Calories

The scientists analysed the average number of calories people burn in their daily lives, among a research group of more than 6,600 people in 29 countries around the world, ranging in age from one week to 95 years. Previous large-scale studies chiefly examined how much energy the body uses to perform basic vital functions, such as breathing, digestion, and blood circulation – in other words, the calories you need to stay alive. This equates to 50 to 70 percent of the calories we burn daily. But none of these studies took into account the energy people expend on everything else: washing dishes, walking the dog, working out at the gym, and even thinking or fidgeting. To arrive at a figure for total daily energy expenditure, the researchers used a tried and tested method.

Heavy water

Doubly labelled water, also known as heavy water, is a so-called stable isotope, which means it doesn’t decay, and therefore doesn’t emit radiation. In the water molecules of the doubly labelled water, common hydrogen (1H) is partially replaced by a two-proton version (2H) and common oxygen with 16 protons by an 18-proton version. When consumed normally, doubly labelled water is non-toxic and can therefore be used in producing urine tests for experiments. Scientists consider the technique to be the gold standard for measuring daily energy use during everyday life. Klaas Westerterp, Professor of Human Energetics at Maastricht University, was first introduced to the method as a biologist during a large-scale study on the energy expenditure of swallows and starlings in the late 1970s. A few years later, he was the first to use the technique to measure the total energy expenditure of people making full physical effort, in tests on riders during the 1985 Tour de France.

Metabolism

Due to the cost, worldwide such studies have been fairly limited. A number of laboratories around the world therefore decided to create a single database collating their data from research dating from the 1980s up to very recently. The publication in Science is based on this data. By pooling the data and analysing energy expenditure over the entire human lifespan, the scientists revealed some surprises. For example, they discovered that infants have the relatively highest metabolism. ‘And that’s not just because babies triple their birth weight in their first year. During the next phase of growth, activity appears to be a determining factor for physical development,’ Professor Westerterp says. ‘From adolescence onwards, on average, energy expenditure remains constant until about the age of 60, but there are wide differences among individuals. This finding offers an opportunity for thorough follow-up research on behaviour and aging.’