Why Calories Count

In speaking about the energy content of foods and the effects of food energy in the body, we try to be consistent about using heat as one form of energy and calories as a way to measure the amount of heat energy. With the main sources of calorie-related confusion

Physiologists have their own language for reporting the results of oxygen uptake experiments. They do not report results in calories. Instead, they use the term metabolic equivalents (METs). METs are rates of oxygen uptake per minute corrected for body weight, sustained by a rate of oxygen consumption of 3.5 ml/kg body weight/min. At a body weight of 125 pounds, 1 MET equals 1.0 calorie per minute (as given in table 10). But at a body weight of 155 pounds, 1 MET equals 1.2 calories, reflecting the higher energy requirement. These numbers are derived like so: 1 liter of oxygen corresponds to about 5 calories, or 0.005 calorie per ml, and 1 MET is defined as 3.5 ml of oxygen/kg/min in adults. Therefore a MET is also 0.0175 kcal/kg/min (3.5 × 0.005). For a 70 kg (155 lb) person, the calculation is 70 × 0.0175 = 1.225 METS. For a 57 kg (125 lb) person, it's 57 × 0.0175 = 0.9975 MET.

Do overweight people gain weight more quickly than so-called normal-weight people?

No, they do not. The number of calories needed to gain weight depends on the initial body weight and body fat content. Once people are overweight, they seem to deposit a larger proportion of excess calories as fat than do lean individuals. Fat tissue takes more excess calories to deposit—it stores more calories—than lean tissues. Overweight people must eat more calories than lean people to increase their body fat stores and gain weight.8

Carnivores and herbivores process foods differently depending on the way their stomachs are organized and where microbial fermentation takes place. But once proteins, fats, and carbohydrates enter the small intestine, digestive enzymes process these molecules the same way in all species

Can I really burn 300 calories in 15 minutes?

Maybe you can. We can't. We've seen such claims on the covers of women's magazines. But if you weigh 130 pounds, jumping rope or running upstairs nonstop for 15 minutes will burn only 150 to 200 calories (see chapter 8). You would have to do something much more vigorous to burn 300 calories in 15 minutes.

In The End of Overeating, David Kessler notes that fast food is deliberately engineered to please people's taste preferences so they will want to eat more of it. Morgan Spurlock, in his film Super Size Me!, gained twenty-five pounds in a month from eating all his meals at McDonald's.

In this and other studies, the calories from snacks were not compensated for by eating less at the next meal, an observation that supports the idea that the more often you eat, the more calories you are likely to consume.6

Nutritionists in Harbin, China, recently counted the number of times obese and lean people chewed their food: the lean subjects chewed their foods longer. The investigators assigned subjects to chew foods either 15 or 40 times before swallowing. The result? The 40-chew groups took in 12 percent fewer calories and displayed lower levels of the “eat more” signal ghrelin and higher levels of the “stop eating” signal cholecystokinin (see table 16 in chapter 12). The investigators concluded, “Interventions for improving chewing activity could become a valuable adjunctive tool for combating obesity.”3 Maybe, if the tedium keeps chewers from overeating (the calories expended in chewing, alas, are trivial). We have our doubts about long-term efficacy, not least because practitioners will drive their dinner partners crazy, but hope Mr. Fletcher is smiling in his grave

observation suggests that eating breakfast might be a useful weight loss strategy. But while some reports say that eating breakfast is satiating and reduces overall calorie intake, others find just the opposite—eating just a little or even skipping breakfast is a good way to reduce calorie intake.4 Our advice? Do what works best for you

Civilization, Wrangham says, became possible once humans began to cook their food. Cooking reduces the energy cost of digestion, leaving more energy available for brain development. Perhaps so, but as we discussed in chapter 5, the energy cost of digestion is quite small—less than 1 percent of the original calories in foods. What cooking does is to improve the digestibility of some plant foods, especially tubers; it makes their calories more accessible so fewer of them are excreted. Cooking particularly improves the digestibility of raw potato starch and also destroys some enzyme inhibitors and plant toxins in other foods. But raw meats are digested as well as cooked meats, and the starch in many uncooked legumes and cereals is efficiently digested. And some anthropologists say that the taming of fire came much later in human evolution than the development of large brains.1