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Understanding Calories | Why All Calories are not Equal

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Discover the workings of the human body in a metabolic chamber. These high-tech facilities, costing millions of dollars and found at 30 locations worldwide (including at the Harvard School of Public Health) track every energy intake and expenditure.

Our research teams use the chambers to study diseases such as obesity and metabolic disorders. The chambers confirm that calories play a crucial role in weight gain or loss. The real question is not if "a calorie is a calorie," but why different calories affect metabolism and hunger differently.

Can't visit a metabolic chamber? Let us guide you on how to make food work for your metabolism.

What Is A Calorie?

A calorie is a unit of energy that represents the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. In terms of food, a calorie refers to the energy that a particular food item provides to the body.

Determining the Number of Calories in Food

There are two main methods for determining the number of calories in food: bomb calorimetry and nutrient composition analysis.

Bomb Calorimetry

Bomb calorimetry involves burning a food sample in a sealed container surrounded by water. The heat generated from the burning process is then transferred to the water, raising its temperature. The rise in temperature is used to calculate the number of calories in the food. This method, although accurate, is no longer the preferred method for determining calorie content.

Nutrient Composition Analysis

Nutrient composition analysis involves determining the calories provided by the protein, carbohydrates, fat, and alcohol in a food item. This method works because the number of calories provided by a gram of each macronutrient remains constant.

The following is a breakdown of the caloric values of each macronutrient:

  • 1 gram of protein = 4 calories
  • 1 gram of carbohydrates = 4 calories
  • 1 gram of fat = 9 calories
  • 1 gram of alcohol = 7 calories

Why Are All Calories Not Equal?

It is important to note that all calories are not equal, as different macronutrients are metabolized differently in the body. Some macronutrients, such as protein, result in a higher calorie expenditure during digestion, while others, such as the fiber found in carbohydrates, affect hunger and appetite.

Balancing one's diet to provide the right amount of energy while also keeping hunger levels in check is crucial for maintaining a healthy diet. However, the number of calories in a particular food item is not the only aspect of calorie-related confusion; it's how our body processes those calories that we eat and digest.

Understanding Energy Balance

To understand weight gain and loss, it's important to grasp the concept of energy balance, which is the balance between the number of calories you consume and the number of calories you burn. Your daily metabolic rate, the number of calories your body burns, is influenced by various factors including:

Basal Metabolic Rate (BMR)

The amount of energy your body needs to function is referred to as the basal metabolic rate (BMR).

Thermic Effect of Food (TEF)

Thermic effect of food (TEF) refers to the amount of energy you burn when you eat.

Exercise and Activity

The calories you burn from movement and exercise make up your physical activity. This can be split into different categories, such as NEAT (non-exercise activity thermogenesis) that includes movements like fidgeting and traditional workouts.

It's crucial to note that 65-80% of the calories burned in a day come from your BMR. Physical activity and the foods you eat make up the remaining portion of your metabolism, but they are equally important.

Metabolism of Macronutrients

Proteins, carbohydrates, and fats are all metabolized differently, which is why all calories are not equal. Eating 100 calories of protein has a different impact on your body compared to eating 100 calories of carbohydrates because proteins have a higher TEF.

When you eat protein, up to 30% of the calories can be burned. For example, if you consume 100 calories of protein, 70 calories will reach your body because 30 calories would be burned due to the protein's high TEF. This is why higher protein diets tend to be associated with weight loss and maintenance.

However, carbohydrates have a TEF of only 5-10%, and fats have a TEF of 3-5%. This is just one of the reasons why all calories are not equal, and it's essential to understand the interplay between macronutrient metabolism and energy balance for a healthy diet.

The Effect of a Protein Rich Diet on Hunger

Protein is a critical factor in controlling your hunger levels and helps keep you feeling full for longer. This is due to its ability to increase satiety, which is the feeling of fullness after eating. A diet that is rich in protein will leave you feeling satisfied, reducing the need to snack and overeat.

On the other hand, fast food, ice cream, and other high-calorie options that lack protein can quickly leave you feeling hungry just a few short hours after eating. This is not just because of their calorie count but also because they do not meet your body's needs for hunger control. When you eat these foods, you are likely to crave more food even when your calorie intake is high.

Hormones that Control Hunger

The hormone ghrelin plays a crucial role in regulating hunger and determining how quickly you feel hungry after a meal. When you consume a high-protein meal, it boosts the release of this hormone, which in turn helps quiet your hunger and keep you feeling full for longer.

The Thermic Effect of Food

One of the key benefits of eating protein is that it has a higher thermogenic effect (TEF) than other macronutrients. This means that your body burns more calories when digesting protein than when digesting carbohydrates or fat. This, in combination with the hunger-curbing effects, helps create a caloric deficit, making it easier to lose weight.

Building Muscle Mass

Protein is essential for muscle recovery and growth, and providing your body with enough protein after strength training can help you build more muscle mass. By making protein a key component of your diet, you can ensure your body has the nutrients it needs to repair and build muscle.

Fiber: The Other Hunger-Controlling Macronutrient

Fiber, which is found in carbohydrates, is also effective at increasing fullness and preventing overeating. Fiber-rich foods have a low energy density, which means you can eat a lot without taking in too many calories. By combining protein and fiber, you are more likely to stay full and avoid overeating.

Finding Balance in Your Diet

The goal of any diet is not to restrict but to provide more freedom. Eating the foods that keep you full and satisfied is the key to achieving this. By focusing on making at least half of your plate consist of protein and fiber, you can create a balanced diet that delivers results and prevents burnout.

The 80/20 Rule

Effective diets typically consist of 80 to 90 percent nutritious foods, such as vegetables, fruits, nuts, seeds, higher-fiber carbs, and protein. The remaining 10 to 20 percent can consist of foods with fewer direct health benefits, such as candy bars or fast food. This balance will provide you with the nutrients your body needs to function at its best, without feeling restricted or deprived.


  1. "The thermic effect of food." Harvard Health Publishing, Harvard Medical School,
  2. "Satiety: Feel Full and Satisfied After Eating." MedlinePlus, U.S. National Library of Medicine, 2 May 2019,
William H. McDaniel, MD

Dr. Robert H. Shmerling is the former clinical chief of the division of rheumatology at Beth Israel Deaconess Medical Center (BIDMC), and is a current member of the corresponding faculty in medicine at Harvard Medical School.

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