Odds are you sometimes think about calories. They are among the most often counted things in the universe. When the calorie was originally conceived it was in the context of human work. More calories meant more capacity for work, more chemical fire with which to get the job done, coal in the human stove. Fat, it has been estimated, has nine calories per gram, whereas carbohydrates and proteins have just four; fiber is sometimes counted separately and gets awarded a piddling two. Every box of every food you have ever bought is labeled based on these estimates; too bad then that they are so often wrong.
A Food is Not a Food—Estimates of the number of calories in different kinds of foods measure the average number of calories we could get from those foods based only on the proportions of fat, carbohydrates, protein and sometimes fiber they contain (In essence, calories ingested minus calories egested). A variety of standard systems exist, all of which derive from the original developed by Wilbur Atwater more than a hundred years ago. They are all systems of averages. No food is average.
Differences exist even within a given kind of food. Take, for example, cooked vegetables. Cell walls in some plants are tougher to break down than those in others; nature, of course, varies in everything. If the plant material we eat has more of its cell walls broken down we can more of the calories from the goodies inside. In some plants, cooking ruptures most cell walls; in others, such as cassava, cell walls hold strong and hoard their precious calories in such a way that many of them pass through our bodies intact.
It is not just cooked vegetables though. Nuts flagrantly do their own thing, which might be expected given that nuts are really seeds whose mothers are invested in having them escape digestion. Peanuts, pistachios and almonds all seem to be less completely digested than their levels of protein, fat, carbohydrates and fiber would suggest. How much? Just this month, a new study by Janet Novotny and colleagues at the USDA found that when the “average” person eats almonds she receives just 128 calories per serving rather than the 170 calories “on the label.”
|[Image 1. Some of the calories our bodies do not digest go to the dung beetles |
and flies whose empire rises around our inefficiencies.
Photo of the species Garreta nitens by Piotr Naskrecki]
It is not totally clear why nuts such as almonds or pistachios yield fewer calories than they “should.” Tough cell walls? Maybe. But there are other options too, if not for the nuts themselves then for other foods.
For one, our bodies seem to expend different quantities of energy to deal with different kinds of food (the energy expended produces heat and so is referred to by scientists as “diet-induced thermogensis”); some foods require us to do more work than others. Proteins can require ten to twenty times as much heat-energy to digest as fats, but the loss of calories as heat energy is not accounted for at all on packaging.
For another, foods differ in how and where they are digested in our guts. Some foods such as honey are so readily used that our digestive system is really not even put to good use. They are absorbed in our small intestines; game mostly over. More complex foods, on the other hand, such as cassava or almonds, have to travel to the colon where they meet up with the largest concentrations of our little friends, the microbes. Digestion continues with the help of our trillions of microbes but nutrients are shared between us and them. The microbes help to break down many compounds our own bodies cannot and in doing so go on to produce a mix of more microbes, gases (such as methane) and then fatty acids. The accounting associated with this process of sharing with the microbes is not considered in calorie counting.
Finally, some foods require our immune system to get involved during digestion in order to deal with potential pathogens. No one has evaluated very seriously just how many calories this might involve, but it might be quite a few. A somewhat raw piece of meat can have lots of interesting species for our immune systems to deal with. Even if our immune system does not attack any of the species in our food it uses energy to take the first step of distinguishing good from bad.
So much is different between the fate of different foods that it is almost certainly rare that the estimate of the number of calories in a food and the true number correspond well. And we haven’t even gotten to the biggest way in which a calorie is not always a calorie, processing.
In a paper published recently in the Proceedings of the National Academy of the Sciences, Rachel Carmody and collaborators at Harvard University examined the effect of the two most ancient forms of food processing–cooking and grinding (technically in their study, pounding)–on the calories available in those same foods.
Carmody knew from her previous work that starches like those in sweet potatoes have more of their calories available to digestion the more they are cooked (at least to a point). As a result, no two sweet potatoes you cook will ever have the exact same number of calories because they grew differently and because you will have cooked them slightly differently. But what, Carmody wondered, about meat? Meat is relatively easily digested; its calories might be just as available in sushi as in a McDonald’s hamburger. Surely, meat is just meat, the one thing that our estimates of calories get right. Wrong.
Digestion is difficult to study. It is hard to make participants, even college students, eat, say, nothing but raw beef for several days. Carmody and her colleagues circumvented this problem by studying mice; they monitored the weight of mice fed different diets. The mice are secretive about their digestion too though so Carmody had to measure how the mice moved and how much weight they gained as an indication of the amount of energy that was not being lost through inefficiency as feces. All things equal, the bigger the mice got on a given diet, the more calories they were getting. Carmody fed adult, male mice organic sweet potatoes (to, in essence, retest what was already known) or organic, lean beef. These foods were served up raw and whole, raw and pounded, cooked and whole, or cooked and pounded. The standard system of calories, the one used to put the numbers on the food you buy in the store, assumes (and hence also predicts) these have no effect on calorie content; but would they? The mice were allowed to eat as much as they wanted and how much they consumed was closely monitored (Carmody had to pick each and every bit of uneaten food up from inside the cage).
|[Image 2. A schematic of a human gut. Photo by Dflock at Wikimedia.]|
The mice on the different diets got about the same amount of exercise. They all had a wheel to run on, and they did not differ one treatment to the next in terms of how inclined they were to take a jog. They did differ, however, in how much they weighed at the end of the study. As predicted, mice lost more than four grams of weight on raw sweet potatoes, but gained weight when given cooked sweet potatoes (whether or not they were pounded). But what about meat? Cooked meat was easier to digest. The mice lost 2 grams of body mass on raw meat but just 1 gram on cooked meat. In retrospect this does not seem surprising. Heat denatures proteins and makes them easier to digest. Heat also kills bacteria and might decrease the immune cost of eating meat by reducing the work the immune system has to do which allows the body to make, well, more body for a given number of calories.