Australasian Science: Australia's authority on science since 1938

Biggest Loser Provides Food for Thought

By Tim Olds

Contestants on The Biggest Loser have provided some startling evidence explaining why it’s difficult to keep weight off after dieting.

About two-thirds of all the energy a human body uses does nothing but stoke the internal furnace – keeping a consistent body temperature, the blood going, the lungs blowing, and the juices flowing.

Resting metabolic rate (RMR) is the rate at which we use energy when we’re sitting or lying down doing nothing (fasted, naked, in a thermoneutral room, as you do). We humans are not exactly powerhouses: we only generate about 80 W, and about half of that is used to keep the brain and liver working.

There are two main ways RMR can be expressed: in absolute terms (as a certain number of kilojoules per day, or litres of oxygen consumed per minute) or in relative terms (kilojoules per kilogram of bodyweight per day, or litres of oxygen per kilogram per minute). Because the main thing driving RMR is the amount of fat-free mass we have, RMR is often expressed as kilojoules per kilogram of fat-free mass.

Other than fat-free mass, a lot of things can affect resting metabolic rate. In relative terms it decreases as we get older. Hormones such as thyroxin can turn up the thermostat.

The genetic lottery plays a role. After adjusting for age, sex and fat-free mass, about 40% of the variance in RMR can be put down to genes.

Now, two recent studies on RMR have really put the obesity cat among the metabolic pigeons. One study followed contestants on the reality show The Biggest Loser. The weight loss was indeed spectacular: from an average of 149 kg down to 91 kg. Alas, 6 years later, 41 kg of the 58 kg they had lost had been regained. Their percentage body fat had rollercoasted from 49% down to 28%, and back up to 45%.

This was not the worst of it. Before their weight loss, average RMR was 10,900 kJ/day. After weight loss it had gone down to 8400 kJ/day. This was expected: they lost not only fat but fat-free mass (muscle and organ mass) –, and fat-free mass is what eats up the calories.

The truly shocking part of this study was that 6 years after their weight loss, their RMR had gone down even further, to 7950 kJ/day, even though they had regained most of their weight and almost all of their fat-free mass. This sustained reduction in metabolic rate meant that they would have needed to walk an extra 90 minutes each day to make up for the lost energy use. Either that or forego five glasses of champagne a day. It’s a tough choice.

So what is going on here? Dieting has consistently been shown to reduce resting metabolic rate by up to 10% even when expressed as per kilogram of fat-free mass. The theory is that the body acts to defend its energy stores – that is, fat. When the energy gets low (or lower than what its used to), it turns down the furnace. In the feast-or-famine environment we probably evolved in, this makes a lot of sense.

The really surprising thing is how long the body takes to adapt to the new reality. Even after 6 years the home fires were burning on a low flame. The body has an elephantine metabolic memory.

The other study followed a group of children from age 7 to age 16, and each year measured, among other things, their resting metabolic rate. It was unsurprising to find that their relative (per kilogram) RMR fell as they got older – children’s furnaces are turned up very high and gradually subside as they enter adolescence.

But this study found that their absolute RMR also fell briefly between the ages of 11 and 15 – close to the time when they were growing at the fastest rate. This is truly surprising given that the kids get much bigger over this time, and their fat-free mass is increasing.

The study’s lead, Prof Terry Wilkin, suggests that this dip in RMR is also an evolutionary adaptation designed to spare energy when children are growing the fastest. In today’s world, however, it might spur a surge in fatness in adolescents – the dreaded “pre-pubertal fat wave”.

The alert reader will recall that a large part of the body’s resting energy goes to the brain. For parents of teenagers, this may explain an awful lot.

Is there anything that we can do to prevent this metabolic Catch 22? When we start to lose weight, it makes it even harder to lose weight. Losing weight through diet and exercise combined, or even using weight training (which helps to maintain fat-free mass), doesn’t appear to make much difference.

The best advice I can give is:

  • don’t get fat in the first place; and
  • don’t develop a liking for champagne.

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