Are Kilojoules Fattening? The Truth About Energy, Diet, and Weight

November 2, 2025 •

5 min read

Excess energy consumption, whether from kilojoules or other sources, is stored as fat. So, are kilojoules fattening? The truth is more nuanced, depending on the balance between energy intake and expenditure, and the source of those kilojoules.

Quick Summary

Kilojoules themselves are not inherently fattening; weight gain results from consuming more energy than you expend. The type of food, overall energy balance, and nutrient density are the key factors.

Key Points

Energy Balance is Key: Weight gain is caused by a positive energy balance (consuming more kilojoules than you burn), not by kilojoules themselves.
All Kilojoules are Not Equal: The nutritional value and impact on satiety vary greatly between foods with similar kilojoule counts, a concept known as energy density.
Nutrient Density Matters: Prioritise nutrient-dense foods like fruits and vegetables, which provide more nutrition for fewer kilojoules, increasing fullness and health benefits.
Macronutrients Affect Energy Density: Fat is the most energy-dense macronutrient, containing more than double the kilojoules per gram compared to protein and carbs.
Inactivity is a Major Contributor: A sedentary lifestyle, alongside dietary choices, is a significant factor in weight gain by reducing energy expenditure.
Read Food Labels Carefully: Pay attention to serving sizes and kilojoule counts on packaging to make informed dietary decisions and avoid overconsumption.
Exercise is Crucial: Combining a balanced diet with regular physical activity is the most effective and sustainable approach to managing weight.

What Are Kilojoules and How Do They Relate to Weight?

Kilojoules (kJ) are a metric unit of energy, used in countries like Australia to measure the energy content of food and drinks. A kilojoule is a measure of energy, just as a kilometre is a measure of distance. Your body needs this energy to fuel its basic functions, such as breathing and circulation, and to power all your daily activities, from walking to thinking. When it comes to weight management, the relationship is based on a concept called energy balance. If you consume more kilojoules than your body uses, the excess energy is stored as fat, leading to weight gain. Conversely, if you use more kilojoules than you consume, your body uses stored energy, leading to weight loss. It is this imbalance, not the kilojoules themselves, that is the cause of weight change.

The Science of Energy Balance: It's Not Just About Kilojoules

The human body operates on a principle of energy balance: energy in versus energy out. While the energy in side is determined by your food and drink, the energy out side is influenced by several factors, including your Basal Metabolic Rate (BMR) and physical activity levels.

Positive vs. Negative Energy Balance

Positive Energy Balance: This occurs when your kilojoule intake is greater than your energy expenditure. The surplus energy is stored in the body, primarily as fat, leading to weight gain. Persistent positive energy balance is the root cause of being overweight or obese.
Negative Energy Balance: This is achieved when you burn more kilojoules than you consume. To meet its energy demands, the body taps into its stored reserves, resulting in weight loss. A negative energy balance is the goal for anyone seeking to lose weight.

How Your Body Uses Kilojoules

Basal Metabolic Rate (BMR): This is the energy your body needs to maintain vital functions while at rest. It accounts for the largest portion of your daily energy expenditure. Factors like age, sex, body size, and muscle mass affect your BMR.
Physical Activity: This includes all movement, from structured exercise to daily activities like fidgeting and walking. The amount of physical activity is the most variable part of your energy expenditure.
Thermic Effect of Food (TEF): This is the energy your body uses to digest, absorb, and process the nutrients in your food. It accounts for a small but relevant portion of your daily energy expenditure.

Are All Kilojoules Created Equal? Nutrient Density vs. Energy Density

This is a crucial concept often overlooked in weight management. Not all kilojoules are the same when it comes to their impact on your body and your sense of fullness. This is because the overall nutritional value of a food matters, not just its energy count.

For example, an avocado and a chocolate bar can have a similar kilojoule count, but their effect on your body is vastly different. The avocado is packed with healthy fats, fibre, and vitamins, providing long-lasting energy and satiety. The chocolate bar offers a quick, but fleeting, energy spike due to its high sugar content and provides very little nutritional value.

High vs. Low Energy-Dense Foods

This distinction helps explain why you can eat a large bowl of salad for fewer kilojoules than a small serving of fried food.


