Book: Nutrition: A Very Short Introduction (Very Short Introductions)

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Historically, a high body weight was considered to be desirable. Only wealthy people could afford to eat excessive food and gain body weight, so what we would now call overweight or obesity was a visible sign of wealth and prosperity. In evolutionary terms there was a survival advantage in having sufficient reserves of fat to survive periods of food shortage and famine. However, from the beginning of the 20th century, insurance companies started to collect data on body weight and life expectancy, showing that excessive weight was associated with earlier death, and therefore a poorer risk for life insurance.

Western society’s attitude to obesity has changed, and obesity is now considered to be undesirable, and places a considerable financial strain on health services. Fashion emphasizes slimness, often using models who are not just slim, but frankly underweight. Because of this, many overweight and obese people have problems of a poor self-image, and low self-esteem. They are certainly not helped by the all-too-common prejudice against them, the difficulty of buying clothes that will fit, and the fact that they are often regarded as a legitimate butt of crude and cruel jokes. Overweight children are reviled by their leaner peers and are often the subject of bullying. This may lead to a sense of isolation and withdrawal from society, and often will result in increased food consumption, for comfort—resulting in yet more weight gain, a further loss of self-esteem, further withdrawal, and more eating for compensation.

The psychological and social problems of the obese spill over to people of normal weight as well. There is continual advertising pressure for ‘slimness’, and newspapers and magazines are full of propaganda for slimness, and ‘diets’ for weight reduction. This may be one of the factors in the development of major eating disorders such as anorexia nervosa and bulimia.

Desirable body weight and body mass index

Data from life insurance companies, then later from prospective studies in which large numbers of people were followed for many years, after being classified by body weight at the beginning of the study, have allowed us to define ranges of desirable and undesirable weight. A person’s weight obviously depends on their height. Early tables showed weight for height. However, it is now more usual to use ranges of body mass index (BMI)—the ratio of body weight/height2 (kg/m2). This is sometimes called Quetelet’s index, after the Belgian mathematician Adolphe Quetelet (1796 –1874) who first calculated it, and showed that it gave an index of weight that was independent of height.

A desirable range of BMI is 20 to 25 kg/m2. This is associated with optimum life expectancy.

BMI 25 to 30 (about 5 to 15 kg, 10 to 33 lb, over desirable weight for height) is classified as overweight, and is associated with a 10 to 35 per cent increased risk of early death. In some countries the range of BMI 25 to 27 is classified as ‘acceptable but not desirable’, but still carries a 10 to 15 per cent increased risk of early death.

BMI 30 to 40 (about 15 to 25 kg, 33 to 55 lb, over desirable weight for height) is classified as obesity, and carries a 35 to 135 per cent increased risk of early death. BMI greater than 40 (more than 25 kg, 55 lb, over desirable weight for height) is classified as severe obesity, and carries an even higher risk of early death.

The health risks of obesity

The major causes of early death associated with obesity are cancer (especially breast, prostate, and colorectal cancers); atherosclerosis, coronary heart disease, high blood pressure and stroke; type II diabetes mellitus and its complications; and respiratory diseases. The risk of developing type II diabetes is 20 to 80 times higher in obese people than lean people.

In addition to the diseases caused by, or associated with, obesity, obese people are considerably more at risk of death during surgery and of developing post-operative complications. Surgery takes longer and is more difficult when the surgeon has to cut through large amounts of subcutaneous and intra-abdominal adipose tissue. More importantly, anaesthesia depresses lung function (as does being in a supine position), in all people. Obese people suffer from impaired lung function under normal conditions; because of the burden of fat in the upper part of the body, their total lung capacity may be only 60 per cent of that in lean people, and the workload on the respiratory muscles may be twice that of lean people. Therefore, they are especially at risk during surgery. Because of this impaired lung function, obese people are more at risk of respiratory distress, pneumonia, and bronchitis than are lean people.

Excess body weight is associated with increased morbidity from such conditions as arthritis of the hips and knees, associated with the increased stress on weight-bearing joints, and varicose veins and haemorrhoids. Obesity in childhood and adolescence is associated with lower bone mineral density and increased risk of developing osteoporosis in later life.

These health risks of obesity have serious implications for health services. In the UK it is estimated that treating obesity and its related diseases costs the National Health Service some £4.2 billion a year—almost 4 per cent of total health service spending.

