Balancing kilojoules

We all need energy to survive - to breathe, move, pump blood etc - and we get this energy from our food.

To measure energy, most countries now use the metric term kilojoule (kJ) instead of the older term, calorie. Kilojoules represent the amount of energy available in food – this is calculated by measuring the heat that would come from that food when our bodies burn, or metabolise, it. Example: If a packet of cereal has 672kJ, this means if we set it on fire and burnt it completely, the reaction would produce 672kJ, which is enough energy to raise the temperature of 160kg of water 1ºC.

Ways we produce energy from food

There are 3 main ways we burn our food to produce energy: when we are at rest (kilojoules burnt by organs and cells); when we are physically active (the part we are most able to increase or decrease); and during digestion and absorption of food.

The process of burning food is called metabolism. This is carried out by enzymes that break carbohydrates into glucose and other sugars, fats into glycerol and fatty acids and proteins into amino acids. All these molecules are carried by the blood to all the cells in our bodies, where they are absorbed, stored, reassembled or react with oxygen to release their stored energy.

How do kilojoules affect weight?

If you eat more kilojoules than you need, the extra energy is stored as fat, and you gain weight. If you eat fewer kilojoules than you need, you will burn up stored fat and lose weight. The more physical activity you do, the more energy you burn.

To maintain a constant weight you have to balance the energy content of what you eat with your energy output. Most adults need to consume between 6,000 and 12,000kJ per day to remain healthy, but for the last 10 years the average adult New Zealander has been gaining about 1g per day, which suggests we’re taking in a little more energy than we need.

Become aware of the number kilojoules you eat

Becoming familiar with how many kilojoules you eat each day, balanced against how much exercise you do, you can have better control over your weight. Eating a balanced diet and paying attention to serving sizes, means you have to rely less on ‘kilojoule counting’ to achieve this.

We can estimate the amount of kilojoules in certain foods by knowing that 1g carbohydrate has 16.8kJ (4 calories), 1g protein has 16.8kJ (4 calories, and 1g fat has 37.8kJ or 9 calories – but it’s much easier to read food labels where manufacturers have noted how much energy the food contains.

To lose weight: eat fewer kilojoules in food and, or, burn more kilojoules.
To gain weight: eat more kilojoules in food and, or, burn less kilojoules.

Not being aware of these things can mean we eat too many high-kJ foods before our body signals that we’re feeling full and should stop eating. We can also easily eat too much when we rely on eating foods prepared outside of the home - we don’t have a god idea of how much energy is in the food and drink we consume.

Some high-kJ food traps

• Eating high-kJ foods between meals for snacks.
• Eating high-kJ food because you really like it, not because you’re hungry.
• Eating everything on your plate, whether or not you feel full.
• Taking a bigger portion size than usual.
• Choosing such foods over fruit and vegetables.
• Using food to cheer you up or relax our, or for because you are bored.

Note: Energy expenditure is calculated for a 74kg person. Energy costs of activities per kg body weight = walking 25 kJ.min-1, wallpapering 15 kJ.min-1, chopping wood 26 kJ.min-1, cleaning windows 18 kJ.min-1, painting 24 kJ.min-1, raking leaves 17 kJ.min-1, ironing 10 kJ.min-1. Source: Southern Cross Healthcare, NZ

* Source: Auckland Dietetics Service