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Counting calories in
Going in, even professionals find counting nearly impossible. Restaurant calorie counts are off by plus or minus about 30 percent because portion size makes such a difference, and in the US the labels have been shown to be a worthless exercise. Ultra-processed products list calories equal to whole foods, but they are ground up and highly refined. Ground almonds and whole almonds are an example: there are perhaps 30 percent fewer available calories in the whole almonds.
Counting calories out
Going out is no clearer. Everyone's metabolism is different. A figure like 2,500 calories a day is an average, and one person who tested his own found that his was much lower.
Why the industry likes it
Calorie counting also treats every calorie as the same, whether it comes from fat, carbs, or a low-calorie soda. The food industry favors counting calories, fat content, and sugar because they are easy to control, and it keeps attention off the quality of the food.
How accurate are restaurant calorie counts?
A 2011 study in JAMA by Lorien Urban and colleagues at Tufts University measured the energy in quick-serve and sit-down restaurant foods and found that actual calories averaged 18 percent above the stated values. US rules have long allowed nutrition labels on packaged foods to be off by as much as 20 percent before enforcement. Portion size also shifts from plate to plate with whoever is serving.
Where did the calorie numbers on labels come from?
Chemist Wilbur Atwater at Wesleyan University developed them in the late 1800s. He burned foods in a calorimeter and subtracted the energy that volunteers lost in feces and urine. He settled on about 4 calories per gram for protein and carbohydrate and 9 for fat. Food labels still rely on those general factors.
How is daily calorie burn measured?
Researchers use doubly labeled water, in which a person drinks water made with heavy isotopes of hydrogen and oxygen. The hydrogen leaves the body as water, while the oxygen leaves as both water and carbon dioxide. The gap between the two rates reveals how much carbon dioxide the person produced, and so how much energy they burned, over one to two weeks. Nathan Lifson at the University of Minnesota developed the method in the 1950s, and Dale Schoeller adapted it for people in the 1980s.
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