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Do diets work?

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VOL: 97, ISSUE: 44, PAGE NO: 46

Lesley Milne is dietitian, Obesity Research Centre, Luton and Dunstable Hospital

Despite the array of diets and weight loss products on the market the incidence of obesity is rising. In the UK between 1980 and 1996 the prevalence of obesity (defined as a body mass index greater than 30kg/m2) more than doubled. Its incidence in men has increased from 6% to 16%, and in women from 8% to 17.3% (Department of Health, 1993).

Despite the array of diets and weight loss products on the market the incidence of obesity is rising. In the UK between 1980 and 1996 the prevalence of obesity (defined as a body mass index greater than 30kg/m2) more than doubled. Its incidence in men has increased from 6% to 16%, and in women from 8% to 17.3% (Department of Health, 1993).

Several factors are thought to act synergistically to promote weight gain. For some it may be due to a sedentary lifestyle, while others may have an excessive energy intake. Genetic factors may also contribute, as seen by the familial propensity to obesity. There are serious health concerns associated with obesity. These include an increased risk of heart disease, stroke and diabetes.

With the many pressures to reduce weight it is perhaps understandable that the promise of weight loss through more unusual dietary manipulation appeals. The vast array of books, magazine articles and TV advertising illustrates the interest in the subject and how financially lucrative the weight loss industry is.

Unfortunately, there are no 'miracle foods' that will lead to weight loss. There is a simple equation underlying all successful weight loss plans: if energy output (activity) exceeds energy input (food) then weight loss will occur. Various factors influence an individual's requirements, including age, sex and activity levels, but if input equals output weight will be stable; if it exceeds output, weight will be gained.

To achieve and maintain weight loss, long-term lifestyle and dietary changes must be encouraged. Realistic weight loss targets (usually one to two pounds per week), healthy eating and the benefits of exercise are frequently overlooked in short-term diet plans. It is also vital to change long-term eating patterns if weight loss is to be successfully maintained.

The pattern usually seen among dieters is initial weight loss followed by weight gain, as original eating patterns are re-established - frequently the amount of weight regained exceeds the amount originally lost. Body composition is also altered, with increased fat mass and decreased lean body mass, a change that can be detrimental to overall health.

Very low-fat diets
Fat contains nine calories per gram, making it the most energy-dense food of all groups. Government guidelines advise that fat should not provide more than 35% of total daily energy. However, some low-fat plans (Conley, 2000) are much stricter, advising fat should not provide more than 25% of total daily energy.

But what does that mean in real terms? If the estimated average requirements (EAR) or total daily recommended energy intake is 2,550 calories for men (aged under 60) and 1,940 calories for women (aged under 50), 25% of this equates to 638 and 485 calories respectively. This, in turn, means daily fat intake should not exceed 71g (men) and 54g (women), representing a very restricted intake that allows only the leanest poultry, meat or fish and the lowest-fat dairy foods. The very restrictive nature of this type of diet may affect long-term maintenance of weight loss.

Do they work?

Due to its calorific density, reducing fat quantities in the diet will lower overall energy intake, and if this is below the body's energy requirements it will lead to weight loss. However, because fat provides essential fatty acids and fat-soluble vitamins, fat-free diets, or those allowing intakes below 1,200 calories a day, are not recommended.

Reduced fat diets
Again these diets are based on the fact that fat is the most energy-dense food group. Reduced fat diets recommend avoiding high-fat foods, ensuring that fat does not provide more than 35% of total daily energy.

Do they work?

This type of diet provides a recognised way to reduce overall energy intake. Providing the diet is balanced, containing foods from all the groups, it is considered the preferred way to help reduce and maintain weight loss.

Low carbohydrate diets
This type of diet claims to change the way the body deals with food. It claims that one of the contributory factors leading to weight gain is higher than normal circulating insulin levels, a hormone that can lead to energy storage. Restricting carbohydrate intake can reduce that insulin level (McCullough, 1997).

Protein foods, fruit and vegetables are recommended in place of carbohydrate food (Atkins, 1999; Eades and Eades, 1997). However, low carbohydrate diets do not follow healthy eating guidelines. It has been shown in numerous research papers that, regardless of how much insulin is circulating, the body will not store energy unless overall intake exceeds requirements. Again the underlying principle is that weight loss and maintenance can be achieved only by ensuring energy output exceeds intake.

There is no scientific proof that weight loss can be achieved by restricting carbohydrate intake. In addition, there is much evidence indicating that obesity causes excessive insulin production, rather than carbohydrate intake, resulting in increased insulin levels and leading to weight gain. Any weight loss achieved by following this diet will be due to decreased overall energy intake.

Do they work?

Diets providing less than 50-100g of carbohydrate per day can lead to gluconeogenesis, causing the body to use fat and protein stores as its main energy source rather than carbohydrate. This leads to the production of ketones, high levels of which can cause ketosis. Mild symptoms include nausea and halitosis. Low carbohydrate diets have a tendency to be low in fibre, leading to constipation.

