From crash diets to drinking water, the bestselling author of the Fast Diet explains the truth behind the myths

On my first day at medical school a hundred of us gathered in a lecture theatre to be greeted by the dean. He talked for an hour but there are only two things he said that I still remember. The first was that, based on previous experience, four of us in that room would marry each other.

He was right; I met my future wife that day. The other thing he said was that while we would learn an enormous amount over the next five years, within ten years of graduating much of what we had learnt would be out of date.

Medicine is constantly changing and unless you keep up you are doomed to cling to outmoded ideas. This is particularly true in the field of human nutrition and dieting. So what are some of the most common and firmly held dieting myths?

Myth 1 Always eat breakfast

We are often told that eating a good breakfast is a simple way to control your weight. If you skip breakfast then you will get hungry later in the day and snack on high-calorie junk food. Eating breakfast revs up your metabolism, preparing you for the day.

There have certainly been plenty of studies that have compared people who skip breakfast with people who don’t and the breakfast-eaters are often found to be slimmer and healthier. This could be for the reasons stated above, or it could be that breakfast-skippers are generally less healthy individuals.

In a recent study published in the American Journal of Clinical Nutrition — “The effectiveness of breakfast recommendations on weight loss: a randomised controlled trial” — researchers tested the merits of the two claims by taking breakfast-skippers and breakfast-eaters, and making them swap habits. They got 300 overweight volunteers and asked the skippers to eat breakfast, while those who routinely ate breakfast were asked to skip the meal for the duration of the trial.

They weighed the volunteers beforehand and then at the end of 16 weeks. The skippers who had made themselves eat breakfast lost an average of 0.76kg. The eaters, who had spent 16 weeks skipping breakfast, lost an almost identical amount, an average of 0.71kg.

The researchers concluded that, contrary to what is widely believed, a recommendation to eat breakfast “had no discernable effect on weight loss in free-living adults who were attempting to lose weight”.

A similar randomised study done years ago, but with smaller numbers, came to a similar conclusion. The researchers thought that making people change their habits was what made the difference, and got better results.

I certainly think that children should eat breakfast, however — and if you want to keep fuller for longer then the evidence is clear that you should eat a breakfast that is rich in protein, like eggs, ham or fish, rather than sugary cereals or toast, as protein is more satiating than carbohydrates.

If you are one of those people who don’t like eating breakfast and who, perhaps, find that eating breakfast first thing makes you hungrier, then there seem to be no compelling scientific reasons to eat it.

Myth 2 Set moderate weight goals

This seems like a reasonable assumption. But is it right? A review article — “Myths, Presumptions and Facts about Obesity” in the prestigious medical journalThe New England Journal of Medicine — put this claim firmly into the “myths” category.

As they point out, “several studies have shown that more ambitious goals are sometimes associated with better weight-loss outcomes”. In one of those studies — “Weight loss goals and treatment outcomes among overweight men and women” — nearly 2,000 overweight men and women were asked about their goals before they started on a weight-loss programme. They followed them for two years and found that, with women, “less realistic goals were associated with greater weight loss at 24 months”. For men there was no link, one way or the other, between how realistic their goals were and whether they succeeded.

Myth 3 Crash diets are less successful than steady weight loss

This is another of those claims that seems to be self-evidently true but which the obesity researchers behind “Myths, Presumptions and Facts about Obesity” describe as a myth. Or as they put it, “Within weight-loss trials, more rapid and greater initial weight loss has been associated with lower body weight at the end of long-term follow-up.”

Very-low-calorie diets (VLCDs), based on consuming less than 800 calories a day, have been used since the 1970s to induce rapid weight loss, but the assumption is that once you stop you will simply put it all back on. In a thorough meta-analysis, “The evolution of very-low-calorie diets: an update ” (University of Pennsylvania School of Medicine), reviewers looked at the results of six randomised trials that had run for at least a year comparing very-low-calorie diets with standard
low-calorie diets.

