Nutrition-wise blog

Calories reconsidered: Old assumptions questioned

By Jennifer K. Nelson, R.D., L.D. and Katherine Zeratsky, R.D., L.D. March 27, 2013

This week's topic is calories. It seems that predicting the number of calories you need to cut — or to burn — to lose weight is being questioned. Furthermore, the number of calories thought to be in foods may or may not be what's actually in the food. Let's look a bit deeper.

Calories and weight loss
The traditional calorie calculation goes like this: 1 pound of body fat equals 3,500 calories. So if you decrease your intake — or burn — 500 calories a day, you should lose 1 pound a week (500 x 7 = 3,500).

Although this equation works mathematically, researchers are noticing that it doesn't seem to work as well in the messier real world.

Nutrition experts agree that many factors — in addition to calories — affect the rate of weight loss. These include your genetics, your metabolic rate, what your body is losing (fat, muscle or water), how your body adjusts to fewer calories and more exercise, and even how much sleep you get. All these factors can alter the prediction of weight loss. And that makes it difficult to predict how fast you or anyone else will lose weight.

Calories in food
Also being questioned is the century-old Atwater method for calculating the number of calories found in foods according to their carbohydrate protein and fat contents. It's long been thought that if you know the grams of carbohydrate, protein  and fat in a food you can get a fairly good estimate of the total calories in that food. So, for example, 1 gram of carbohydrate has 4 calories and 1 gram of fat has 9 calories — knowing how many grams of each are in the food, you can estimate the total calories.

However, food and nutrition experts are having to rethink this assumption. It appears that the Atwater method overestimates the number of calories in a food or mixed diet by as much as 25 percent compared to more precise measurements that mimic what a person may actually digest, absorb and metabolize. If that's the case — and that hasn't been proven yet — it may mean that food labels and tables listing calorie content of foods might be off.

These questions fascinate nutrition experts, who welcome reexamination of old assumptions. But what does it mean if you're trying to educate people about losing weight? What does it mean when your attempt to lose weight isn't as easy or speedy as you'd like? Be reassured that the age-old recommendation to "eat less and exercise more" is still in effect. Steady, life-long commitment remains important.