cooking for forty humans

catching_fire_coverAbout 20% of the calories we burn go straight to our brain. How did our small-brained, not particularly bright australopithecine ancestors evolve into self-aware, large-brained primates?

By one theory, brain growth was driven by meat consumption. When human forebears made primitive tools, or even just grabbed a rock, they could smash the bones that other animals wouldn’t eat (except the terrifying, bone-dissolving bearded vulture). Inside those bones? Sweet, sweet, calorie-dense bone marrow. That marrowy goodness was one part of a feedback loop: more calories were available to a growing brain, and with the growing brain came more complex cognitive skills like tool use, hunting strategies, communication, and harnessing fire—skills that made it easier to find even more calories. Thus, by this view, did eating meat make us human (alternative theories hold that fish or tubers were the calorie-rich foods that kickstarted brain growth).

In Catching Fire, Wrangham argues that the explosion in brain growth wasn’t from eating meat, but cooking it.

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Cooking, it turns out, does more than kill bacteria or make food palatable: it also improves metabolic efficiency. Raw foods require more time and more energy to digest than cooked foods, in which the starches have been gelatinized and broken down and proteins have been denatured (in fact, soft foods are generally easier to digest than hard foods, and given a choice, most animals opt for soft foods over hard ones). Consequently, the body nets more calories (and gets them faster) from cooked compared to raw meat: 95% of the protein in cooked eggs is digested, versus just 50% from raw eggs. In dietary studies, people on raw-food-only diets can lose weight and can end up malnourished, even while people eating the same number of cooked calories will gain weight. In an ancestral environment where food sources were not abundant, maximizing metabolic efficiency was critical—you don’t have to find more food if you can make better use of what you do find.

Cooking helped our ancestors more efficiently convert potential food energy to realized food energy, which had ancillary effects. Our digestive system shrank, jaw muscles decreased in size and strength, and teeth got smaller, since we no longer needed the anatomical equipment for digesting raw foods (great apes use 10% of their daily energy fueling digestion). We also lost the musculature associated with tree climbing, possibly because fire could deter predators even better than heights. In addition to brain growth, cooking helped our digestive apparatus become less ape-like.

Another, and I think weaker, branch of Wrangham’s theory is that cooking underlies common aspects of human social structure, including gendered division of labor and large codependent social groups. Like brain size, these were classically thought to arise solely from meat-eating: big burly manly men hunt for elusive but nutritional meat, while delicate women gather the ubiquitous fruits and grains. Large communities enact that reciprocity on a broad scale, spreading the risk of hunger across an entire social unit, rather than an individual pair.

But, Wrangham says, that logic only holds when food is cooked. Why? Because of chewing. Raw food has to be chewed. That’s trivial when you’re consuming a raw banana, but most primates eat hard, pulpy fruits that take time and effort to pick apart, chew, swallow, and digest. Chimps spend more than six hours a day chewing; extrapolating by body size humans would need to spend 40% of their time chewing to take in a subsistence level diet of uncooked fruits and grains (though it’s unclear if this is current humans, or ancestral hominids with digestive systems equipped for raw foods). Without cooking, hunters who come back to camp bereft of mammoth meat might not enough have enough time to chew all the raw fruits and grains they need to survive. And because cooking usually meant foods were brought to a central location to be prepared—as opposed to eating berries right off a tree or bone marrow off a found carcass—cooking also created an impetus for group living. And that is how cooking made us human. In theory.

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I’ve been thinking a lot about the idea of metabolic efficiency and nutrition. The calorie counts on foods are potential energy—how much energy is released when the food is thrown in a bomb calorimeter and incinerated. But there’s not a strict correspondence between calorie counts on packaging and calories actually realized by your body. Eating 100 calories of Oreos are like the power laces from Back to the Future II: they require little work and are immediately available. In  contrast, 100 calories of cooked steak is harder to digest, so we net fewer calories and get them slower. That food is metabolized at different rates is (generally) the underlying principle of glycemic index: “bad” foods spike your blood sugar, whereas good foods (like whole grains) release their energy slowly. I keep thinking that, in a certain sense, processed foods aren’t so much processed as “pre-digested.”

This all reminds me of something learned in a long-ago bio class: folk wisdom of metabolism is backwards. We tend to think thin people who eat a lot have a “fast” metabolism, but really they are inefficient: they aren’t getting (or storing) as many calories as possible from what they eat. Nutritionists say that weight loss is a matter of calories in being less than calories out—losing weight on an all-Twinkie diet proves it—but maybe the proliferation of different diets (Atkins, South Beach, zone, juicing) and the variability in their success owes partially to how we don’t often account for the difference between calorie counts on packages and the calories we actually get from consuming the food. That might not be entirely possible—we all have different metabolisms and there’s evidence that metabolic efficiency can change depending on the ratio of proteins, carbs, and fats consumed. But maybe the Atkins diet doesn’t work so much because you eat a lot of protein and not a lot of carbs per se, but because it’s a diet where you body works harder to metabolize the calories you ingest.

Final musing: When rats eat sugar substitutes but not sugar, their digestive system learns that sweet things have no calories. When later given access to real sugar, these rats go hog-wild and gain all kinds of weight. In fact, their metabolism is altered such that they gain more weight than normal rats eating that much sugar. I wonder whether the ubiquity of processed foods has had conceptually similar effects: that is, do diets high in sugar or processed starches affect the brain’s “expectation” of how much energy is available in food? And if so, does that have long-term consequences on how food is metabolized and the excess energy is stored?

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The book: Catching Fire: How Cooking Made Us Human, Richard Wrangham (2009)

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