They found that the VLCDs led, not surprisingly, to much bigger weight loss in the short term and though the dieters did, on average, later put back on much of the weight they had lost, so did those on the standard diet. In the long term there didn’t seem to be any significant difference between these approaches. The researchers confirmed that “cycles of weight loss and regain do not seem to have the adverse health and metabolic consequences once feared”.

Myth 4 It is better to eat several small meals a day for weight loss

A common belief is that if you spread out your food into lots of small meals this will increase your metabolic rate, keep you less hungry and help you lose weight. In a recent study researchers at the Institute for Clinical and Experimental Medicine in Prague decided to test this idea by feeding two groups of type 2 diabetics meals with the same number of calories but taken as either two or six meals a day. Each group ate about 1,700 calories a day.

The group eating two meals a day ate their first meal between 6am and 10am and their next meal between noon and 4pm. The others ate at regular intervals throughout the day.

Despite eating the same number of calories, the two-meals-a-day group lost, on average, 1.4kg more than the snackers and about 1.5in more from around their waists. The six-meals-a-day group felt less satisfied and hungrier than those on two meals.

Myth 5 Very low calorie eating swiftly slows your metabolism down

Fear of going into “starvation mode” is common and yet, at least from an evolutionary perspective, it makes little sense. Our remote ancestors often had to go without food for a while and if, every time this happened, they had simply curled up on the floor of their cave and waited for pizza to be delivered they would have become extinct. Only during periods of prolonged famine would it make sense to slow the metabolism down and wait for better times to come.

The myth seems to be based, in part, on the Minnesota starvation experiment, a study carried out during the Second World War in which young volunteers lived on extremely low-calorie diets for up to six months. After prolonged starvation there was a drop in body temperature and heart rate, suggesting that their basal metabolic rate (the energy burnt by your body when you are at rest) had fallen. This, however, was an extreme situation.

A more recent experiment carried out by the Department of Internal Medicine at the University of Vienna, on the effects of short-term calorie restriction, “Resting energy expenditure in short-term starvation”, produced very different results. In this experiment they took 11 healthy volunteers and asked them to live on nothing but water for 84 hours.

The researchers found that the volunteers’ basal metabolic rate went up while they were fasting. By day three it had risen, on average, by 14 per cent.

One reason for this may have been the significant rise that they detected in a catecholamine, called noradrenaline, which is known to burn fat.

If they had continued then, I’m sure, the volunteers’ metabolic rates would eventually have fallen. Yet, in the short term, there is no evidence that starvation mode is anything other than a myth.

Myth 6 Diets based on fasting will make you feel faint

In the trial I mentioned above they also measured the volunteers’ blood glucose levels. They found that their blood glucose levels did slowly fall over the three days, from 4.9mmols/l on day one to 3.5mmols/l by day four. These, however, are the sort of levels you might expect to see in a healthy individual who had their blood taken before breakfast. They are not, in any sense, abnormally low. At the same time the levels of fatty acids in their blood shot up, showing that their bodies had switched into major fat-burning mode.

Your body evolved to cope with periods without food. Modern humans, however, are used to eating lots of regular meals and there is evidence that the hunger hormone, ghrelin, rises simply in anticipation of a meal. Intermittent fasting can be tough, but there is no evidence that it will cause you to faint.

Myth 7 Juicing diets are a good way to lose weight

There are juice diets out there promising that you can lose “7lb in 7 days”, but are they credible?

Let’s start by looking at some of the numbers. There are about 3,500 calories in a pound of fat and the average woman consumes 14,000 calories a week (2,000 a day). So if you ate nothing at all for a week and lost all your weight as fat (which you won’t), then the absolute maximum amount of fat you could lose is 4lb.

