Calories: A complicated story

Since the early 20th century, doctors and nutritionists have been giving the same basic advice: we gain calories by eating, we lose calories by exercising or otherwise burning them off, and the remainder ends up on our hips, stomach and thighs.

Except, it turns out, the story is a lot more complicated than that.

The current and only system for measuring food energy, which is based on the work of Wilbur Atwater during the late 1800s, is so oversimplified compared to the reality that it’s “surely wrong,” according to Harvard evolutionary biologist Richard Wrangham.

A calorimeter burns foods or other fuels and measures the heat, or energy, that is generated. In the case of foods, this means it’s measuring the total number of calories it would be possible for your body to absorb from a food item. Photo: Akshat Goel, Wiki Commons

A problem has been quietly sneaking up on the diet and nutrition world. The obsession over the last 50 years with energy balance—that equation of calories in minus calories out—has popularized low-calorie and low-fat diets without taking into account what happens to calories after you swallow them.

Calories are measured today using a calorimeter, essentially a closed chamber that burns up the food and measures the amount of heat energy generated.

It’s as if your food is the fuel and your body is a fireplace. But that’s not what the human digestive system is like at all: the calorimeter measures the potential energy in the food, not the actual amount of energy you take in.

Other factors come into play, including the structure of the food, the energy your body uses to digest it, and the types of carbohydrates it contains.

Digestion: What’s going on in the body?

To physically break down food, the body has to generate hydrochloric acid and pump a high concentration of it into the stomach. Creating and maintaining a high concentration of anything anywhere in the body takes work, University of Alabama biologist Steven Secor says.

The more intact and structurally tough the food is, the more acid and energy it takes to break down. The next stop is the small intestine, where enzymes to digest the food must be made and transported. After the food is broken down into component nutrients, it’s pumped into the bloodstream.

“And once things get into the bloodstream, there’s the cost of what you do with it next,” Secor says. A lot of it is going to be restructured into something else. Amino acids are getting restructured into proteins, fats are getting reconstructed into fat bodies, and there’s a cost to all of that.

Experts say a lot more research is needed to find out just how far off we are when it comes to counting calories. But when the complicated confounding factors are sorted out, the whole system of nutritional measurement—from the Canada Food Guide rainbow to the nutrition labels on grocery store shelves—might need an overhaul.

It takes energy to make energy

After a meal, the metabolism, the body’s internal combustion engine, kicks into overdrive. It takes energy to extract energy from your food, and a larger amount of food requires more energy.

In fact, according to University of Alabama biologist Steven Secor, there’s a reason competitive eaters who scarf 20 hotdogs turn red and break a sweat. Their bodies are working hard to break down all that food, and the heat that is released on the skin is a product of the body burning calories.

The human body works harder to break down protein such as beef, and other types of red meat. Photo by Robin Grant.

The net amount of calories those eaters absorb from their food varies greatly from food to food, but people spend about eight to ten per cent of the calories from any given meal during the process of digesting it, Secor says. A bigger meal would cost more.

Crushing and cooking=calories

In today’s world, food processing companies do some of the digestion for us, so our bodies don’t have to work as hard or use as many calories breaking down food.

And that could be having a big effect on our energy balance and ultimately our weight.

A large-scale study in this area has yet to come. However, all the evidence suggests the impact of food processing, especially cooking, on available calories is an “iconic representation of the failure of the Atwater system,” Wrangham says.

A small study of 17 adults published in the Journal of Clinical Investigation provides a good example. On the first day, they each were given a cheese sandwich with white bread and processed cheese. On the second day, it was multigrain bread and natural cheddar cheese.

“Consumption of a processed diet leads to weight gain that is not predicted by food labels.” — Rachel Carmody.

The two sandwiches were precisely identical in terms of protein, fat, carbohydrates, and calories as measured by the Atwater system. The subjects’ metabolic rates were measured carefully for hours after each meal. When they ate the healthy sandwiches, their internal engines worked a lot harder and burned more calories—and average of 64 more per sandwich.

For a 260-calorie lunch, that’s significant.

Canada’s food guide. Courtesy of Health Canada’s Food Guide.

On ongoing experiment by human biologist Dr. Rachel Carmody at Harvard University is showing even more striking results in mice.