The distribution of fat is important

There are two types of adipose tissue in the body: subcutaneous fat reserves under the skin, which is mainly, but not entirely, on the hips and thighs, and fat within the abdominal cavity, between the organs, which is mainly seen as increased waist diameter or waist:hip ratio.

Subcutaneous fat acts mainly as a reserve of metabolic fuel and is relatively metabolically inactive. By contrast, intra-abdominal adipose tissue is metabolically more active, and releases fatty acids directly to the liver, where they stimulate the production of glucose, whether or not there is a need for increased blood glucose. Intra-abdominal adipose tissue also secretes hormones that antagonize the actions of insulin, so leading to the development of type II diabetes.

In evolutionary terms, intra-abdominal adipose tissue developed to metabolize fatty acids as a means of maintaining body temperature, and was originally the very metabolically active brown adipose tissue—so called because it has a red-brown colour, while storage adipose tissue is white or pale yellow. However, in response to a high fat diet (and the reduced need for heat production when we have better heating in homes) it has differentiated into storage adipose tissue, but continues to be more metabolically active than subcutaneous adipose tissue.

While there is no doubt that any excess body fat is a cause of ill health and premature death, intra-abdominal adipose tissue is considerably more hazardous than subcutaneous fat. Measurement of waist circumference gives an indication of intra-abdominal adipose tissue; desirable sizes are waist circumference less than 102 cm (40 inches) for men or less than 88 cm (35 inches) for women.

The obesity epidemic

Over the past half century there has been a considerable increase in the prevalence of obesity in all developed countries. In 1980, 39 per cent of men and 32 per cent of women in Britain had a BMI above 25, and 6 per cent of men and 8 per cent of women were clinically obese with a BMI above 30. This was sufficiently worrying for the Department of Health to set a goal in its ‘The Health of the Nation’ policy document to halve obesity within a decade. In fact, by 1991, 53 per cent of men and 64 per cent of women had a BMI above 25, and 13 per cent of men and 16 per cent of women were clinically obese. By 2003, 67 per cent of men and 60 per cent of women had a BMI over 25, and 24 per cent of men and 26 per cent of women were clinically obese. There was a three-fold increase in obesity in 23 years. The encouraging news is that since 2003 there seems to have been little further increase, although it is not clear whether this is the result of health promotion activities by general practitioners and the Department of Health, or whether it reflects the premature death of overweight and obese people.

There has also been an increase in the numbers of overweight and obese children. In 1995, 24 per cent of boys and 25 per cent of girls ages between 2 to 15 were overweight or obese; 10.9 per cent of boys and 12 per cent of girls were obese. By 2006, this had increased to 30.6 per cent of boys and 28.7 per cent of girls being overweight or obese, and 17.3 per cent of boys and 14.7 per cent of girls being obese. There is some good news here, in that the levels of overweight and obesity among children peaked in 2004, and showed a small decrease from this peak by 2006 (the most recent date for which figures are available). Since premature death is not the problem for overweight children, this suggests that the health promotion activities to target children are having some success.

Britain is not alone in this. All developed countries have seen similar increases in overweight and obesity, although the increases started earlier in the USA and some other countries than in the UK, and later in other countries. In most developing countries, where undernutrition is the main problem, rates of obesity are also increasing, so that while many in the population suffer from the disease of hunger, many others are suffering from the problems of overnutrition. Globally the ratio of overweight: underfed people is 1.4:1. In developed countries this ratio is 11.8:1, and in developing countries it is 0.7:1.

The causes of obesity

The three-fold increase in obesity in less that a quarter of a century cannot be the result of genetic change in the population. Rather, it is the result of increased availability and consumption of food, coupled with decreased physical activity.

In most countries, the amount of food available per head of population has increased over the last three to four decades. Of course, this does not take account of wastage, which may be a considerable proportion of food that is purchased. At the same time, the cost of food has decreased in real terms in most developed countries—people have to spend a smaller percentage of their income on food than half a century ago. Perhaps more importantly, the consumption of supermarket prepared meals and food from fast food restaurants has increased, and many of these foods are high in fat and sugar, yielding more calories per serving than many home-prepared foods. It is therefore very easy to over eat and to have an excessive calorie intake. One calculation suggests that in the USA the percentage of food spending on meals outside the home increased from 30 to 40 per cent between the 1970s and 2000, but the percentage of calories consumed outside the home increased from 15 to 38 per cent over the same period.

