1 February 2013

Food for Thought

Prof Richard Wrangham on why cooked food provides a lot more energy than eating the same food raw. 

Prof Richard Wrangham
Photograph: Stephanie Mitchell/Harvard News Office

‘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. 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.

Rachel Carmody’s mouse study found that when the food was cooked the mice gained more weight (or lost less weight) than when it was raw. Over 40 days, two groups of mice were fed a series of diets that consisted of either meat or sweet potatoes prepared in four ways – raw and whole, raw and pounded, cooked and whole, and cooked and pounded. Over the course of each diet, the researchers tracked changes in the body mass of the mice, controlling for how much they ate and ran on an exercise wheel. The results clearly showed that cooked meat delivered more energy to the mice than raw meat. The same was true for sweet potatoes. In both foods, the energy gains from cooking were greater than those from pounding, and cooking increased the energy gained from pre-pounded foods. Preference tests also revealed that hungry mice strongly preferred cooked foods suggesting that the energetic benefits of a cooked diet were obvious to the subjects (i.e. the mice) themselves.

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 carbs 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.

Meat cooking

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 carbs, 9 kcals for fats … 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.

The bottom line: 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.’

Richard Wrangham is a professor in the Department of Human Evolutionary Biology at Harvard University and the author of the MUST-READ Catching Fire: How Cooking Made Us Human.