The science of fighting fat

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We’re big fans of the BBC’s science offerings – especially when it’s about weight loss. This month our pals at BBC Focus magazine, the BBC’s science and technology mag, have very kindly shared this fascinating article with us all about the science of losing weight…


Has Christmas taken its toll on your waistline? We navigate through the minefield of misinformation to find out what the experts really say about losing weight.

The statistics tell their own story. One in four people in England are now classified as obese, compared with one in six in the 1990s. Fifty-eight percent of women and 68 percent of men are now overweight. Being overweight makes us less healthy: a new study published in Lancet Public Health shows a clear relationship between hospital admissions and body weight. But it also matters because being overweight makes many people unhappy.

A British Social Attitudes survey revealed that people who are overweight suffer significant stigma, and that 53 percent of the British public are intolerant, believing that most overweight people could lose weight if they tried. But the science shows that it’s not simply a matter of being weak-willed.

“There are very clear reward pathways for food in the brain, and so if something is rewarding and constantly available, why wouldn’t you?” explains Prof Susan Jebb, a nutrition scientist at Oxford University. “In busy and stressed lives, you have to make a constant conscious effort to say no.” Fortunately, science is now providing some answers on weight control. Just a decade ago, there weren’t enough scientific diet trials to allow doctors and dietitians to provide evidence-backed advice. Now, there are clear scientific pointers on how to fight fat, and what the studies find may surprise you.


When it comes down to it, the science of dieting is simple: eat less. You can do it with a low-fat diet (like the raw food diet), or a low-carb diet (like the Atkins or paleo diet). But the problem with diets is not so much losing weight, but finding a way to do it that is effective, safe, fits in with your lifestyle, and is sustainable so that your weight doesn’t rocket up again.

Diet academics (as opposed to product pushers) avoid prescriptive advice because different diets fit different people’s lifestyles and personalities. But recent research indicates that one particular group of diets is most effective for the greatest number of people. These are the supervised diet programmes, like the Cambridge Weight Plan, LighterLife and Optifast diets, consisting entirely of pre-prepared snack bars, shakes and other food products. You might assume these fast-acting diets would be condemned by scientists as drastic, unhealthy and gimmicky. Yet research is finding that these very low-calorie diets, also known as total food replacement diets, are effective and safe if applied correctly.

A major analysis of trials last year, headed by Birmingham University’s Centre for Obesity Research, showed that these diets brought an average weight loss of 10kg after 12 months. This compares with research showing that behavioural programmes (focused on changing eating habits and exercising), such as Slimming World and Weight Watchers, bring a weight loss of 4kg after one year.

Jebb explains that, although research indicates that all dieters gain weight afterwards (no matter which regime you embark on), the more weight you lose the longer you stay beneath the ‘obese and unhealthy’ bar. “The research shows that the food replacement programmes which provide weekly or monthly behavioural support are associated with the best long-term success,” she says.

And though food replacement diets may look extreme, they do contain a balance of nutrients that some do-it- yourself diets – for example, the milk diet or the lemonade diet – might not.

“Food replacement diets are easy, and if you want to lose weight, why not do it quickly? What’s not to like?” Jebb says.

Verdict: Try a supervised diet programme to safely shed excess weight.


It depends what you mean by ‘crash diet’. There is evidence that supervised food replacement diets work very well for many people. But what about the more DIY crash diets that claim to make your weight plummet? Diets like the cabbage soup diet, the grapefruit diet, and juicing and cleansing diets?

The evidence behind these is currently slim. However, there is less scientific opposition to losing weight quickly than there used to be. Australian research has indicated not only that more people achieve their weight loss goals if they lose weight fast, but also that losing weight quickly doesn’t mean you’ll regain it quickly as well. Rapid weight loss can motivate people to stick with some programmes, the researchers suggest.

But maintaining a healthy nutritional balance while on these diets can be a problem: advice from the NHS is still that “crash diets make you feel very unwell and unable to function properly… crash diets can lead to long-term poor health”.

And both our biology and lifestyles may condemn many extreme crash diets to failure. Dr Giles Yeo, principal research associate at Cambridge University’s Institute of Metabolic Science, specialises in the molecular mechanisms underlying the control of food intake and body weight.

“If you want to try and sustain your weight loss, the worst thing you can do is try and starve yourself for three weeks,” he says. “Rather than taking a huge pendulum swing that will inevitably swing back in the other direction, I think people have to find some balance to lose weight long-term.”

In particular, we have to address the fact that crash diets generally make us feel hungry. Yeo’s research examines how the brain responds to hormones and nutrients that are released from the gut into the blood. These reflect the body’s nutritional status, and the brain turns them into what we experience as ‘fullness’ or ‘hunger’.

“One of the universal truths of weight loss is that if you want to eat less then you have to have a strategy to make you feel more full, otherwise you are simply fighting hunger for the rest of your life,” Yeo says. “What we now know is that the longer something takes to be digested, the fuller it makes you feel – because as food goes down the gut, different hormones keep being released, most of which give us a feeling of fullness. That’s why high-protein diets can work, because protein is more complex than fat or carbohydrate and goes further down the gut before it’s broken into its constituents.”

