- The brain tunes the body to store more fat thinking that the body is in famine when a person is on special diets.
- People who don't follow weight-loss diets are less likely to store excess body fat.
- Over eating and weight gain is often seen in those who stop following a low calorie diet.
When you are on repeated dieting, the brain interprets the diet as short famines and urges the body to store more fat for future shortages, leading to weight gain.
This may be the reason for over eating high calorie foods in people who try low-calorie diets when not dieting and so don't keep the weight off.
By contrast, people who don't diet will learn that food supplies are reliable and they do not need to store so much fat.
‘Regular exercise and slightly reducing the usual quantity of food is a safer way to reduce weight gradually as it does not involve food craving and overeating.’
Today, people can get into a vicious cycle of weight gain and ever more severe diets - so-called yo-yo dieting - which only convinces the brain it must store ever more fat. Humans evolved in a world where food was sometimes plentiful and sometimes scarce - and in the latter case those with more fat would be more likely to survive. With more and more people becoming obese, scientists are looking for evolutionary reasons to explain why many find it hard to resist overeating.
The study, published in the journal Evolution, Medicine and Public Health, is based on observations of animals such as birds.
Animals respond to the risk of food shortage by gaining weight, which is why garden birds are fatter in the winter when seeds and insects are hard to find.
The authors studied a mathematical model of an animal that knows whether food is currently abundant or limited, but does not know when things will change.
The model shows that during dieting or if food supply is often restricted an optimal animal - the one with the best chance of passing on its genes - should gain excess weight between food shortages.
Dr Andrew Higginson, Senior Lecturer in psychology at the University of Exeter, says: "Surprisingly, our model predicts that the average weight gain for dieters will actually be greater than those who never diet. This happens because non-dieters learn that the food supply is reliable so there is less need for the insurance of fat stores."
The researchers' model predicts that the as a diet goes on, the urge to eat increases hugely and this urge won't diminish as weight is gained because the brain gets convinced that famines are likely.
"Our simple model shows that weight gain does not mean that people's physiology is malfunctioning or that they are being overwhelmed by unnaturally sweet tastes," says Professor John McNamara, of the University of Bristol's School of Mathematics.
Uncertainty about the food supply triggers the evolved response to gain weight though the brain could be functioning perfectly.
"The best thing for weight loss is to take it steady. Our work suggests that eating only slightly less than you should, all the time, and doing physical exercise is much more likely to help you reach a healthy weight than going on low-calorie diets," Dr Higginson says.
- Dr Andrew Higginson et al. Yo-yo dieting might cause extra weight gain, Evolution, Medicine and Public Health (2016)