by Richard Wrangham, as told to Discover’s Veronique Greenwood.
Wrangham is the chair of biological anthropology at Harvard University, where he studies the cultural similarities between humans and chimpanzees—including our unique tendencies to form murderous alliances and engage in recreational sexual activity. He is the author of Catching Fire: How Cooking Made Us Human.
When I was studying the feeding behavior of wild chimpanzees in the early 1970s, I tried surviving on chimpanzee foods for a day at a time. I learned that nothing that chimpanzees ate (at Gombe, in Tanzania, at least) was so poisonous that it would make you ill, but nothing was so palatable that one could easily fill one’s stomach. Having eaten nothing but chimpanzee foods all day, I fell upon regular cooked food in the evenings with relief and delight.
About 25 years later, it occurred to me that my experience in Gombe of being unable to thrive on wild foods likely reflected a general problem for humans that was somehow overcome at some point, possibly through the development of cooking. (Various of our ancestors would have eaten more roots and meat than chimpanzees do, but I had plenty of experience of seeing chimpanzees working very hard to chew their way through tough raw meat—and had even myself tried chewing monkeys killed and discarded by chimpanzees.) In 1999, I published a paper [pdf] with colleagues that argued that the advent of cooking would have marked a turning point in how much energy our ancestors were able to reap from food.
To my surprise, some of the peer commentaries were dismissive of the idea that cooked food provides more energy than raw. The amazing fact is that no experiments had been published directly testing the effects of cooking on net energy gained. It was remarkable, given the abiding interest in calories, that there was a pronounced lack of studies of the effects of cooking on energy gain, even though there were thousands of studies on the effects of cooking on vitamin concentration, and a fair number on its effects on the physical properties of food such as tenderness. But more than a decade later, thanks particularly to the work of Rachel Carmody, a grad student in my lab, we now have a series of experiments that provide a solid base of evidence showing that the skeptics were wrong.
Whether we are talking about plants or meat, eating cooked food provides more calories than eating the same food raw. And that means that the calorie counts we’ve grown so used to consulting are routinely wrong.
Part of the reason this had never been addressed before was that several areas of research had been overlooked, even by nutritionists. Especially fascinating were the physiological studies on people who subsist only on raw foods. I was impressed to learn that raw-foodists are thin compared to those eating cooked diets, given that in most cases they are eating domesticated foods with lots of nutrients, are processing them in machines like electric blenders, and of course, living as most do in the developed world, never suffering through seasonal food shortage. Yet despite all these advantages over anyone who might try eating wild foods raw, the average woman on a 100% raw diet did not have a functioning menstrual cycle. About 50% of women entirely stopped menstruating! When a raw-foodist’s reproductive system does not allow her to have a baby even when her diet is composed of processed, high-quality, agricultural foods, the obvious explanation is that she is not getting enough calories.
The fact that these studies weren’t generally known may have been because cooking seems to fall into a gap between research fields. For instance, there has been a journal called Meat Science for more than 70 years and textbooks and multi-volume books called Meat Science, which study the production of meat all the way to the consumer’s mouth. But there the science stops. As a meat scientist wrote to me, “The problem of how cooking affects the caloric value of meat does not seem have been of much interest to meat scientists.”
Yet there were signs that cooking did affect the calorie counts of some foods. Starches, for instance, like those in wheat, barley, potatoes, and so on, are composed mostly of two sugar-based molecules, amylopectin and amylose, which, when raw, are tightly packed and inaccessible to digestive enzymes. Studies have found that cooking gelatinizes starch, which means that amylopectin and amylose are released and exposed to enzymes. Thus, cooked starches yield more energy than raw ones.
To study how cooking (and processing, like pounding or chopping) affected calories, we turned to mice. They are a good species for this because their diet choices are rather similar to human food preferences. They like grains, roots, fruits and even meat; in the wild, there are populations of mice that get most of their food by eating live albatrosses [video]. Rachel Carmody led a study in which mice were given regular mouse pellets for six days at a time, interrupted by four days of eating sweet potatoes or beef. Half the time the sweet potato or meat was presented raw, and half the time cooked; half the time it was also pounded and half the time unpounded. She and Gil Weintraub carefully measured the exact amount of food eaten by the mice, and then calculated the animals’ gain or loss of weight over four days as a function of the weight of food eaten, using both wet weights and dry weights of food to check the results. For both meat and sweet potato, Rachel found that when the food was cooked the mice gained more weight (or lost less weight) than when it was raw. Pounding had very little effect.
We suspect that there are two major reasons for cooked beef providing more calories than raw beef. In cooked beef, the muscle proteins, like the sugars in cooked starch, have opened up and allowed digestive enzymes to attack their amino acid chains. Cooking also does this for collagen, a protein that makes meat difficult to chew because it forms the connective tissue wrapped around muscle fibers. However, we do not know the exact mechanisms. What we do know, though, is that the mice had a spontaneous preference for eating cooked meat over raw meat, and their choice made sense, given that they fared better on it.
Mechanism aside, though, what the experiments indicated was some serious discrepancies in how calorie counts are measured. The USA uses the Atwater Convention for assessing calories in food, a century-old system that treats food as being composed of a certain number of components, each of which has a fixed calorie value–such as 4 kcals for a gram of protein, 4 kcals for a gram of sugars, 9 kcals for fats [ed: kcals are popularly called "calories"]. Modifications to the original convention allow advances in nutritional knowledge to be incorporated, such as better estimates for some specific types of carbohydrate. The system gives a good approximation for foods that are highly digestible and demand very little work by the digestive system, such as candy bars. It is convenient because it produces standardized numbers that everyone can agree on.
But the Atwater Convention has two big flaws. First, it pays no attention to the extent to which food has been processed. For example, it treats grain as the same calorie value whether it is eaten whole or as highly milled flour. But smaller particles are less work to digest, and therefore provide more net energy. Second, it treats foods as equally digestible (meaning, having the same proportion digested) regardless of processing. But cooked foods, as we’ve seen, are more digestible than raw foods.
These flaws matter. According to the Atwater Convention raw foods have equal calorie content to cooked foods. So people can be deluded into thinking that feeding their children on 100% raw foods is a healthy practice, whereas I believe it would be dangerous for them.
The more highly processed our foods, the more calories we get out of them. If you want to gain weight, make sure you eat highly processed and well-cooked meals. If you want to lose weight, do the opposite. You can eat the same number of measured calories, but if the foods vary in how finely they have been ground or whether they have been cooked, the calorie counts will not tell you what you want to know.
The next wave of research will decide how profound the effects of cooking are. My best guess, based on studies of the increased digestibility of starch or eggs eaten cooked compared to raw, is that the increase in net calorie gain from cooking will prove to be in the region of 25**–50%. That is only a guess, but I am confident it will be much higher than 10%. It is going to be exciting to find out.