Which nutrient has the highest number of kilojoules per gram?

December 7, 2025 •

3 min read

Fat contains approximately 37 kilojoules per gram, according to sources like the Food and Agriculture Organization (FAO), making it the most energy-dense macronutrient. This high concentration is why small amounts of fatty foods can contribute significantly to daily kilojoule intake.

Quick Summary

Fat is the nutrient providing the most energy per gram. Its chemical structure makes it the most energy-dense macronutrient, delivering more than double the energy of protein or carbohydrates by weight.

Key Points

Fat Provides Most Kilojoules: Fat contains 37 kJ per gram, making it the most energy-dense macronutrient.
Alcohol is Also Energy-Dense: Alcohol provides 29 kJ per gram, which is more than protein or carbohydrates but less than fat.
Protein and Carbs Offer Less Energy: Both protein and carbohydrates provide approximately 17 kJ per gram, offering less than half the energy of fat by weight.
Energy Density vs. Nutrient Density: High energy density does not equal unhealthy; nutrient-dense foods with high fat (like nuts) provide crucial vitamins and minerals.
Implications for Weight Management: Diets high in energy-dense foods can lead to higher overall kilojoule intake because people often eat a consistent weight of food, regardless of its energy density.
Lowering Energy Density: You can reduce overall kilojoule intake by increasing the proportion of water-rich foods, such as fruits and vegetables, in your diet.

Understanding the Energy in Macronutrients

All food provides energy, measured in kilojoules (kJ), but not all nutrients are created equal when it comes to energy density. The amount of energy a food provides depends on its macronutrient composition—specifically the proportions of fat, protein, and carbohydrates. Understanding the energy density of these core nutrients is fundamental to comprehending nutrition labels and how different foods impact energy balance.

The Energy Hierarchy: A Comparative Breakdown

The main energy-providing components of food are fat, protein, and carbohydrates. Alcohol also contributes a significant amount of energy, though it is not considered a nutrient in the same vein as the others.

Fat (37 kJ/g): With 37 kilojoules per gram, fat is the most concentrated source of dietary energy. This high energy density results from fat molecules having more carbon-hydrogen bonds and less oxygen than carbohydrates and protein, which allows them to be oxidized more efficiently for energy.
Alcohol (29 kJ/g): While not a nutrient, alcohol provides 29 kilojoules per gram, which is considerably more than protein or carbohydrates. The body prioritizes metabolizing alcohol for energy, which can lead to the storage of other nutrients as fat.
Protein (17 kJ/g): Protein provides 17 kilojoules per gram, the same amount as carbohydrates. However, the body primarily uses protein for building and repairing tissues and only uses it for energy when other sources are insufficient.
Carbohydrates (17 kJ/g): Carbohydrates are the body's preferred and quickest source of energy, offering 17 kilojoules per gram. This includes sugars and starches. Some carbohydrates, like dietary fiber, offer less energy (around 8-13 kJ/g) because they are not fully absorbed.

Why Fat is the Most Energy-Dense Nutrient

From a biochemical perspective, the difference in kilojoule content is a matter of molecular structure. Carbohydrates and proteins contain more oxygen atoms in their molecular makeup than fats. This means they are already partially oxidized and, therefore, require less energy to break down for fuel. Fat molecules, specifically triglycerides, are composed of long chains of fatty acids rich in carbon-hydrogen bonds. Breaking these bonds through oxidation releases a much larger amount of energy. This high energy concentration is why the body evolved to use fat as its primary storage form of energy.

The Importance of Nutrient Density

While kilojoule count is important for managing energy intake, it's crucial not to mistake energy density for nutritional value. A food with high energy density isn't always unhealthy, nor is a low-energy food always the best choice. For example, nuts and avocados are high in energy due to their healthy fat content but are also packed with essential vitamins, minerals, and fiber. This makes them nutrient-dense foods. In contrast, a chocolate bar might have a similar kilojoule count but offers far fewer beneficial nutrients, making it an 'empty-kilojoule' food. Focusing on overall dietary balance and nutrient intake is more beneficial for long-term health than simply counting kilojoules.

Impact on Diet and Weight Management

This difference in energy density has significant implications for diet and weight management. A diet high in energy-dense foods (e.g., fats and processed foods) can lead to consuming excess kilojoules without feeling full, as research has shown that people tend to eat a consistent weight of food, not a consistent energy intake. Conversely, incorporating more low-energy-dense foods, such as fruits and vegetables (which have high water content), can help manage hunger and reduce overall kilojoule intake.


Macronutrient	Kilojoules per gram (kJ/g)	Kilocalories per gram (kcal/g)
Fat	37	9
Alcohol	29	7
Protein	17	4
Carbohydrate	17	4

Practical Lists for Applying this Knowledge

High Energy-Dense Food Examples (Prioritize Portion Control)

Oils and butter
Nuts and seeds
Avocados
Fried foods
Baked goods
Full-fat dairy

Low Energy-Dense Food Examples (Eat in Reasonable Portions)

Most fruits and vegetables
Broth-based soups
Lean proteins
Legumes
Whole grains

Conclusion

Fat is the nutrient with the highest number of kilojoules per gram due to its concentrated chemical structure. While this fact is important for understanding nutritional science, it is only part of the bigger picture of healthy eating. A balanced diet should consider both the energy and nutrient density of foods. Integrating healthy fats in moderation and prioritizing nutrient-rich, lower-energy-dense foods like fruits and vegetables is a sustainable approach for managing body weight and promoting overall health. For further information on the metabolic effects of macronutrients, consult reputable health and nutrition guides, such as those from the FAO: CALCULATION OF THE ENERGY CONTENT OF FOODS.

Frequently Asked Questions

Fat molecules are more energy-dense than protein or carbohydrates because their chemical structure contains more energy-rich carbon-hydrogen bonds and less oxygen, allowing for a more efficient release of energy upon metabolism.

One gram of protein contains approximately 17 kilojoules of energy. The body primarily uses protein for building and repairing tissues, but it can be used for energy if needed.

Yes. The body processes carbohydrates more quickly for immediate energy. Fat is a slower, more concentrated source of energy, often stored in the body for long-term fuel, while protein is primarily used for tissue maintenance.

Alcohol is mentioned because it also contains energy, providing 29 kilojoules per gram. The body prioritizes metabolizing alcohol, which can affect the way it uses energy from other nutrients.

Energy density refers to the amount of kilojoules per gram of food, whereas nutrient density refers to the amount of nutrients (vitamins, minerals, etc.) per kilojoule. For example, an avocado is high in both energy and nutrient density, while a candy bar is high in energy density but low in nutrient density.

By choosing foods that are less energy-dense but high in nutrient density, such as fruits, vegetables, and lean proteins, you can eat a larger volume of food to feel full without consuming an excessive number of kilojoules.

Cooking methods can affect a food's kilojoule content, particularly if oils or fats are added. For example, frying food significantly increases its energy density due to the added fat, whereas steaming or boiling does not.