Feature	Low Energy-Dense Foods	High Energy-Dense Foods
Kilojoules	Lower kJ per gram or serving	Higher kJ per gram or serving
Composition	High in water and fibre	Low in water and fibre; high in fat, sugar, or refined carbs
Satiety	More filling; helps control hunger	Less filling; can lead to overconsumption
Examples	Vegetables, fruit, broth-based soups, lean protein	Fried foods, sweets, processed snacks, pastries

The Role of Macronutrients: How Different Foods Impact Satiety

Kilojoules come from the three main macronutrients: carbohydrates, protein, and fat. Each provides a different amount of energy per gram, influencing the overall kilojoule content of your food.

Fat: Provides the most energy, at approximately 37 kJ per gram. This is why fatty foods can contribute to a high kilojoule intake very quickly.
Protein: Offers a moderate amount of energy, about 17 kJ per gram. Protein is highly satiating, meaning it helps you feel full, which is beneficial for weight management.
Carbohydrates: Also provide approximately 17 kJ per gram. Complex carbohydrates, particularly those high in fibre, promote fullness and aid in digestion.

Practical Strategies for Managing Kilojoule Intake

To successfully manage your weight, focusing solely on the number of kilojoules is not enough. You must also consider the source of those kilojoules and your physical activity levels. Here are some actionable tips:

Prioritise Nutrient-Dense Foods: Fill your plate with vegetables, fruits, whole grains, and lean proteins. These foods are lower in energy density and higher in fibre and other vital nutrients, helping you feel fuller for longer.
Watch Your Portion Sizes: Be mindful of how much you're consuming, especially with energy-dense foods. Even healthy high-fat foods, like nuts and avocado, can lead to weight gain if portion sizes are too large.
Increase Physical Activity: Exercise burns kilojoules, helping to create the necessary negative energy balance for weight loss. Regular activity, including moderate and vigorous exercise, is key.
Stay Hydrated: Water contains zero kilojoules and can help you feel full, preventing overeating. Sugary drinks are often surprisingly high in kilojoules and offer little nutritional value.

Understanding Food Labels

Learning to read nutrition information panels is a valuable skill. Here's what to look for:

Kilojoules (kJ): This is the total energy content. Check both the per serving and per 100g columns.
Serving Size: Be aware that the serving size listed on the package may be smaller than what you typically consume. If you eat two servings, you double the kilojoule intake.
Macronutrients: Compare the amounts of fat, protein, and carbohydrates to understand the energy density of the product.
Sugar: Look for the added sugar content, especially in beverages and processed snacks.

Conclusion: The Final Word on Fattening Kilojoules

Ultimately, no single food or energy unit is inherently fattening. It is the overall balance between the kilojoules you consume and the kilojoules you expend that determines whether you gain or lose weight. While a high kilojoule intake can lead to fat storage, the quality of your food choices is just as important as the quantity. Focusing on nutrient-dense foods, managing portion sizes, and staying active are the most effective strategies for long-term weight management and overall health. For more information and resources on making healthier dietary choices, visit the Better Health Channel.

Frequently Asked Questions

A kilojoule (kJ) is a unit of energy used to measure the energy content of food and drinks. It is the metric equivalent of a calorie, with one calorie equaling approximately 4.2 kilojoules.

Kilojoules don't inherently cause weight gain. Weight gain occurs when you consistently consume more kilojoules than your body burns for energy. The excess energy is then stored in your body as fat.

They measure the same thing—the energy in food—but are different units of measurement. In Australia and other metric-system countries, kilojoules are the standard unit, while calories are used in other regions, including the USA.

No, not all kilojoules are created equal. The nutritional value and source of the kilojoules matter. Kilojoules from nutrient-dense foods, like vegetables, provide more satiety and health benefits than the same amount of kilojoules from highly processed, energy-dense foods.

Energy balance is the relationship between the kilojoules you consume and the kilojoules you expend. It is crucial for weight management because it dictates whether you gain, lose, or maintain your weight.

Effective strategies include prioritizing nutrient-dense whole foods, controlling portion sizes, limiting sugary drinks and processed snacks, and staying hydrated with water. Making small, sustainable changes is key.

Energy density is the concentration of kilojoules in a given amount of food. Foods with low energy density allow you to eat a larger volume for fewer kilojoules, which helps increase fullness and manage weight. High energy density foods can lead to overconsumption.

Physical activity increases the amount of energy your body burns, which helps offset kilojoule intake and contribute to a negative energy balance. The more you move, the more kilojoules you expend.