DO CRASH DIETS WORK?

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’. You can find more advice for feeling fuller for longer at the end of this feature.

“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.”

DO INTERMITTENT FASTING DIETS WORK?
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.”

CAN YOU BE FAT AND FIT?
For decades, scientific debate has raged about the role of exercise in diet 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 ed ca y neglected (who are also oftften 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 per cent increased risk of coronary heart disease, 7 per cent increased risk of stroke, and 96 per cent increased risk of heart failure.

ARE ANTIBIOTICS MAKING US FAT?
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.

DO FAT-BURNING TABLETS WORK?
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.