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From cabbage soup to detox: dispelling the diet myths

Sarah Schenker
Nutrition Scientist British Nutrition Foundation

With the increase in overweight and obesity rates in the UK, it is increasingly important to be able to offer consistent, realistic advice to people wanting to lose weight.
Here are some of the most popular dieting myths:

  • Carbohydrates are fattening and should be ­eliminated from the diet.
  • The "detox your system" diets not only lead to weight loss but also banish cellulite and cleanse the body.
  • Particular foods or combinations of foods such as those found in cabbage soup can boost ­metabolism and speed up weight loss.
  • Poor digestion of foods or "allergy" to foods can cause weight gain.
  • Protein-containing foods and carbohydrate-­containing foods should not be eaten at the same meal.
  • Your blood group or any other physiological ­characteristics can dictate which foods you should and shouldn't eat.

Diet myth no. 1 - the evils of carbohydrates
Carbohydrates have become the villains of the nutrition world, as Hollywood celebrities banish them from their diets in favour of more protein and fat. The diets are high in protein and very low in carbohydrate - you are encouraged to eat all the meat, fish, eggs and cheese you want but have to cut out potatoes, bread, pasta and fruit. Even the slimmer's favourite energy snack, the banana, is banned. The theory goes that it is sugar and not fat in the diet that makes a person fat. Sugar is rapidly absorbed into the bloodstream, causing blood glucose levels to increase. Insulin is then required to bring levels back to normal. Insulin also happens to be the hormone responsible for promoting fat deposition. So far so true, but irrelevant to dieters. By taking in less energy (calories) than are being used up, fat stores will be used as a source of energy. Cutting out starchy carbohydrates and fruit leads to the diet becoming unbalanced, with poor intakes of fibre, B vitamins and antioxidants, as well as iron and calcium from fortified flour products (it is compulsory in the UK to fortify white flour with niacin, thiamine, iron and calcium). Not only do low-carbohydrate diets go against all current healthy eating advice, but also, if taken to extremes, can lead to ketosis. This is a potentially dangerous state for the body to be in as the pH of the blood can drop to a dangerous level, leading to unconsciousness and eventually coma. Furthermore, anyone trying to exercise while following this diet will find it a real problem: stored carbohydrates are the best source of energy for exercise. This diet is likely to make the person feel tired, lethargic and irritable.

Diet myth no. 2 - the detox
Many celebrities embark on detoxing diets that exclude wheat, dairy products, sugar, caffeine and alcohol in the belief that this regimen will cleanse their blood, flush out toxins, aid weight loss and get rid of cellulite. Cellulite, as ugly and horrid as it might appear, is not a unique type of fat that requires special treatment, and it is not caused by toxins - it is a build-up of fat in areas where people are predisposed to lay down fat. For women this tends to be the hips, thighs and bottom. It may seem unfair that when a person diets they lose weight from all the places they don't want to lose it from, but unfortunately nothing can change your genetic makeup. Regular exercise and healthy, balanced eating will eventually lead to weight loss, but a few unsightly wobbly bits may still remain. Exercises targeted at specific muscles groups in problem areas are the best remedy.

Diet myth no. 3 - cabbage soup
Soup diets promise a miraculous instant weight loss of up to 10lb in the first week and attribute this result to the special combination of ingredients. Very often when people start to diet, they find that they can lose a relatively large amount of weight in the first week. Then the weight loss slows down and they start to feel demoralised, and often this is when they give up, only to repeat the cycle a few weeks later. There is no magic, rapid way that fat stored in our bodies can be lost. During the first week of a diet, especially an extreme diet such as fasting, people usually lose carbohydrate stored as glycogen and the water that has been absorbed with it. We have approximately 500g of stored glycogen (that will last a sedentary person about 3-4 days), and each gram is stored with 3g of water, thus when this is lost there is a dramatic effect on the scales. Once this is lost, and if it is not replaced, fat loss will dominate, but fat being much more energy dense than carbohydrate (9kcal/g versus 3.75kcal/g) it requires a greater energy deficit to be used up. It is a simple fact that to lose weight a person needs to take in less energy than they use up throughout the day. Even a daily deficit of 50 calories will make a difference over time. Potential slimmers should be prepared for slow, steady weight loss, achieved through an increase in activity and by making small changes to their diet that they can live with in the long term.
There are no special combinations of foods that will affect the metabolism one way or the other. Overweight people do not have slow metabolisms - in fact, quite the opposite. The bigger a person is, the harder the body has to work to pump blood around the body and carry the large frame when they move, so the higher the metabolism and the energy requirement. This means that a very large person does not have to drop to an impossible 1,000 calories per day to lose weight - they could easily lose weight on 1,500-1,800 calories per day if they are using up about 2,500. As they become lighter their calorie requirement will drop, but this can be compensated by increasing the exercise level and lowering intake to maintain the deficit. The weight loss will be slow but the weight will stay off.

Diet myth no. 4 - food intolerance
People suffering from food allergies or food intolerances do not gain weight by eating the culprit foods - in fact, quite the reverse. Food intolerances usually cause sickness and diarrhoea. Food allergies can have very serious consequences resulting in anaphylactic shock; milder reactions include asthma and eczema. None of this is pleasant, but it is hardly implicated in weight gain.

Diet myth no. 5 - food combining
Dr William Hay invented "food combining" at the beginning of the last century. He believed that disease resulted from the accumulation of toxins and acid waste in our bodies. The way to cure disease was to avoid eating "foods that fight". He advocated not ­mixing proteins and carbohydrates in the same meal, and eating foods that restore the body's natural balance between acids and alkalis. To date there is no scientific evidence to support this theory. The rules of food combining are rather complicated to observe and inherently contradictory. It is possible to lose weight observing the regimen, but this is because it greatly increases the intake of fruit and vegetables at the expense of more calorific foods. So while it is not harmful, people should not be fooled into thinking that food combining per se offers any great nutritional benefit.

Diet myth no. 6 - blood group diets
Diets based on the idea that a person's physiological characteristics, such as eye colour, hair type or blood group, can dictate which foods are likely to cause weight gain have no scientific basis. The idea behind blood group diets is to split foods into groups of "highly beneficial", "neutral" and "avoid", depending on type. For instance, people with blood group O are told to avoid sweetcorn as this will cause weight gain, whereas it acceptable for those with a different blood type; people with blood group A are told kidney beans will encourage weight gain. Both sweetcorn and kidney beans are low in fat and high in fibre, so it is difficult to understand why this should be the case. A person may well lose weight on this diet because it is so restrictive, but in being so it is also likely to make the diet unbalanced. Daily consumption of fruit and vegetables of all types should be encouraged to help prevent diseases such as heart disease and cancer.

Last year the British Dietetic Association (BDA) called for a halt to the continuous stream of celebrity-endorsed diets and health claims that do not stand up to scientific scrutiny. The BDA also made the point that food supplements were foods, not drugs, and as such remain under the auspices of food law and not the Medicines Act, despite the pseudomedicinal claims made for their usage. The British Nutrition Foundation shares these sentiments and agrees that, despite the hype, key advice on healthy eating has changed very little over the past few decades. The best way to maintain a healthy weight is to eat a diet that includes a wide variety of foods with plenty of fruit and vegetables, modest amounts of fat and alcohol, ­combined with regular activity - and that is a fact!

British Dietetic Association
British Nutrition Foundation
The Nutrition Society