Doughnuts: Or Why Quidditch Players Are Thin

We, which is to say, we the donut eaters of the world, like to think that we can work off the calories in a donut with a bit of exercise. True enough, but how much exercise? An elite athlete can generate something like 200 Watts of mechanical work for an hour. This mechanical work is generated at an efficiency of about 25%, i.e., by burning about 800 Watts worth of stored energy. That amounts to about one Calorie (i.e., 1 kilo calorie) every five seconds, which means that he could work off that donut in 20 minutes or so. Those of us who actually eat donuts are likely to take a lot longer.

Bar tailed godwits are a lot more impressive burners of calories. During their fall migration from Alaska to New Zealand (nonstop!) they burn off about 58% of their body weight, including all the fat, most of the muscle, and big parts of every organ except brain, bone and feather. So, how many Calories could I burn by flying to, say, Cozumel?

Bird arcana courtesy of The Simple Science of Flight, by Henk Tennekes.

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