6 Reasons Why a Calorie Is Not a Calorie

August 12, 2015 Updated: June 16, 2016

There are many ridiculous myths in nutrition.

The “calorie myth” is one of the most pervasive… and most damaging.

It is the idea that calories are the most important part of the diet, that the sources of those calories don’t matter.

It is true that all “calories” have the same amount of energy. (ValuaVitaly/iStock)


“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 is true that all “calories” have the same amount of energy. One dietary Calorie contains 4184 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.

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 we eat can have a huge impact on the biological processes that govern when, what and how much we eat.

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

1. Fructose vs Glucose

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

These two seem almost identical. They have the same chemical formula and weigh the exact same.

But to your body, the two are completely different.

hand holding soda can pouring a crazy amount of sugar in metaphor of sugar content of a refresh drink isolated on blue background in healthy nutrition, diet and sweet addiction concept
Even though fructose and glucose have the same chemical formula, fructose has much more negative effects on hormones, appetite and metabolic health. (iStock)

Glucose can be metabolized by all of the body’s tissues, but fructose can only be metabolized by the liver in any significant amount.

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 we’re hungry and down after we’ve eaten. One study shows that fructose leads to higher ghrelin levels (more hunger) than glucose.
  • Fructose does not stimulate the satiety centers in the brain in the same way as glucose, leading to reduced satiety.
  • A high consumption 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.

Same number of calories, vastly different effects on hunger, hormones and metabolic health. Because a calorie is not a calorie.

Keep in mind that this applies to fructose from added sugars only, not the fructose from fruit. Fruits also have fiber, water and significant chewing resistance, which mitigate the negative effects of the fructose.

Bottom Line: Even though fructose and glucose have the same chemical formula, fructose has much more negative effects on hormones, appetite and metabolic health.

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 energy is used for work and less is dissipated as heat.

Protein takes more energy to metabolize. (klenova/iStock)

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 the protein calories are lost as heat when it is 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.

This is the thermic effect of different macronutrients:

  • Fat: 2-3%.
  • Carbs: 6-8%.
  • Protein: 25-30%.

Sources vary on the exact numbers, but it is clear that protein requires much more energy to metabolize than fat and carbs. 

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

Studies show that high protein diets boost metabolism by 80 to 100 calories per day, compared to lower protein diets.

Put simply, high protein diets have a “metabolic advantage.”

There is also one study that compared two sandwich meals that had the same number of calories and macronutrients.

However, one sandwich was made with whole grains and cheddar cheese, while the other was made with refined grains and processed cheese.

Those who ate the whole grain sandwich burned twice as many calories digesting the meal.

Bottom Line: Protein calories are less fattening than calories from carbs and fat, because protein takes more energy to metabolize. Whole foods also require more energy to digest than processed foods.

3. Protein Kills Appetite and Makes You Eat Fewer Calories 

The protein story doesn’t end with increased metabolism.

It also leads to significantly reduced appetite, making you eat less calories automatically.

The studies show that protein is the most fulfilling macronutrient, by far.


If people increase their protein intake, they start losing weight without counting calories or controlling portions. Protein puts fat loss on autopilot.

In one study, those who increased their protein intake to 30% of calories automatically started eating 441 fewer calories per day and lost 4.9 kg (11 lbs) in 12 weeks.

If you don’t want to go on a “diet” but simply tip the metabolic scales in your favor, then adding more protein to your diet may be the simplest (and most delicious) way to cause “automatic” weight loss.

It is very clear that when it comes to metabolism and appetite regulation, a protein calorie is NOT the same as a carb calorie or a fat calorie.

Bottom Line: Increased protein can lead to drastically reduced appetite and cause automatic weight loss without the need for calorie counting or portion control.

4. The Satiety Index

Different foods have different effects on satiety.

It is also much easier to overeat on some foods than others.

For example, it may be quite easy to eat 500 calories (or more) of ice cream, while you’d have to force feed yourself to eat 500 calories of eggs or broccoli.

