The sound track for yesterday’s run was a talk by primatologist Richard Wrangham1, author of Catching fire: how cooking made us human. Cooking, he says, has long been thought to be an optional cultural practice, like wearing jewelry. But really, he argues, cooking was the essential technological innovation that enabled us to produce the metabolic energy we needed to become human.

How? Cooked food is more digestible than raw food. And not just by a little, but by a lot. Learn how to control fire, use it to cook your food, and you free up extra energy — plus time that would otherwise be spent masticating. Spend that time hunting, and your metabolic equation gets even better.

Wrangham has fascinating things to say about how this surplus time and energy explains such cultural universals (or former universals) as marriage, sexual division of labor, and the family dinner. Whether you agree or disagree with this analysis, though, it’s supported by an attention-grabbing claim. Everything we thought we knew about absorption of energy from food is wrong.

To this day, Wrangham says, the USDA website2 publishes tables that make no distinction between the nutritional value of cooked and raw food. On this page, for example, the energy content of one large raw egg is given as 75 kcal. The value for one large hard-boiled egg is almost the same: 78 kcal.

This is wrong, Wrangham says. A cooked egg delivers way more energy than a raw egg. How could this be? And how could we not know it?3

Here’s the explanation. We have traditionally measured the energy content of food by comparing input (the food we eat) and output (the feces we excrete). Burn both in a calorimeter, subtract, and the difference is the energy that was extracted from the food.

Yes, but extracted by whom? Or rather, by what? The energy that we humans take from our food has almost all been extracted by the time it reaches the end of the small intestine. But it has a long way to go yet. It must also pass through the large intestine, where dwell a myriad of gut flora. And they, Wrangham says, are hungry. If you eat a raw banana you only get some of its energy, and they get most of the remainder. If you eat a cooked banana, though, you get a lot more of its energy and leave less for them. The end result looks the same, but the internal distribution is quite different.

So you need to compare the energy in food entering the mouth to the energy remaining in the digestive products leaving the small intestine.4 Only then does the dramatic difference between the energies we get from raw versus cooked food become evident.

This is a great parable about instrumentation, measurement, knowledge, and epistemology. What other profound errors of basic understanding arise from misplaced instrumentation? And what might we learn by making simple — and in retrospect obvious — adjustments?


1 Yet another podcast from KUOW’s Speakers’ Forum, which has become one of my most reliable sources of audio brain food.

2 A sad reminder that government website and chamber of horrors are still, too often, synonymous.

3 The error, if it is indeed an error, propagates to WolframAlpha, which sources the USDA data. Compare 100 g of raw egg to 100 g of cooked egg.

4 How do you tap in at that point? Recruit people who have had ileostomies.