What determines the speed of your metabolism?

The idea of a ‘fast’ or ‘slow’ metabolism is a myth, says Rick Miller, principal independent dietitian at King Edward VII’s Hospital, London. ‘Unless you have an overactive or underactive thyroid, BMR is quite consistent between people with similar muscle mass.’

What you burn off and store is mostly down to daily activity, he says. ‘A lot of people who are naturally quite slim are active all the time – eg constantly moving and fidgeting; this lack of sedentary behavior helps to increase the energy burn,’ says Miller. This is known as non-exercise activity thermogenesis (NEAT); it includes anything from walking to work and gardening to typing and fidgeting. While BMR is quite fixed between people of similar builds, NEAT can vary by as much as 2,000kcals a day in people of similar sizes. It accounts for between six and 10 per cent of daily energy expenditure in those with a mainly sedentary lifestyle and up to 50 per cent in highly active subjects.