Look out for Dr Giles Yeo in the new series of Trust Me, I’m A Doctor, starting on BBC Two this January.

Verdict: Crash diets are not nutritionally balanced and will make you feel awful.

The science of fighting fat


Intermittent fasting diets – for example, the Fast Diet and 5:2 diet – revolve around eating what you want some days a week, and then eating very little on the other days. They have become popular over the past five years. But are they more effective than other weight loss diets? The latest research suggests not.

A study published in an American Medical Association journal in 2017 found that, after a year, weight loss was not significantly different than for daily calorie-restricted diet groups.

Supporters of fasting diets claim they provide health benefits beyond weight loss. Indeed, animal studies have indicated that fasting prolongs life and reduces the risk of diabetes, cancer, heart disease and Alzheimer’s disease. But human studies are scarce and contradictory.

A University of Southern California study of 71 adults published recently found that intermittent fasting reduced blood pressure and risk factors for cardiovascular disease, cancer and diabetes, and reduced body fat too. But another new study, from the University of Illinois, suggests it improves cardiovascular risk no more than any other diet.

What is undoubtedly true is that intermittent fasting diets suit many people because they don’t disrupt lifestyles or family meals too much. “It’s not particularly dangerous because you’re essentially not changing what you’re eating on most days, yet over a week you end up eating less,” says Yeo. “They are very effective for some people.”

Verdict: Intermittent fasting is no more effective than other calorie-restricted diets, but it works for many people as it isn’t too disruptive.


For decades, scientific debate has raged about the role of exercise in weight loss. Today, there is greater scientific consensus that food intake is more important than exercise for losing weight. But the debate goes on about whether being fit mitigates the health risks of being overweight.

Central to the controversy is research from the Cooper Institute for Preventive Medicine in Dallas, which shows that over-60s who exercise have lower mortality regardless of how much body weight they carry. American health psychologist Dr Traci Mann from the University of Minnesota is currently the most prominent figure in asserting that overweight people can live healthy lives as long as they exercise.

She says there is no evidence that overweight people have shorter lifespans, there is just evidence that people who are sedentary, poor and medically neglected (who are also often obese) live shorter lives. “Obesity only really leads to shorter lifespans at the very highest weights,” she says.

There is no point in dieting, she claims. “To reduce your risk for cardiovascular disease and diabetes, you don’t actually have to get thin, you just have to exercise.”

But the ‘fat but fit’ camp has few supporters in the UK, and the theory has received a new setback from a recent study of 3.5 million GP records by the University of Birmingham. This found that ‘healthy’ obese people, who had normal blood pressure and cholesterol levels, were still at higher risk of serious disease than healthy people of normal weight. The obese people had 49 percent increased risk of coronary heart disease, 7 percent increased risk of stroke, and 96 percent increased risk of heart failure.

Verdict: Obese people with healthy blood pressure and cholesterol still have an increased risk of heart problems and strokes.


The past five years have seen interest in the idea that our gut bacteria play a crucial role in regulating weight, and killing them off with antibiotics is causing obesity.

The most recent evidence is fascinating but inconclusive. Studies in prestigious medical journals have produced contrasting results. One found that three courses of antibiotics before the age of two was associated with increased risk of early childhood obesity, while the other found that exposure to antibiotics in the first six months of life was not associated with early childhood weight gain.

Yet recent research is indicating a link between gut fauna and our body mass index. People with higher levels of Christensenellaceae bacteria – one in 10 of us – appear less likely to put on weight than those with lower amounts. Scientists from King’s College London have found that levels of this bacteria are partly genetically determined.

According to Yeo, who investigated the possibility of microbial transplants to cure obesity for a BBC programme, this new field is important and requires research. “But I have yet to see convincing evidence that there are lean bacteria and obese bacteria,” he says.

Verdict: More research needs to be done, but our gut bacteria may affect how readily we put on weight.


Dozens of ‘metabolism-boosting’ supplements – including ingredients such as caffeine, capsaicin, L-carnitine and green tea extract – claim to stimulate energy processing in the body, increasing the rate at which we burn calories. But there’s little evidence that these products work, and most of their claims are not subject to scientific scrutiny because they are classed as food supplements rather than medicines.

Some studies have indicated that people burn more calories when they take caffeine but, according to the Mayo Clinic, this doesn’t appear to have any significant effect on weight loss. There is little data on most other ‘fat-busting’ pill ingredients, although there is some evidence from small studies that capsaicin, which is found naturally in chillies, can promote loss of abdominal fat and make people feel fuller.

There is a constant stream of news stories about food types that can apparently provide a shortcut to weight loss by boosting metabolism, reducing fat levels or promoting healthy gut bacteria. Cayenne pepper, apples, cider vinegar and cinnamon have all been in the news recently. The problem is that most of these stories are based on small or isolated studies, often in rodents not humans. There may be something in them, but it’s still very early days.

Verdict: There is no easy fix for burning fat. Sorry!

Thanks to BBC Focus magazine for letting us republish their article – you can find it in the newest issue, which is out today.

What do you think of these findings? Was there anything that surprised you? Let us know over on the forum



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