Scientific evidence increasingly points to a far deeper problem that confronts dieters: cutting out calories changes your metabolism and brain, so your body hoards fat and your mind magnifies food cravings into an obsession.
Slimmers have often feared this was somehow true, but now science confirms this cruel fact of nature. New research shows dieting raises levels of hormones that stimulate appetite – and lowers levels of hormones that suppress it.
Meanwhile, brain scans reveal that weight loss makes it harder for us to exercise self-control and resist tempting food. Worse still, the more people diet, the stronger these effects can become, leaving some almost doomed to being overweight as a result of their attempts to become slim.
And as research lays bare the dangers of yo-yoing weight, some experts argue it would be better not to diet at all.
Researchers, including Joseph Proietto, a professor of medicine at the University of Melbourne, have uncovered one of the main possible reasons. Two years ago, his team recruited 50 obese men and women, and coached them through eight weeks of an extreme 500-to-550-calories-a-day diet (a quarter of the normal intake for women).
At the end, the dieters lost an average of 30 lb. Joseph Proietto’s team then spent a year giving them counseling support to stick to healthy eating habits. But during this time, the dieters regained an average of 11 lb. They also reported feeling far hungrier and more preoccupied with food than before losing weight.
As the researchers reported in The New England Journal of Medicine, the volunteers’ hormones were working overtime, making them react as though they were starving and in need of weight-gain. Their levels of an appetite-stimulating hormone, ghrelin, were about 20% higher than at the start of the study. Meanwhile their levels of an appetite suppressing hormone, peptide YY, were unusually low.
Furthermore, levels of leptin, a hormone that suppresses hunger and raises the metabolic rate, also remained lower than expected.
Joseph Proietto describes this effect as “a co-ordinated defense mechanism with multiple components all directed toward making us put on weight”. In other words, the body had launched a backlash against dieting.
The team’s landmark study reinforces a belief among biologists that the human body has been shaped by millennia of evolution to survive long periods of starvation.
The human frame contains around ten times more fat-storing cells in relation to its body weight than most animals (polar bears, which have to endure long stretches when prey is unavailable, are similarly fat-rich).
Our calorie-hoarding frames have strong mechanisms to stop weight loss, but weak systems for preventing weight gain. If you manage to lose 10% of your weight, your body thinks there’s an emergency. So it burns less fuel by slowing your metabolism.
The body learns to function on fewer calories, resetting your metabolism. The problem is if you then stop dieting and start eating more again, those extra calories are stored as fat.
This effect kicks in after around eight weeks of dieting – and can last for years. Studies by Columbia University show this metabolic slowdown can mean that just to maintain a stable weight, people must eat around 400 fewer calories a day post-diet than before dieting.
Why would this be so? Muscle samples taken before and after weight loss show that once a person drops weight, the fibres may change to become more fuel-efficient – burning up to a quarter fewer calories during exercise than those of a person at the same weight naturally.
How long this state lasts isn’t known, though some research suggests it might be up to six years.
It’s also thought the brain changes in the way it reacts to food. This wilts our willpower, according to Michael Rosenbaum, a researcher at Columbia University Medical Centre who studies the body’s response to weight loss.
“After you’ve lost weight, there’s an increase in the emotional response to food,” he says, adding that there is also “a decrease in the activity of brain systems that might be more involved in restraint”.
In 2010, Michael Rosenbaum and his colleague, Joy Hirsch, a neuroscientist at Columbia University Medical Centre, scanned the brains of people before and after weight loss while they looked at objects such as grapes, chocolate, broccoli and mobile phones.
After losing weight, the scans showed a greater response in the areas associated with reward and a lower response in those associated with self-control.
And last year, scientists at the Albert Einstein College of Medicine in New York discovered that when starved of food, brain cells actually consume each other. This causes the release of fats, which in turn results in higher levels of a powerful brain chemical that stimulates appetite, the journal Cell Metabolism reports. All bad news for dieters, as going without food could make them even hungrier.
All of this helps to explain why an analysis of 31 long-term clinical studies found that diets don’t work in the long run. Within five years about two-thirds of dieters put back the weight – and more. The researchers from the University of California found that dieting works in the short term, with slimmers losing up to 10% of their weight on any number of diets in the first six months of any regimen. But after this, the weight returns, and often more is added, says their report in the journal American Psychologist.
The analysis concluded that most volunteers would have been better off not dieting. Their weight would be pretty much the same and their bodies would not have wear and tear from yo-yoing.
This backfire effect is worst among teenagers: people who start habitually dieting young tend to be significantly heavier after five years than teens who never dieted. This mix of biology and psychology translates into a sobering reality: once we become overweight, most of us will probably remain that way.
Certainly, we should all be worried about what dieting does to our health. Restricting calories may increase the risk of heart disease, diabetes and cancer, according to a study from 2010 in the journal Psychosomatic Medicine.
Ultimately, of course, we should be more wary of piling on the pounds, than relying on diets to reverse the damage.