Thermic Effect of Food (TEF) | Calories Not Fully Utilised | Macronutrient Differences |  FAQ |

Quick Answer

Not all calories you eat are fully used by your body. Some energy is lost during digestion (the thermic effect of food), and a small portion of food is not digested at all, especially for fat, fibre and protein.

Introduction

While “calories in calories out” is a helpful starting point for understanding weight management, the reality is more complex. Your body uses some of the calories you eat just to digest, absorb, and process food, and not every calorie from food is absorbed. The type of macronutrient—fat, carbohydrate, or protein—affects how many calories you actually get from your food.

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Understanding the Science Beyond Calories In Calories Out

Thermic Effect of Food (TEF)

The thermic effect of food is the energy your body uses to digest, absorb, and metabolise nutrients. Protein has the highest TEF, meaning you burn more calories digesting protein than carbs or fat. This is why high-protein diets can slightly increase your calorie burn.

• Fat provides 9 calories per gram, with a TEF of 0–3%.
• Carbohydrates provide 4 calories per gram, with a TEF of 5–10%.
• Protein provides 4 calories per gram, with a TEF of 20–30%.

Calories Not Fully Utilised

Not all the calories you eat are absorbed. A small percentage of fat and protein passes through your digestive tract without being digested, while digestible carbohydrates are almost fully absorbed. Fibre, a type of carbohydrate, is not digested and does not contribute to calorie intake.

• About 1–5% of fat you eat may not be digested and is excreted.
• 0% of digestible carbohydrates are typically not digested, but fibre is not absorbed.
• 0–1% of protein is generally not digested.

Macronutrient Differences

The table below shows how many calories the body typically utilises from 1000 calories of fat, carbs, and protein when accounting for the thermic effect of food and incomplete digestion.

• Protein results in the lowest net calories due to its high thermic effect and incomplete digestion.
• Carbohydrates are almost fully absorbed except for fibre.
• Fat is energy dense, but a small amount is not digested

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In Summary

Not every calorie you eat is fully absorbed or used by your body. Protein burns the most calories during digestion, while fat and carbs are more efficiently stored. Focusing on whole foods, protein, and fibre can help you manage your calorie intake more effectively.

FAQ

How does the thermic effect of food influence weight loss?

The thermic effect of food slightly increases the number of calories you burn, especially when you eat more protein. This can help with weight management, but the effect is relatively small compared to total calorie intake and activity.

Why isn’t all the food I eat digested?

Some fat and protein are not fully digested and pass through the digestive tract, especially if you eat large amounts or have digestive issues. Digestible carbohydrates are almost completely absorbed, but fibre is not.

Does eating more protein help burn more calories?

Yes, protein has a higher thermic effect, so your body uses more energy to digest and process it compared to carbs and fat. This means fewer net calories from protein-rich foods.

Should I focus only on calories, or also on macronutrients?

While calorie balance is key for weight management, the type of macronutrients you eat affects how many calories are actually absorbed and used. A diet higher in protein and fibre can help you feel fuller and burn more calories.