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Dear Diabetes: How Are Calories Calculated?

How food manufacturers calculate calories per serving

A recent New York Times story raised questions about the nutritional information listed on food labels, suggesting that for nuts and other high-fiber and high-protein foods, the calorie counts may be overstated by as much as 25 percent. As the article points out, this may be a particular concern for people with diabetes who are counting calories while following a low-carbohydrate diet, since the more your diet comes from protein and fiber, the more likely the calorie count is to be off. (Be sure to check with your diabetes care team before making any changes to your meal plan.)

There are a number of reasons that the calorie estimates on food labels are unlikely to be accurate. Let’s start with science: the technical definition of a kilocalorie (which are the “calories” on food labels) is the amount of energy it takes to raise a kilogram of water one degree centigrade – the word itself comes from calor, which is Latin for “heat.” To determine how many of these calories a particular piece of food contains, scientists use updated versions of methods developed by nutritional chemists in the late 1800s, including burning a sample of food in a sealed container submerged in water and measuring how much heat is produced. We also calculate calories based on the assumption, introduced around the same time, that fat always provides 9 calories per gram, and protein and carbohydrates provide 4.

The problem is that the human body isn’t as efficient as these scientific methods. We’re pretty good at absorbing simple carbohydrates, the type that are common in starchy and refined foods – which makes the calorie counts on those foods relatively accurate. But other foods like meat require us to burn extra calories just to digest them, reducing the total number of calories that we get from them. High-fiber foods, like nuts, may be difficult to fully digest, so some of their calories pass through our bodies unabsorbed. What’s more, calories from cooked foods are potentially more easily absorbed than those from raw foods, and sometimes other factors – like microbes in our guts – may affect how many calories are available for our bodies to use.

But those aren’t the only reasons that calorie counts on food labels may not be entirely accurate. Food manufacturers are allowed to calculate the information on Nutrition Facts panels from a wide variety of sources. One of the most reputable sources is the National Nutrient Database for Standard Reference, which is maintained by the US Department of Agriculture’s Nutrient Data Laboratory and contains detailed nutritional information for more than 8,000 foods. But even the National Nutrient Database cannot reveal the calorie content of your particular slice of pizza. Its values, like those of most databases, may be based on averages derived from multiple samples.

And lastly, there’s a regulatory issue: the FDA allows manufacturers to understate the number of calories listed on the label by up to 20 percent, which means that your 240-calorie candy bar could actually have 288 with no penalty, and even the most conscientious dieter could be consuming 20 percent more calories than they think each day. (Another important detail for people with diabetes: the product must contain at least 80 percent of the carbohydrate amount on the label. There’s no upper limit. This means that a food listed as 100 grams of carbohydrate per serving could have as few as 80, or far more than 100 – there’s no way to tell for sure.)

So what are you supposed to do if you’re trying to count calories? The best advice is to not obsess over precise numbers. Instead, evaluate foods in relation to each other. A bowl of strawberries is far lower in calories than a bowl of ice cream, even if the nuts in the Rocky Road aren’t fully absorbed. Try to keep an eye on the big picture – and take all nutritional information with a big (and calorie-free) grain of salt.

Catherine Price is a freelance journalist and type 1 diabetic who has written for The New York Times, Slate, Popular Science and O magazine, among others. Her newest book, Vitamania: Our Obsessive Quest for Nutritional Perfection, is available from Penguin Press. She blogs about diabetes at and you can follow her on Twitter @Catherine_Price. Price is a paid contributor for The DX. All opinions contained in this article reflect those of the contributor and interviewee, and not of Sanofi US, its employees, agencies, or affiliates.

© 2015 The DX: The Diabetes Experience

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