Six Reasons Why a Calorie is Not a Calorie

Of all the nutrition myths, the calorie myth is one of the most pervasive and most damaging.

It’s the idea that calories are the most important part of the diet — that the sources of these calories don't matter.

"A calorie is a calorie is a calorie," they say — that it doesn't matter whether you eat a 100 calories of candy or broccoli, they will have the same effect on your weight.

It’s true that all calories have the same amount of energy. One dietary calorie contains 4,184 Joules of energy. In that respect, a calorie is a calorie.

But when it comes to your body, things are not that simple. The human body is a highly complex biochemical system with elaborate processes that regulate energy balance.

Different foods go through different biochemical pathways, some of which are inefficient and cause energy (calories) to be lost as heat (1).

Even more important is the fact that different foods and macronutrients have a major effect on the hormones and brain centers that control hunger and eating behavior.

The foods you eat can have a huge impact on the biological processes that control when, what and how much you eat.

Here are 6 proven examples of why a calorie is not a calorie.


1. Fructose vs Glucose

The two main simple sugars in your diet are glucose and fructose.

Gram for gram, the two provide the same number of calories.

But the way they are metabolized in the body is completely different (2).

Glucose can be metabolized by all of your body's tissues, but fructose can only be metabolized by the liver in any significant amount (3).

Here are a few examples of why glucose calories are not the same as fructose calories:

Ghrelin is the hunger hormone. It goes up when you’re hungry and down after you've eaten. One study showed that fructose leads to higher ghrelin levels — that is more hunger — than glucose (4).
Fructose does not stimulate the satiety centers in your brain in the same way as glucose, leading to a reduced feeling of fullness (5).
Consuming a lot of fructose can cause insulin resistance, abdominal fat gain, increased triglycerides, blood sugar and small, dense LDL compared to the exact same number of calories from glucose (6).

As you can see: the same number of calories — vastly different effects on hunger, hormones and metabolic health.

Judging nutrients based on the calories they provide is way too simplistic.

Keep in mind that fructose only has negative effects when eaten in excessive amounts. Added sugar and candy are its largest dietary sources.

Don’t be discouraged to eat plenty of fruits. While they contain fructose, they’re also rich in fiber, water and provide significant chewing resistance, which mitigates the negative effects of the fructose.

2. The Thermic Effect of Food

Different foods go through different metabolic pathways.

Some of these pathways are more efficient than others.

The more efficient a metabolic pathway is, the more of the food’s energy is used for work and less is dissipated as heat.

The metabolic pathways for protein are less efficient than the metabolic pathways for carbs and fat.

Protein contains 4 calories per gram, but a large part of these protein calories is lost as heat when it’s metabolized by the body.

The thermic effect of food is a measure of how much different foods increase energy expenditure, due to the energy required to digest, absorb and metabolize the nutrients.

Here is the thermic effect of the different macronutrients (7):

Fat: 2–3%
Carbs: 6–8%
Protein: 25–30%

Sources vary on the exact numbers, but it’s clear that protein requires much more energy to metabolize than fat and carbs (8).

If you go with a thermic effect of 25% for protein and 2% for fat, this would mean that 100 calories of protein would end up as 75 calories, while 100 calories of fat would end up as 98 calories.

Studies show that high-protein diets boost metabolism by 80–100 calories per day, compared to lower-protein diets (8, 9).

Put simply, high-protein diets have a metabolic advantage.

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