According to anthropologist Herman Pontzer of Duke University, when physical activity is increased in the long term, daily energy expenditure also rises – but less than one would expect. Even as activity increases, the body’s total daily energy expenditure rises more and more slowly, until the number of calories burnt daily becomes virtually constant.
In other words, the body adapts to changing circumstances. If expenditure is more or less constant, then, the body compensates for increased physical activity by reducing the energy it spends on other, nonessential functions.
Pontzer’s hypothesis, which was developed working alongside a team of doctors, would explain why I tend to feel cold – even on very hot days – and why I feel colder on mornings when I’ve exercised intensively. What I make my body pay for in exercise, my metabolism “charges” me for by reducing heat generation.
So while moderate physical activity leads to a reduction in nonessential physiological activities, the reduced functionality triggered by high levels of physical activity may actually be harmful to human health.
Research shows that heavy physical activity can alter the ovarian cycle in women, diminish sperm production in men, lower levels of sexual hormones in the blood and reduce sex drive.
Under conditions of very high activity, the effects on reproductive function become more obvious – consider the delayed onset of puberty in young gymnasts, for example. The immune system suffers, as does the body’s ability to repair damaged tissue.
So while it is healthy for many reasons to regularly engage in moderate physical activity, that activity does not have the slimming effects often attributed to it. And increasing the amount you exercise won’t change that.
You don’t want to hear this any more than I do, but controlling calorie intake – difficult as that is – is a more effective way to control body weight.
The body adapts to a lack of food, as it does to increased physical activity. In this case, it saves energy by reducing the speed of various physical processes, including metabolic activity – but not enough to offset the reduction in calories.
When you eat less, body temperature is also affected: Reducing food intake tends to make people feel colder. Eating less entails a slower physiological life and, to a certain extent, a more efficient one – that’s why, scientists are fairly confident, eating less is tied to longevity.
It’s not my place to advise anyone on their habits. But here’s how I’ve chosen to change my behavior based on this information.
First, I eat less than I used to and am more careful about what I eat. But I confess that I still overindulge occasionally. I’ve also moderated my physical activity. These days, I do about 100 miles a week on the stationary bike and walk whenever I get the chance.
So far I have not found life unbearable. Type 2 diabetes is still hanging over my head like the sword of Damocles, but I’m confident I’m on the right path.
