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Why we can’t rely on counting calories

Rob Dunn’s Scientific American article focuses on how counting calories is overly simplistic, and why it may not help us lose weight.

nutrition label
“Digestion is far too messy a process to accurately convey in neat numbers.”

Among the many interesting articles in Scientific American’s special September food issue is one by biologist-journalist Rob Dunn (“The Wild Life of Our Bodies”) on how and why the calorie counts ascribed to various foods are wrong.

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The article is not about debunking the conventional belief that obesity primarily results from individuals taking in more calories than they expend. Gary Taubes, a science writer and co-founder of the Nutrition Science Initiative, does that in a separate article.

Dunn’s article focuses instead on another reason counting calories is overly simplistic, and thus why it may not help us lose weight — or improve our health: “Digestion is far too messy a process to accurately convey in neat numbers.”

Based on 19th-century experiments

Wilbur Olin Atwater

A calorie, Dunne dutifully notes, is “the amount of energy required to heat one kilogram of water by one degree Celsius.” More than a century ago, an American chemist, Wilbur Olin Atwater, appropriated that term to devise a system for determining the average number of calories in one gram of fat (nine), protein (four) and carbohydrate (four).

Incredibly, we still use Atwater’s system today. Writes Dunn:

Every calorie count on every food label you have ever seen is based on these estimates or on modest derivations therefore. Yet these approximations assumed that the 19th-century laboratory experiments on which they are based accurately reflect how much energy different people with different bodies derive from many different kinds of food.

New research has revealed that this assumption is, at best, far too simplistic. To accurately calculate the total calories that someone gets out of a given food you would have to take into account a dizzying array of factors, including whether that food has evolved to survive digestion, how boiling, baking, microwaving or flambéing a food changes its structure and chemistry, how much energy the body expends to break down different kinds o food; and the extent to which the billions of bacteria in the gut aid human digestion and, conversely, steal some calories for themselves.”

The effect of heat

Of those factors, how a food is cooked may skew calorie counts the most, says Dunn:

Rachel N. Carmody, a former graduate student in Wrangham’s lab, and her collaborators fed adult male mice either sweet potatoes or lean beef. She served these foods raw and whole, raw and pounded, cooked and whole, or cooked and pounded and allowed the mice to eat as much as they wanted for four days. Mice lost around four grams of weight on raw sweet potatoes but gained weight on cooked potatoes, pounded and whole. Similarly, the mice retained one gram more of body mass when consuming cooked meat rather than raw meat. This reaction makes biological sense. Heat hastens the unraveling, and thus the digestibility of proteins, as well as killing bacteria, presumable reducing the energy the immune system must expend to battle any pathogens.

Carmody’s findings also apply to industrial processing. In a 2010 study people who ate 600- or 800-calorie portions of whole-wheat bread with sunflower seeds, kernels of grain and cheddar cheese expended twice as much energy to digest that food as did individuals who consumed the same quantity of white bread and “processed cheese product.” Consequently, people snacking on whole wheat obtained 10 percent fewer calories.

Personal variations

Dunn also offers descriptions of some intriguing (and odd) findings about how differences in the size and composition of our gut (stomach and intestines) may determine the number of calories we individually consume:

Measuring people’s colons has not been popular for years, but when it was the craze among European scientists in the early 1900s, studies discovered that certain Russian populations had large intestines that were about 57 centimeters longer on average than those of certain Polish populations. Because the final stages of nutrient absorption occur in the large intestine, a Russian eating the same amount of food as a Pole is likely to get more calories from it.  …

People differ immensely as well in what scientists have come to regard as an extra organ of the human body — the community of bacteria living in the intestines. In humans, two phyla of bacteria, Bacteroidetes and Firmicutes, dominate the gut. Researchers have found that obese people have more Firmicutes in their intestines and have proposed that some people are obese, in part, because the extra bacteria make them more efficient at metabolizing food: so instead of being lost as waste, more nutrients make their way into the circulation and, if they go unused, get stored as fat.  …

Because many modern diets contain so many easily digestible processed foods, they may be reducing the populations of gut microbes that evolved to digest the more fibrous matter our own enzymes cannot. If we continue to make our gut a less friendly environment for such bacteria, we may get fewer calories from tough foods such as celery.

What to do?

Dunn points out that coming up with a new system of calorie counts wouldn’t give us truly accurate numbers because of the many complex factors that are involved.

But there is one thing we can do, he says, if we want to eat healthier (and take in fewer calories): Consume more whole and raw foods — and fewer processed ones.

Dunn’s article is behind a paywall online, but you can find it on the newsstands (along with a slew of other provocative articles) in the September “food” issue of Scientific American.