So why do the scales sometimes drop far more than that? Well, excess glucose is stored in your muscles and liver in a form called glycogen. This also binds water. When you stop eating your body burns through the glycogen stores, releasing the water. After a week you may well have lost 7lb but much of that weight will be water, and also some muscle, since, on these diets you’re eating little or no protein. The body doesn’t store protein, so after 24 hours without it in your diet your body will start to cannibalise itself. Not surprisingly, once you start eating normally again your body will replenish its water and glycogen stores and much of the weight will come back on.

Myth 8 Two litres of water a day will help weight loss

This myth dates back to the 1940s, when researchers calculated that two litres was how much water someone’s body used in 24 hours. However, the researchers also said (and this gets ignored) that we obtain much of the water we need each day from our food. Drinks such as coffee and tea also count, despite what many people believe.

Drinking eight glasses of water a day could help you lose weight if you drink it ice cold, simply because you use a few calories to raise it to body temperature.

Water can help you to lose weight, however, if it is in the form of soup. If you drink water with your meal then the food will be kept in your stomach for digestive juices to do their bit, but the water passes straight through the stomach and into the intestines, where it is absorbed, and makes little difference to how hungry you feel a couple of hours later. However, Ultrasound and MRI studies have shown that if you take the chicken and vegetables and blend them with water then the stomach will stay fuller for longer, staving off hunger pangs.

Myth 9 Being overweight is unhealthy

The accepted wisdom is that you should aim for a body mass index (BMI) of 18.5-24.9 and that anything more than that is considered “overweight” and is associated with a shorter life.

In January 2013 a review in the Journal of the American Medical Association caused a stir because the scientists involved claimed to have found evidence, based on 97 studies involving 2.9 million people, that people with a BMI of between 25 and 30 were actually 6 per cent less likely to die than people considered to have a healthy BMI.

The criticisms were vigorous but a more recent study suggests they may have been on to something, at least if you are a bit older. An Australian research team from Deakin University in Melbourne reviewed a large number of studies that have looked at BMI and risk of death in older people. They found that if you are over 65 then it is better to be slightly outside the “normal” range. The risk of death was lowest if you had a BMI of about 27.5, which is currently considered “overweight” and those with a “healthy” BMI of 21 to 22 were actually 12 per cent more likely to die.

Carrying a little bit of weight may actually be protective, unless it is round the gut. Whatever your BMI you should aim to have a waist measurement (measured round the belly button) that is less than half your height.

Myth 10 Any exercise will lead to weight loss

It seems obvious. Do some exercise, burn some calories, lose weight. However, as Dr Stephen Boutcher, of the University of New South Wales, wrote in a recent review article, “Most exercise programmes designed for weight loss have focused on steady state exercise of around 30 minutes at a moderate intensity on most days of the week. Disappointingly, these kinds of exercise programmes have led to little or no loss.”

Part of the problem, as mentioned earlier, is that fat is very energy dense. You would need to run for about 36 miles to burn off a single pound of fat. The other problem is that people reward themselves for doing exercise by having a treat. If you run for a mile you will burn about 120 calories; if you then decide to eat a small bar of chocolate you will consume 240 calories.

An alternative approach, which I am a fan of, is to make your exercise regimens short and intense. I have written extensively about the science behind this in Fast Exercise, but I was pleased to see yet another study recently published that suggests intensity is the way to go if you want to burn fat.

In this study, “Effect of high and low intensity aerobic exercise on the body fat of overweight young men”, carried out in Bangalore, 80 men were randomly allocated to either 30 minutes a day of moderate intensity cycling or 15 minutes a day of moderate cycling interspersed with short, intense bursts. They did this five days a week for 12 weeks. At the end of three months, only those doing the high-intensity bursts had lost any fat.

That said, exercise in any form will almost certainly help you keep the weight off once you have lost it, and being fit will improve the quality of your life whatever your age. It’s been estimated that if you are a couch potato then doing 20 minutes of brisk activity a day will add about an hour to your life, which is a good investment.

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