Mostly the brain does this without troubling our consciousness, but Buddhist monks, through deep meditation, can reduce their metabolic rate to