The other factor in obesity is low energy expenditure. The average physical activity level (PAL) of adults in UK is only 1.4 x Basal Metabolic Rate (BMR). A desirable PAL for fitness is 1.7 x BMR, and this is achieved by only 22 per cent of men and 13 per cent of women.

Physical activity has decreased considerably over the last half century, for a number of reasons. Most leisure activities now involve spectator sports rather than playing on the field—and more people watch sport from armchairs in front of television sets than in the stands at the sports venue. Perhaps more worryingly, children spend more leisure time with computers and games consoles than playing outdoors. One study has linked an increase in rickets among adolescents to such indoor activities.

Most people now walk less and use cars or public transport more. Either because of laziness or because of real or perceived fears about safety, most children are now driven to school rather than being allowed to walk. Many people will wait for several minutes to take a lift for one or two flights of stairs rather than performing even this moderate exercise.

At the same time, with some very obvious exceptions, most work is now less physically demanding than it was half a century ago. There is increased mechanization in factories, more jobs are sedentary, with less need to walk around an office or factory. Automated washing machines need far less effort than doing laundry by hand; powered hedge trimmers and lawn mowers reduce the physical exertion of gardening.

How can overweight people lose weight?

There is an apparently simple answer to this question—eat less and exercise more. The increase in obesity tells us that the solution is not so simple. In fact, there are two separate but related problems: to reduce weight to within the desirable range; and to maintain that desirable weight afterwards.

There are many diets and slimming regimes that will allow someone to lose weight—if they follow the diet correctly. Long-term maintenance is more of a problem, and many of the popular weight reducing diets do not educate people in how to eat sensibly after they have achieved their target weight. The diet for maintenance of desirable weight is the same as the prudent diet discussed in : a total energy intake to permit maintenance of body weight, with 30 per cent of energy from fat, and only one-third of that fat as saturated fat, and 55 per cent of energy from carbohydrates (mainly as starches, with only 10 per cent of energy from sugars).

Adipose tissue contains about 15 per cent water, 5 per cent protein, and 80 per cent fat. This means that a gram of adipose tissue yields 7.4 kcal. We can calculate from this that a deficit of 500 kcal per day will lead to utilization of 68g of adipose tissue per day, or 476g in a week. If we assume a total energy expenditure (and therefore requirement) of 2,500 kcal/day, then even with total starvation the maximum possible weight loss will be 2.4kg (5 lb 5 oz) per week. It is important to bear this in mind when evaluating the claims for weight loss that are made for some diets. While the initial rate of weight loss as food intake is reduced may be higher in the first week, as a result of losing the water that is associated with tissue reserves of carbohydrates, after this even total starvation will only give a loss of 2.4kg (5 lb 5 oz)/week.

It is relatively easy for a dietitian to formulate a diet for weight reduction, by measuring or estimating the patient’s current energy intake, then reducing it by about 10–20 per cent until the target weight has been achieved. It is less easy for someone to formulate his or her own weight reducing diet without having recourse to tables of food composition and performing relatively complex calculations. Labelling of foods with energy yield and fat and sugar content helps, but it is still tedious, and there are ways of making life easier for the would-be slimmer.

Prescriptive menus. Many obese people have relatively poor appetite control and are helped by having very prescriptive menus—with little or no choice of what to eat at each meal, and no choice of how much to eat. A number of companies now make this easy for the dieter by providing precisely formulated pre-prepared meals, either available from supermarkets or delivered to the home.

Traffic light lists of foods. A simple way of reducing energy intake is to have three lists of foods, which are coded like traffic lights. Red means high calorie foods, and foods high in fat and sugars, which should be eaten only in small amounts. Amber means foods that can be eaten in relatively larger amounts, but still with caution. Green means foods than can be eaten (within reason) in unlimited amounts—especially vegetables and other foods that are high in water and low in calories.

