Unless you enjoy your beef patties uncooked and straight from the fridge, there may be more calories hiding in that hamburger than you think. Harvard researcher Rachel Carmody says that our standard method of measuring calorie content doesn't account for the ways heat changes food. Cooking adds calories, Carmody says, and she's got some Atkins-adherent mice to back her up.
The calorie numbers on food labels are calculated according to how many grams of fat, carbohydrate, and protein the food contains, and how calorie-dense each of those nutrients is. It's simple math and chemistry. But since calories are a unit of energy--how much energy you, as an eating animal, manage to extract from your meal--biology should be a part of the equation, too. Our standard calorie math doesn't consider how much energy we expend chewing and digesting our food, or what components of our meal go toward feeding our gut microbes instead of our bodies. It also ignores the effect of cooking: heat breaks down starches and unravels proteins, making those molecules easier for our bodies to absorb.
Carmody used mice to study the effects of an all-cooked or all-raw diet. Mice, like humans, are natural omnivores. Unlike humans, they will allow you to feed them nothing but raw sweet potato for four days.
Adult male mice were put on a diet of either sweet potato or beef, raw or cooked. (The researchers also studied the effect of pounding the food, which makes it easier to chew but doesn't otherwise have much effect.) The mice could eat as much of their one food as they wanted. They were also free to exercise, running on magnetic wheels that recorded how much use they got. The researchers measured how much food their subjects ate, how much they exercised, and how much weight they had gained or lost after four days.
Since previous research had shown that cooked starches provide more energy, the potato-eating mice were expected to get more calories from their food when it was cooked. Obligingly, the mice maintained their original weights on a cooked-potato diet but lost weight on a diet of raw potato. Besides getting more energy out of each bite of food, the cooked-potato mice also ate more. The raw-diet mice, on the other hand, apparently weren't able to choke down enough sweet potato to keep up their weight. Both groups of mice exercised the same amount.
Mice being fed lean beef were also expected to lose weight, lacking necessary fat and carbohydrates in their diet. (The authors point out that in humans, eating nothing but lean meat leads to a condition called "rabbit starvation." You can go ahead and cross that all-rabbit diet off your list of resolutions for 2012, because it's said to cause diarrhea, headache, and "vague discomfort.")
All of the meat-eating mice lost weight. But those mice eating cooked meat lost significantly less weight, demonstrating that they were able to get more calories out of their food. Their amount of exercise was the same as the raw-meat mice. And unlike the mice fed on sweet potato, the meat-eating mice actually ate less of their food when it was cooked. This suggests that they weren't enjoying their diet very much, but it also suggests that the cooked beef was even more calorie-rich than the weight results would imply. If the mice eating cooked beef had swallowed the same quantities as their raw-diet counterparts, they might have lost even less weight, or not lost weight at all.
There are several factors that could make cooked meat more energy-rich. High heat unwinds (or "denatures") protein molecules, making them easier to digest. Since cooked meat is usually softer, we need to expend less energy chewing it up and breaking it down inside our bodies. Additionally, cooking kills the pathogens that like to hang out on raw meat. When we ingest E. coli or Salmonella along with our meal, we have to divert extra energy to our immune systems to keep those bacteria at bay. Calories spent chewing, breaking down, or disinfecting our food cancel out the calories of energy we're taking out of it.
We started using fire at least 300,000 or 400,000 years ago. For reference, that's before modern humans even existed. As long as we've been Homo sapiens sapiens, we've lived with fire. Once we figured out how to cook our food, which included a lot of meat, we would have seen the benefits: more energy for making tools, raising families, and growing those big brains.
Now that meat is available to many of us in the form of daily Double Whoppers, and not just the occasional mastodon steak, the question of how many calories are really in our food isn't a trivial one. Even the most careful calorie counters may be taking in more energy than they think. A fast-food taco, or a trough of starchy pasta at a restaurant, could hold even more calories than the menu says.
We need better math, for everyone's sake. For starving and malnourished populations, understanding how cooking increases calorie content could help people glean more sustenance from their limited resources. For populations struggling with obesity, better food labeling could allow people to take control of their calorie intake before we all have to go on a rabbit diet.
Carmody, R., Weintraub, G., & Wrangham, R. (2011). Energetic consequences of thermal and nonthermal food processing Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1112128108