In a ‘cruel twist’ for dangerously fat people, a new study has found that exercising when obese reduces the number of calories burnt while sitting still.
This means that for someone who’s obese, losing weight through physical exercise is likely to be substantially harder than it is for a lean person.
People who exercise burn fewer calories on ‘body maintenance’ – the ongoing processes in the body that happen even when we are fully at rest, experts say.
The thing is, this unfortunate metabolic phenomenon adversely impacts obese people the most.
Essentially, it means people living with obesity reduce their resting metabolism – the total number of calories burned when the body is completely at rest – the more active they are.
You may have to work extra hard to lose weight if you’re obese, a new study finds. This is because obese bodies are particularly good at holding on to fat stores
The result is that for every calorie they spend on exercise they save about half a calorie on resting – a cruel twist for obese individuals.
The study was conducted by researchers from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences and the University of Roehampton in London.
Study author Professor Lewis Halsey from the University of Roehampton explained body maintenance, which he likened to a stationary car that has the engine still running.
‘In this state, the body is still burning energy on functions such as protein synthesis, the immune system, potassium-sodium pumps in our cells, kidney function, the heart beating, the brain thinking, ventilation,’ he told MailOnline.
‘These functions don’t happen only at rest but the energy expenditure of body maintenance can be measured at rest because by definition no other processes are happening at that time.’
For the study, the team investigated the effects of activity on energy expenditure and how these effects differ between individuals.
They performed an analysis based on data from 1,750 adults in the IAEA’s doubly labelled water database – a free-to-use collection of more than 6,500 measurements of daily energy expenditure from people of all sizes living globally.
‘When enrolled into exercise programs for weight loss, most people lose a little weight,’ said study author Professor John Speakman at SIAT.
‘Some individuals lose lots, but a few unlucky individuals actually gain weight.
‘This analysis using data from the DLW database shows how individuals are not all the same in the way they budget their energy use.
‘People living with obesity may be particularly efficient at hanging onto their fat stores, making weight loss difficult.
Researchers found that the reduction in energy burned at rest was most pronounced in individuals with obesity and also – to a lesser extent – in older adults.
In individuals with the highest body mass index (BMI), 51 per cent of the calories burned during activity translated into calories burned at the end of the day.
For those with normal BMI, however, 72 per cent of calories burned during activity were reflected in total expenditure.
Professor Halsey explains: ‘If you do exercise on a given day and burn 300 kilocalories (kcal), you might expect that at the end of that day the total calories you’ve burned are 300 kcal more than you would have otherwise have burned.
‘However, over the long term, the body reduces the calories burned on other processes to partially compensate for those burned on activity.
‘So the 300 kcal burned on exercise doesn’t “translate” into an additional 300 kcal burned that day, but perhaps only 200 additional kcal burned that day, with 100 kcal less having been burned on other processes in compensation.’
The observed discrepancy between those with high and low BMI is probably because of what are called compensatory mechanisms.
Both obese people and older people have these compensatory mechanisms, researchers say.
These mechanisms include eating more food because exercise stimulates our appetite, or reducing our expenditure on other components like our resting metabolism, so that the exercise is in effect less costly.
The upshot is that losing weight by increasing activity is likely to be substantially harder than for a lean person, whose compensation is much less and whose need to lose weight is much lower.
‘Around the world, national guidelines tend to recommend a 500–600 calorie deficit through exercising and dieting to lose weight,’ said Professor Halsey.
Being overweight or obese results in about 2.8 million deaths a year, according to the World Health Organisation (WHO). Pictured, an obese woman
‘However, they do not take into account the reduction of calories being burned in the most basic of human functions as the body compensates for the calories burned on the exercise.’
Recent studies revealed 1.9 billion adults worldwide are overweight (with a BMI of between 25 and 29.9) and 650 million are obese (with a BMI of 30 or more).
Being overweight or obese results in about 2.8 million deaths a year, according to the World Health Organisation (WHO).
It is also estimated about 57 per cent of the world’s adult population will be overweight or obese by 2030.
This new study has been published in the journal Current Biology.