Food combining
This dietary plan, also known as the Hay diet, claims to promote health and assist in regaining optimal body weight. It is promoted in numerous dietary books (Habgood, 1997; Grant, 1993).

Food combining advises that particular food groups should not be eaten at the same time. In particular, a meal should not contain both proteins, such as meat, fish, cheese or eggs, and carbohydrate, such as bread, rice, pasta or potatoes. It encourages the consumption of food only from whole sources, with no refined starches or sugars and no additives. Vegetables and fruit may be eaten with either food type.

Do they work?

There is little scientific evidence to support food combining, but this does not seem to affect its popularity. Many individuals report health benefits and weight loss when food combining, but this is likely to be because avoiding refined starches and sugars significantly reduces overall energy intake.

Cabbage soup diets
These diets continue to attract interest (Danbrot, 1999). There have been numerous, different versions of this diet, all of which are based on following a restrictive, very low-fat food intake for up to a week. Food intake is limited to a vegetable-based soup which, as the name suggests, contains cabbage, although there are various, slightly different recipes around.

Do they work?

Following a cabbage soup-type of eating plan will lead to weight loss, but, the majority of the weight lost will be fluid. This is because the body has basic energy requirements which are essential to sustain life - that is, energy to breathe, to keep the heart beating, to enable the brain to function and so on. If the diet provides insufficient energy to meet these requirements, the body will initially use energy stores in the form of glycogen, which is stored in the liver. For every 1g of glycogen used 3g of water are also used, hence the weight loss. But as soon as energy intake increases, glycogen stores are replaced and with them the water stores.

This type of eating plan does not change long-term eating behaviour or lifestyle. It is intended as a short-term measure and as soon as a normal diet is resumed, weight will increase - often to more than it was originally. The importance of exercise and a balanced nutritional intake is not usually considered in these diets, which are not recommended.

Blood group diets
Blood types have been linked to many health concerns, from disease susceptibility to weight gain. Publicity involving appropriate diets for particular blood groups has attracted much attention, particularly in the USA.

Those with blood group type O, known as 'the hunter', are advised to eat lean meat and fish regularly, and to avoid dairy and wholewheat products, avocados, cabbage, sweet corn, aubergine, potatoes and sprouts. Coffee should be replaced with green tea.

Type A, 'the cultivator', should avoid all meats, whole milk products and processed cereals. They should eat fruit three times a day and include apricots, figs, grapefruit and plums but avoid bananas, coconuts, melon, oranges and rhubarb.

Type B, 'the nomad', is advised to eat plenty of lean red meat such as lamb, mutton or rabbit several times a week but should avoid chicken. They can include a wide variety of dairy foods, especially if low fat, but should avoid wheat and rye, although they can include millet, oat bran and oatmeal. Five portions of fruit should be included daily, but coconut and rhubarb should be avoided.

Finally, type AB, 'the enigma', should include some meat, including lamb, mutton, beef and turkey. They are particularly suited to cultured or soured products such as yoghurt and cereals such as millet, oats and bran but not cornflakes. Bananas, oranges and mangoes should be avoided, but figs, grapes, grapefruit, plums and pineapple can be included.

Do they work?

Avoiding numbers of foods can lead to an over-restrictive diet and deficiency in nutrients.

De-tox regimens
De-tox diets appear popular, especially at the beginning of the New Year and in preparation for summer holidays. Invariably they involve eating only raw fruit and vegetables and drinking lots of water for various lengths of time. Doing so reportedly rids the body of harmful toxins and 'flushes out the system'.

Do they work?

Weight loss will occur, as energy intake is dramatically restricted to below average energy requirements. However, if there is too great a restriction, glycogen stores will be used, again taking water and resulting in rapid weight loss. If the restriction continues, lean muscle mass will be used as an alternative energy source.

The diets provide high levels of antioxidant vitamins as long as a variety of fruit and vegetables are consumed. They are also high in fibre which, combined with a high fluid intake, will ensure (or improve) bowel movements. However, if the previous diet was relatively low in fibre, sudden increases can lead to stomach cramps, bloating and wind.

The diets do not include starchy carbohydrates and are often very low in protein. Low protein intakes can result in decreased protein turnover and reduction in amino acids used for oxidation, depending on the extent of restriction and the length of time this is followed.

Starvation diets
Starvation diets are often used to achieve the maximum weight loss in the minimum time. They involve avoiding all foods and drinks with the exception of water, diet drinks or tea and coffee without milk or sugar.

Do they work?

Starvation diets can be dangerous and should not be followed, even briefly. Of course weight loss will occur, but at the expense of glycogen and lean body mass stores. Micronutrient depletion will occur if the diet is followed for any significant length of time. When food is reintroduced following a starvation diet, glycogen and water are replaced, so weight begins to increase and body composition tends to change, leading to an increase in body fat and decrease in body proteins.

The only way to achieve real and sustainable weight loss is to ensure that energy output exceeds energy input. This is best achieved through a healthy eating pattern and an active lifestyle.

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