Food is energy for the body. In the nineteenth century, American chemistry Wilbur Atwater measured the available energy in food by calculating the average number of calories in fat, protein and carbohydrates. We still use his averages today. However, as biologist Rob Dunn makes clear in his feature for Scientific American[br]special issue on food, there is no such a thing as an average food[br] or an average person. How many calories we extract from food depends on the biology [br]of the species we are eating, how we cook and process our food and even on the different bacterial communities in different people’s guts. Standard calorie counts don’t take[br]any of these factors into consideration, resulting in numbers [br]that are slightly inaccurate, at best, and sometimes rather misleading. For example, let’s take about the difference[br]eating foods and vegetables. Fruits are generally soft and sweet because they evolved to attract hungry animals that inadvertently [br]helped plants to spread their seeds In contrast the stems, leaves and roots that we call vegetables, [br]are often tough and fibrous. Seeds are also rather tough because they have to survive digestion[br]in order for a plant reproduce. Because we spend considerable energy breaking down hard vegetables[br]and seeds in our intestines, we generally get fewer net calories[br]from them than from tender fruits. Cooking plants completely changes the equation[br]by making them much easier to digest. Steamed broccoli yelds more calories[br]than raw broccoli, for instance. That’s also true of cooked meat and fish. It takes more energy to break down[br]a firm piece of sushimi than a flake of big salmon. Even if two people sit down[br]for exactly the same meal, prepared in exactly the same way, they won’t leave [br]with the same number of calories. In addition of all kinds[br]of anatomical and metabolic differences, each of us have a unique community of bacteria living in our guts. Some of these bacteria help us break down[br]tough plant fibers. But they also take some of the calories[br]for themselves. Other bacteria might be a little too helpful making certain people so efficient[br]at metabolizing food, that they absorb more calories than they need[br]and gain weight. As you can see, digestion turns out to be such a messy affair that we’ll probably never have[br]precise calorie counts for all the different foods[br]we’d like to eat and prepare in so many different ways. But at least we can remember[br] to take standard calorie counts[br]for the grain of salt. For Scientific American Instant Egghead,[br]I”m Ferris Jabr.