“A calorie is a calorie.” Or so the debate goes. And it’s true. Until you chew that calorie up and swallow it. Then it’s a whole other story. The calorie myth stops here! Yes, calories have the same “thermodynamic” value—meaning, they burn at the same rate on a Bunsen burner. But your body is not a Bunsen burner.
So even though one gram of protein has four calories and one gram of fat has nine calories, once they’re ingested they have different functions, absorption rates, and physiological effects. Thus, not all calories are equal. There. We said it. So what’s going on behind the scenes?
Debunking The Calorie Myth
“One gram of sugar from a cookie has just as many calories as one gram of sugar from a carrot,” writes Kathryn Seigel of Greatist. But, “it’s how the body uses calories from these sources that makes them different.” She explains:
- Protein rebuilds damaged tissue and creates certain hormones and enzymes.
- Fat coats our organs in protective cushioning and is necessary to absorb fat-soluble vitamins A, D, E, and K.
- Carbohydrates convert to glucose for accessible energy.
- Trans fats can raise bad cholesterol and increase risk of heart disease.
How Calories Burn
Because these macronutrients play such different roles inside the body, they are distributed and burned at different rates. So, what’s the breakdown?
- Protein calories have the highest satiability—meaning they make you feel fuller and more satisfied. That’s because the body burns these calories slower, and for longer.
- Carbohydrate calories are the body’s main source of glucose(energy). These calories are burned first for fast, efficient energy.
- Fat calories are the most concentrated source of energy. These calories are the last source of energy for your body.
How Calories Absorb (or Don’t)
Also, the calorie count on the nutrition label isn’t telling the whole story. Not only do calories burn at different rates, but they are also absorbed differently. Nuts, for example, are very high in fat and protein. That explains why one handful of almonds has super appetite-quelling power. But, at 150 calories in 20 nuts, you don’t want to get crazy with trail mix. And yet, we might be wrong about that.
As the New York Times reported, “only about three-quarters of the calories [nuts] contain are absorbed. The rest are excreted from the body unused. So the calories listed on their labels are not what the body is actually getting.”
Plus, there’s our hero: fiber. It aids digestion, nutrient absorption, and detoxification. And get this: fiber is only partially digested. That means the calories from high fiber foods may deliver even fewer calories than the label indicates.
As health columnist Mark Bittman explains, “Most [fiber] calories don’t get into the body, which is one reason why fruits and vegetables, which are high in fiber, help with weight loss.” You might be consuming 100 calories per banana, but the insoluble fiber sweeps through your system unabsorbed. This fact isn’t accounted for in the calorie count.
Why We Want More
The glycemic index (GI) also illuminates the superficiality of the all-calories-are-equal theory. As the New York Times reports in a recent study from Dr. David Ludwig, “high glycemic foods (such as sugar, bread and potatoes) spike blood sugar and stimulate hunger and cravings, which can drive people to overeat.”
So, for example, while a bowl of steel-cut oatmeal may have the same amount of calories as a blueberry muffin—those calories behave vastly differently in the body. That high-GI muffin will spike and drain your insulin levels, making you hungrier faster, and more likely to eat more during the day. The low-GI carbs of the oatmeal burn slower, so you feel satisfied longer.
How Quality Counts
The calorie-is-a-calorie theory breaks down with dieting, too. Yes, the old adage holds true: burn more calories than you consume, and you’ll lose weight. But, it’s not a simple calorie-in/calorie-out equation. Dr. Ludwig’s paper found low-fat dieters burned fewer calories at rest.
The experiment compared three diets: low-fat, low-GI, and low-carb: essentially pitting the 1980s against The Zone and Dr. Atkins. And who came out on top? Participants on the low-GI diet burned up to 300 more calories a day, even though they were consuming the same number of calories as low-fat dieters.
Thus, it’s not the amount of calories that matter, it’s the source. Ludwig’s low-fat dieters were restricted from eating high-fat foods (often higher in protein), causing their metabolism to plummet. As you can see in the chart above, the low-GI (LGI) and very low carb (VLC) burned more calories at rest, and in total, because the protein and fat calories they were eating burned slower and longer.
This case isn’t closed. Research continues to illuminate the different roles calories from fat, protein, and carbohydrates play during metabolism.