Australasian Science: Australia's authority on science since 1938

The Truth about the Average Adult Energy Intake

By Tim Olds

Nutritional labels on food state that the average adult consumes 8700 kJ/day, but isn’t this a bit low?

The Tour de France is an extraordinary sporting event, with some of the world’s fittest men embarking on a challenge that is on the very edge of human endurance capacity. This year 198 cyclists set out to cycle 3664 km in 21 days.

I often watch the Tour with a bottle of beer, the label of which tells me that it contains 6% of the “average adult diet” of 8700 kJ. But where does this figure come from? How do we know how many kilojoules a person consumes or needs?

The gold standard for measuring energy expenditure is the devilishly clever method of doubly-labelled water, which is water with a special isotope of hydrogen (2H) and a special isotope of oxygen (18O). Doubly-labelled water is eye-wateringly expensive, but you’ll actually find some in the bottom of your kettle. Because it’s weighed down by the extra neutrons, it’s a bit heavier than ordinary water and it remains in your kettle while ordinary H2O is fractionally distilled as it boils.

Here’s how it works. After taking a baseline urine sample, you get someone to drink a glass of doubly-labelled water, which enriches the natural concentrations of 2H and 18O in their body water. Over a period of, say, a week, you take regular urine samples. Now the labelled hydrogen is eliminated from the body only in the form of water (sweat, urine, faeces, breath) while the labelled oxygen is eliminated both as water and as carbon dioxide.

So if we know the differential rate of removal of labelled hydrogen and oxygen (which we get by putting the urine samples through a mass spectrometer) we can tell how much CO2 a person has produced over the week. And because almost all energy produced in the body results in the production of CO2, we can estimate energy expenditure.

Many studies have been conducted using doubly-labelled water and other methods that are a bit more speculative, so we have a pretty good idea of how much energy different types of people use, and we can even make estimates for extinct populations. The upper limit that is sustainable over a long period appears to be about 4.5 times resting metabolic rate – in animals as well as in humans. This limit can be achieved by Tour de France riders on mountain stages, and by Polar explorers and cross-country skiers.

Table 1 shows how much energy different groups of people can use (and consume) over long periods. Since energy requirements vary with body weight, I’ve calculated these for a 70 kg person.

So what does this tell us about the food labelling? It tells us that 8700 kJ/day seems awfully low. According to the responsible body, Food Standards Australia and New Zealand (FSANZ), this estimate is “based on the average energy consumption/day for adults and children over 4 years of age in Australia and New Zealand”. That helpful piece of information certainly doesn’t appear on the label, which tells us it is a percentage of the “average adult diet”.

If we take the average Australian male (86 kg) with the average daily metabolic rate (1.69 times resting metabolic rate), we arrive at an estimated requirement of 15,400 kJ/day – more than 70% higher than the FSANZ figure – to remain weight-stable. If he followed the label recommendations and ate only 8700 kJ/day we can calculate that the poor bloke would be dead after 380 days, weighing just 41 kg. His wife (71 kg,

1.64 times resting metabolic rate) would lose 28 kg before her weight stabilised at 43 kg after 5 years on these starvation rations. Even their 12-year-old son (45 kg, 1.7 times resting metabolic rate) would lose 7 kg before his weight stabilised after a year.

So if, as any reasonable person reading the label would assume, the 8700 kJ referred to adults it would be misleading. If it can refer to anyone aged from 4 year old girls (average energy requirement 4800 kJ/day) to adult males (average requirement 15,400 kJ/day) it is completely useless.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.