THE BLOG
12/31/2015 01:03 pm ET Updated Dec 06, 2017

Which Fat Are You Burning?

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Low-carb advocates have stated that low-carb diets have some metabolic advantage when it comes to fat loss and insulin metabolism. They claim a low-carb diet is better because your body becomes a more efficient fat burner.

Scientists can determine the ratio of fat, carbs, or protein someone is burning by measuring the amount of oxygen and carbon dioxide they exhale and how much nitrogen is in their urine. These tests have consistently shown that people burn more fat on low-carb diets.

The question is, which fat are they burning?

A recent study helped to illustrate an important point: Burning fat for fuel is not the same as burning fat from your body. [1]

A study published in the journal Cell Metabolism looked at the changes in fat loss, fat metabolism and body weight in overweight people whose controlled diets were either low in fats or low in carbs, but with the same number of calories.

The study showed that low-carb diets caused participants to burn more fat. However, people on the low-fat diet lost 80 percent more body fat.

How is this possible?

The idea of low-carb diets causing you to burn fat is partially true but not useful. It's partially true because when you eat more fat, you burn some of the fat you ate. It's not useful because burning fat from your diet is not the same as burning fat from your body. Your body is able to burn whatever you eat, whether it's protein, fat, or carbs.

The people on the low-carb diet ate more fat and burned more dietary fat as fuel. The people on the low-fat diet ate less dietary fat and burned more of their body fat as fuel. In the study, participants' diets were set at 1,800 calories per day. Everyone lost weight. However, the low-carb group lost 245 grams of body fat while the low-fat group lost 463 grams of body fat.

The scientific community is taking this study seriously since it was conducted in a metabolic ward where all meals were prepared and carefully measured by nutritionists. Any uneaten food was also measured and accounted for. Furthermore, the participants only had access to the foods given to them during the period of the study.

Did you know almost all diet studies base their conclusions on surveys? Imagine someone asking you, "In the last year, what have you eaten for lunch and how large were the portions?" Do you think you could tell them the exact foods to the gram accuracy? I sure couldn't. Researchers know that data from surveys may not be meaningless, but it is close.

We are omnivores. This means we can burn fat, carbs, or protein as fuel in a pinch. Be aware that burning fat from your meal has nothing to do with burning fat from your belly.

What is a dieter to do?

As always, if what you're doing is working, you're healthy, you have great energy levels, and your blood tests are good, don't change anything. However, if your weight is stuck, or you find yourself too tired to exercise, you may be better with less fat and increased good carbs as the participants in this study were.

1. Hall, Kevin D. et al, "Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity," Cell Metabolism, Volume 22 , Issue 3 , 427-436.