Counting Calories

Scientific American: The hidden truths about calories. An excerpt:

Amazingly, there are more ways in which a calorie is not a calorie. Even if two people were to somehow eat the same sweet potato cooked the same way they would not get the same number of calories. Carmody and colleagues studied a single strain of heavily inbred lab mice such that their mice were as similar to each other as possible. Yet the mice still varied in terms of how much they grew or shrank on a given diet, thanks presumably to subtle differences in their behavior or bodies. Humans vary in nearly all traits, whether height, skin color, or our guts. Back when it was the craze to measure such variety European scientists discovered that Russian intestines are about five feet longer than those of, say, Italians. This means that those Russians eating the same amount of food as the Italians likely get more out of it. Just why the Russians had (or have) longer intestines is an open question.