Is a Calorie a Calorie? I was looking forward to "Why Calories Count," the new book by Marion Nestle and Malden Nesheim. I figured gaining an advanced education in calories might allow me to better understand diet and weight gain. These two are not faddists but clear thinkers: actual scientists. But of course there is more to weight gain than the calorie. This was obvious from the moment I asked Ms. Nestle a key question: "Is a calorie a calorie?" This sounds simple, and if the answer is "yes," all you do is take in fewer calories than you expend and you'll lose weight. It need go no further than that. It might help to first define a calorie, and that's easy: it's a measure of the energy derived from a food source. A gram of fat has been determined to have nine calories and a gram of protein or carbohydrate four calories; so for any given measure, fat has more than twice as many calories as protein or carbs. Those numbers are not perfectly accurate, but they're good enough. A food isn't a food - they're all different - but since a calorie is just a measurement of energy, how can it vary? When I asked my question, Nestle's answer was confounding: "Yes and no," she said, adding, "It's Talmudic." Because calories change as they enter the body, the nine grams for fat and four for everything else turn out to be not very accurate measures at all; besides, foods are only rarely one thing or another. Here's what is true, she said: "The studies that have measured calorie intake, that have put people on calorie-reduced diets and measured what happened, show no difference in weight loss based on composition of the diet." When people are essentially incarcerated, when all intake is weighed and measured, they will lose weight if the calories in their diets are reduced - regardless of the composition of the diet. "That's why we hear a calorie is a calorie," she said. But no one lives under experimental conditions, and foods are complicated mixtures: fiber makes a difference and form makes a difference. (Fiber is special because it's not digested or digested incompletely. Most of its calories don't get into the body, which is one reason why fruits and vegetables, which are high in fiber, help with weight loss.) The "calorie is a calorie" argument is widely used by the processed food industry to explain that weight loss isn't really about what you eat but about how many calories you eat. But if it were just about calories, you could eat only sugar and be fine. In fact, you'd die: sugar lacks essential nutrients. That's an obvious case. But although a calorie may be a calorie when people talk about weight loss and nothing else, there are other factors involved. And once you get past my perhaps lame "Is a calorie a calorie" question, you can begin to see something approaching the truth. For one thing, says Nestle: "There are dozens of factors involved in weight regulation. It's hard to lose weight, because the body is set up to defend fat, so you don't starve to death; the body doesn't work as well to tell people to stop eating as when to tell them when to start. An important question, then, is really something like, "What can I eat to keep from putting on weight?" and here the answer turns out to be not only easy but also expected. "If you're eating a lot of fruits and vegetables," Nestle says, "you're not taking in as many calories as you would if you were eating fast food and sodas." Yes, that's a calorie issue; the latter group is way higher in calories than the former. But though there's a difference between eat less and eating better, "eating better makes it much easier to eat less." Ultimately, the calorie is political: marketing affects instinct, and Nestle and Nesheim really shine in their analysis in this realm. (Their slogan: "Get organized. Eat less. Eat better. Move more. Get political.") When I asked Nestle what she would do, given that people in the United States were obviously eating too many calories and that the resulting excess weight was costing all of us life years and money, she answered quickly: "We need a farm bill that's designed from top to bottom to support healthier diets, one that supports growing fruits and vegetables and making them cheaper. We need to fix school lunches so they're based on fresh foods, and fix food assistance programs so people have greater access to healthier foods. Her list goes on: fix the food-safety system; make it possible for people to get into farming; fix front-of-packaging labeling. And a couple of big ones: "Stop marketing food to kids. Just make it go away. And get rid of health claims on food packages too. "Unless," she says, reverting to her pure science role, "they're backed up by universally accepted science. Which would get rid of all of them. Even if a calorie is a calorie, the situation is not so simple.