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Calories, kilojoules and weight loss

13 May, 2015

Understanding calories and kilojoules

Calories and kilojoules both measure the amount of energy in a food or drink. Up until 1988 in Australia, we were using calories to measure the amount of energy in food or drink. Since 1988, we’ve been using kilojoules. The short version of kilojoules is kJ. One calorie is equal to 4.2 kilojoules. If you want to do a quick conversion when you’re looking at a food label with amounts in kilojoules, you can divide the kilojoule amount by four to get the amount in calories.

How your body uses energy

Most food that you eat contains different proportions of proteins, carbohydrates and or fats. These are known as macronutrients and are the main sources of energy for your body. The amount of energy or kilojoules in each of these nutrients is different. Fat has more than twice as much energy per gram as carbohydrates and protein. They also supply the body with energy at different rates, based on how they are digested. Carbohydrates are the quickest, and fats are the slowest to be digested.

Your body uses this energy for:

  • metabolic processes
  • physical functions
  • muscular activity
  • heat production
  • growth of new tissues

Understanding the basal metabolic rate

The amount of energy your body needs for the above processes in a given period of time is known as the Basal Metabolic Rate (BMR). The BMR accounts for between 45 to 70% of daily energy needs. Your body also needs energy to process foods into nutrients.

How many kilojoules (energy) do we need each day?

An average adult needs an average of 8700 kilojoules each day to maintain a healthy weight. But the amount each individual needs depends on a number of factors. For example if you exercise a lot, or have a lot of muscle, you’re likely to need more. Your age and sex also influence how many kilojoules you need. Generally, if you eat more kilojoules than your body needs, your body will store the excess energy as fat.

To work out how many kilojoules you need, you can use the Energy Needs Calculator from the Better Health Channel.

For more detail, read about dietary energy requirements for different ages at the National Health and Medical Research Council.

How to find out how many kilojoules (energy) are in a food or drink

Packaged products

You’ll need to read the nutritional information panel on the back of a food or drink product to know how many kilojoules are in it. These panels also provide information on the amount of protein, fat, saturated fat, carbohydrates, sugars and sodium in the product.

A food product must list how many kilojoules are in 100g of the food or 100mL of the drink as well as per serve. This can be useful if you want to compare similar products, but keep in mind, you’re likely to consume more than 100g or 100ml of a product. The ‘per serve’ measure could be more useful for working out how many kilojoules you’re likely to consume.

To find out more, use this interactive label reading tool on the Food Standards Australia New Zealand website.

If this is all too much and you need someone to interpret the information for you, try using the Health Star Rating.  The higher the star rating, the healthier the packaged food or drink is compared to other like products in the same category.

Losing weight

If you want to lose weight, you’ll need to consume less energy than you use.

According to the Merck Manual, people should aim to consume 2093 to 4187 fewer kilojoules each day to lose 450 grams to one kilogram per week. 450g of body fat stores about 18,500 – 37,000 kilojoules.

To work out how many kilojoules you need to cut down on to lose weight, you can use the Energy Needs Calculator from the Better Health Channel. By entering your age, sex, physical activity level and desired weight, you can work out roughly how many kilojoules you need to achieve your desired weight.  

Making changes to your diet and follow the Australian Dietary Guidelines

Choosing foods from the Australian Dietary Guidelines will help you choose foods that provide the most nutrients with the least extra kilojoules. They will fill you up, keep you feeling satisfied and stop those hunger pains.

You should aim to fill your meal and snack choices with  foods from the five food groups each day:

  • vegetables and legumes (beans)
  • fruit
  • grains and cereals
  • lean meat, poultry, fish, eggs, tofu, nuts seeds
  • milk, cheese, yoghurt or alternatives

Avoiding fad diets

It’s generally best to avoid fad diets, especially those that promote very fast weight loss or cut out entire food groups. Fad diets can lead to a reduction in your body’s BMR. Your metabolic rate is how quickly your body burns kilojoules. This could mean you might end up putting on weight quickly in the long run. It could also mean that you might experience other symptoms such as fatigue, headaches or even insomnia.

Healthy weight loss diets

You can also try the DASH Diet combined with the Mediterranean diet. It’s high in healthy whole foods like vegetables, fruits and legumes, and low in sweets, red and processed meats, and processed foods generally. As well as helping you to manage your weight, this diet can also reduce your risk of developing heart disease, type 2 diabetes, fatty liver disease, and some cancers.

You can also read our article on tips for losing weight.

Where to get more help

If you’re concerned about your weight, it’s best to see your GP or an accredited practicing dietitian or registered nutritionist. There are also many free services to get you started like the Healthy Weight Guide at the Department of Health.


All information contained in this article is intended for general information purposes only. The information provided should not be relied upon as medical advice and does not supersede or replace a consultation with a suitably qualified healthcare professional.


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