Controlling for classical calorie intake and exercise, Carmody and her team fed male mice a rotating diet of two foods (steak and sweet potato) in four different forms (raw, raw and crushed, cooked, and cooked and crushed). Under the Atwater system, neither cooking nor crushing affects calories. What she found was a surprise.

Carmody says she believes the difference occurred because the cells in both beef and sweet potato are broken down during crushing, and broken down and softened even more during cooking. With foods in a simpler form, the body needs to do less work digesting the food using acid and enzymes.

“Consumption of a processed diet leads to weight gain that is not predicted by food labels,” Carmody says.

The end of “food groups”? Not all carbohydrates are created equal

The laser-sharp focus on calories as the key to nutrition and weight management has overshadowed the need to eat good quality foods, says Geoffrey Livesey, a U.K.-based biochemist who is working on improving nutrition guidelines for consumers.

“We get foods like a low-fat Twinkie being marketed as a health food. In reality, it’s nothing more than an oral glucose tolerance test,” he says.

For decades, recommended diets have been high in carbohydrates and low in fat—a regime that has now been associated with a variety of chronic health problems, Livesey says. Fat-phobia emerged, he explains, because fat has the highest number of calories by weight. He says there’s no reason to restrict fat more than other nutrients, and in fact fat can be good because it helps the body absorb certain vitamins. When slathered on vegetables in the form of cheese, it can convince kids that nutrient-rich broccoli is tasty.

“Refined grains are arguably the most unhealthful food.” — Geoffrey Livesey

There is another big problem with the calorie-cutting paradigm: it puts all carbohydrates—brown rice, white bread, and pop tarts—in the same category.

Replace fat with sugar, under the Atwater system, and you’re coming out ahead. But many refined carbohydrate-rich foods, including snack cakes and white bread, have what’s called a high glycaemia index. Eating them causes the blood sugar to shoot up more than it does for other foods. Persistently high blood sugar is closely related to a number of nasty health conditions, from Type II Diabetes to hardening arteries.

“Refined grains are arguably the most unhealthful food,” Livesey said.

There’s even emerging microbiology research in the journal Nature that shows eating refined sugars and white flour could have an impact on the trillions of friendly bacteria that live in human intestines and help digest food. The altered suite of bacteria seems to have an increased ability to extract calories from the food passing through, which perpetuates the problem even further.

So what should I eat?

Nutrition books, health magazines, and even family doctors still preach that “a calorie is a calorie is a calorie.” So is it true?

Whole foods and high-fibre foods is what people should be eating. Photo by Robin Grant.

“Thermodynamically, yes,” says Ottawa registered dietician Meghan Barnes, who works with clients in the city to help them manage their weight and overall health.

Barnes says she keeps up with current research, and she does use calories to gauge people’s food intake. There’s just not a good, inexpensive alternative that can be used within her office. But Barnes says she’s more concerned that people are healthy, enjoying what they eat, and getting the nutrients they need, instead of consuming a particular number of calories.

“But whole foods, high-fibre foods—absolutely, that’s what people should be eating.” — Meghan Barnes

Livesy is envisioning a new version of the U.S. food pyramid or Canadian food rainbow that takes the emphasis away from calories. He wants to see vegetables on the bottom in the largest serving bracket, lots of healthy fats and “hard-kernel” grains in the middle, and refined flour and sugar on top.

Barnes says the Canada Food Guide often overestimates how many carbohydrates people need, but that it’s a good tool for teaching people about serving sizes, especially now that it’s been revised to emphasize fruit and vegetables more.

As much as she likes the idea of a food pyramid that provides a sliding scale from least to most processed foods, she says “You have to have a system people will use.”

“But whole foods, high-fibre foods—absolutely, that’s what people should be eating.”


  1. Genna Buck | The Science of Calories - July 1, 2013

    […] This is a multimedia piece I co-produced with Robin Grant for Catalyst, Carleton University’s mutimedia science magazine. I reported and wrote this story, and Robin was responsible for the photos and cutlines. I also got a chance to experiment with making infographics from Share this:TwitterFacebookLike this:Like Loading… This entry was posted in Multimedia, Science and tagged Calories, Carleton University, Catalyst, infographics, journalism, multimedia, Science, ThingLink, Web. Bookmark the permalink. […]