The food choices you make can have a huge impact on the total calories you end up consuming. (shironosov/iStock)

This is a key example of how the food choices you make can have a huge impact on the total calories you end up consuming.

There are many factors that determine the satiety value of different foods, which is measured on a scale called the satiety index.

The satiety index is a measure of the ability of foods to reduce hunger, increase feelings of fullness and reduce energy intake for the next few hours. 

If you eat foods that are low on the satiety index, then you will be hungrier and end up eating more. If you choose foods that are high on the satiety index, you will end up eating less and losing weight.

Some examples of foods with a high satiety index are boiled potatoes, beef, eggs, beans and fruits, while foods that are low on the satiety index include donuts and cake.

Clearly… whether you choose fulfilling foods or not will have a major difference on energy balance over the long term. Because a calorie from a boiled potato is not the same as a calorie from a doughnut.

Bottom Line: Different foods have different effects on satiety and how many calories we end up consuming in subsequent meals. This is measured on a scale called the Satiety Index.

5. Low-Carb Diets Lead to Automatic Calorie Restriction

Since the year 2002, over 20 randomized controlled trials have compared low-carb and low-fat diets.

Low-carb diets consistently lead to more weight loss than low-fat diets (olgna/iStock)

The studies consistently show that low-carb diets lead to more weight loss, often 2-3 times as much.

One of the main reasons for this is that low-carb diets lead to drastically reduced appetite. People start eating less calories without trying.

But even when calories are matched between groups, the low-carb groups usually lose more weight, although it doesn’t always reach statistical significance.

The biggest reason for this is probably that low-carb diets also cause significant water loss. Excess bloat tends to go away in the first week or two. 

Another reason is that low-carb diets tend to include more protein than low-fat diets. Protein takes energy to metabolize and the body expends energy turning protein into glucose.

Bottom Line: Low-carb diets consistently lead to more weight loss than low-fat diets, even when calories are matched between groups.

6. The Glycemic Index

There are many controversies in nutrition and the experts don’t agree on many things.

But one of the few things that almost everyone agrees on is that refined carbs are bad.

This includes added sugars like sucrose and high fructose corn syrup, as well as refined grain products like white bread. 

Refined carbohydrates tend to be low in fiber and they get digested and absorbed quickly (nop16/iStock)


Refined carbohydrates tend to be low in fiber and they get digested and absorbed quickly, leading to rapid spikes in blood sugar. They have a high glycemic index (GI), which is a measure of how quickly foods raise blood sugar.

When we eat a food that spikes blood sugar fast, it tends to lead to a crash in blood sugar a few hours later… also known as the “blood sugar roller coaster.” When blood sugar crashes, we get cravings for another high-carb snack.

In a study that served people milkshakes who were identical in every respect except that one had high GI carbs while the other had low GI carbs, the high GI milkshake caused increased hunger and cravings compared to the low GI shake.

Another study found that teenage boys ate 81% more calories during a high GI meal compared to a low GI meal.

So… the speed at which carb calories hit the system can have a dramatic effect on their potential to cause overeating and weight gain.

If you’re on high-carb diet, it is crucial to choose whole, unprocessed carb sources that contain fiber. The fiber can reduce the rate at which the glucose enters your system.

The studies consistently show that people who eat the most high glycemic index foods are at the greatest risk of becoming obese and diabetic. Because not all carb calories are created equal.

Bottom Line: Studies show that refined carbohydrates lead to faster and bigger spikes in blood sugar, which leads to cravings and increased food intake.

Take Home Message

Different calorie sources can have vastly different effects on hunger, hormones, energy expenditure and the brain regions that control food intake.

Even though calories are important, counting them or even being consciously aware of them is not at all necessary to lose weight.

In many cases, simple changes in food selection can lead to the same (or better) results than calorie restriction.

This article was originally published on www.authoritynutrition.com