- Key brain cells act as a trigger to prevent us burning calories when food is scarce
- Neurons in the hypothalamus can be switched on and off to control hunger
- They also burn or spare calories depending on the energy we have already
- ‘Dieting has little effect on its own over a long period,’ said one researcher
- The Cambridge University findings could lead to new treatments for obesity
Claudia Tanner For Mailonline
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There has been much debate over whether ‘starvation mode’ – the idea that the body slows metabolism and conserves energy when you follow a low calorie diet – is fact or fiction.
Now a study may help explain why restricting calories can be an inefficient way to lose weight.
Cambridge University scientists have discovered that key brain cells act as a trigger to prevent us burning calories when food is scarce.
‘Weight loss strategies are often inefficient because the body works like a thermostat and couples the amount of calories we burn to the amount of calories we eat,’ says lead author of the research Dr Clémence Blouet.
‘When we eat less, our body compensates and burns fewer calories, which makes losing weight harder.’
We have known that the brain must regulate this caloric thermostat, but how it adjusts calorie burning to the amount of food we’ve eaten has been ‘something of a mystery’, Dr Blouet said.
Scientists have explained why dieting has little effect on its own over a long period
The research identified a new mechanism in mice through which the body adapts to low caloric intake and limits weight loss.
Mice are are genetically and biologically similar to humans and so are considered a useful model for studying how our bodies work.
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The team tested the role of a region in the brain known as the hypothalamus, which controls hunger and satiety (fullness).
They used a genetic ‘trick’ to switch these neurons ‘on’ and ‘off’ in the mice.
The rodents had their temperature – which is representative of energy expenditure – in different contexts of food availability.
It was demonstrated that ‘agouti-related neuropeptide’ (AGRP) neurons are key players in the caloric thermostat that regulates our weight – and controlling how many calories we burn.
DR MOSLEY ON STARVATION MODE
Best-selling health author Dr Michael Mosley says ‘starvation mode’ is a myth.
‘One of the most popular dieting myths is ‘starvation mode’, the claim that if you stop eating, your metabolic rate immediately slows down as your body tries to conserve your fat stores,’ he says.
‘In a recent experiment, researchers took 11 healthy volunteers and asked them to stay in a metabolic chamber (a room where they precisely measure your metabolic rate – the rate at which the body burns energy while resting) living on nothing but water.
‘By day three their metabolic rates had risen by 14 per cent. This was probably due to a rise in a hormone called noradrenaline, which is known to burn fat.
‘If they had continued, I’m sure the volunteers’ metabolic rates would eventually have fallen, not least because they would have begun to lose significant amounts of weight and become malnourished.
‘But, in the short term, there is no evidence that starvation mode is anything other than a myth. ‘
When activated, these neurons make us hungry and drive us to eat – but when there is no food available, they act to spare energy, limiting the number of calories that we burn and hence our weight loss, the authors suggest.
As soon as food became available again and the mice started eating, the action of the AGRP neurons was interrupted.
This causes their energy expenditure to go back up again to normal levels.
These clever neurons also operate by detecting how much energy we have on-board, regulating their activity and controlling how many calories are burned, say the scientists.
‘Our findings suggest that a group of neurons in the brain coordinate appetite and energy expenditure, and can turn a switch on and off to burn or spare calories depending on what’s available in the environment,’ says Dr Blouet.
‘If food is available, they make us eat, and if food is scarce, they turn our body into saving mode and stop us from burning fat.
‘While this mechanism may have evolved to help us cope with famine, nowadays most people only encounter such a situation when they are deliberately dieting to lose weight.
‘Our work helps explain why for these people, dieting has little effect on its own over a long period. Our bodies compensate for the reduction in calories.’
The study could lead to future therapies aimed at reducing overeating and treating obesity.
The findings were published in the open access journal eLife.
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