Exchange lists and ‘points’. An alternative, which allows considerably more choice than prescriptive menus, but requires more discipline than traffic light lists, is to have lists of foods that are interchangeable, or have ‘points’, based on calorie and fat content. The dieter can now choose what to eat, as long as s/he keeps to the total number of points allowed each day. Of course, it is possible to formulate a very inadequate and inappropriate diet this way, for example by swapping all the points allowed for servings of meat and fish to points from chocolates, biscuits, and cakes.

High fibre diets. Diets that are rich in fruits and vegetables and whole grain cereals are high in dietary fibre, which is not digested and has very low energy yield. However, a high fibre diet will promote a feeling of fullness—many people complain that they feel hungry on slimming diets that are low in fibre.

Very low calorie diets and meal replacements. Consuming no more than 500 kcal/day will certainly permit a good rate of weight loss, and a number of diets achieve this by providing nutrient-rich low-calorie preparations to be eaten or drunk in place of one or more meals each day. However, once the target weight has been reached, the slimmer has not learnt any new eating habits, and is likely to revert to high calorie foods and regain the lost weight.

Very low carbohydrate diets. Very low carbohydrate diets, allowing more or less unlimited fat and protein, do work for weight reduction. This is partly because on a very low carbohydrate diet you become ketotic (producing large amounts of ketone bodies as a fuel for muscle), leading to nausea and loss of appetite. In addition, there is a need to maintain an appropriate level of blood glucose for the brain, and this can be provided by synthesizing glucose from the amino acids provided by protein. This is an energy expensive process, so it increases the utilization of body fat reserves to provide the energy needed.

Help and support. Many general practices now have a dietitian who can not only help people to formulate a diet, but can also arrange for regular meetings to assess their progress. However, this is extremely expensive, either for the patient or for the health service. An alternative, which has great success, is provided by slimming clubs of one kind or another. Typically, you receive appropriate dietary advice and are set a target weight as well as a time in which to achieve it. In return for a modest fee, there are weekly meetings at which you are weighed, and can discuss any problems with fellow slimmers and group leaders. Group leaders are typically people who have successfully lost weight (and maintained a desirable weight) on the same programme, and who therefore know all the problems of compliance that you are likely to encounter.

Sweeteners and fat replacers. For people with a sweet tooth, there are a number of non-caloric sweeteners available. Some are only suitable as ‘table top’ sweeteners to be used in tea, coffee, and low calorie soft drinks; others can be used in cooking to make cakes and desserts, or jams.

Many traditional foods that are high in fat are available as low fat alternatives, such as skimmed milk, low fat cheeses and sausages, as well as spreads to replace butter or margarine, containing only 40–50 per cent fat, and sometimes less—compared with 80 per cent fat in butter or margarine.

There are fat replacers based on protein and carbohydrate that have the same texture in the mouth as fat and can be used in some manufactured foods. Other fat replacers are based on compounds of fatty acids with sucrose; these are not digested at all, so have zero energy yield, but can be used for frying or baking.

Drugs to aid weight loss. Over the years a number of drugs have been developed to aid weight loss. Some of these acted by increasing the metabolic rate so that energy expenditure was increased. The result of this was a low grade fever—hardly a pleasant way of losing weight.

Slimming patches purport to contain iodine from seaweed, although in at least some cases there is no iodine present, because iodine is volatile. The theory behind their use is that if you are iodine deficient you produce little thyroid hormone, your metabolic rate falls, and so that you become overweight and lethargic. This is true, as is the fact that people who over-produce thyroid hormone because of thyroid disease have a high metabolic rate, lose weight, and are thin. However, if you are not iodine deficient and do not have thyroid disease, providing additional iodine does not increase metabolic rate and does not help weight loss.

A number of drugs have been developed to suppress appetite by acting on the appetite control centres of the brain. While they are successful, most of them are potentially or actually addictive, and some have serious side-effects. Many appetite suppressant drugs have now been withdrawn, although some are still available on prescription, and some are available by online purchase (but there is no guarantee of quality or safety with drugs bought online).

Drugs to inhibit carbohydrate digestion have some efficacy, and drugs to inhibit fat digestion are certainly effective, although they have the side effect of causing foul smelling, fatty diarrhoea if you continue to consume a relatively large amount of fat.

Surgery for obesity. It was noted above that obese people are considerably more at risk during surgery than lean people. Any surgical intervention to remove a significant part of the intestinal tract or part of the stomach is irreversible. This means that surgery must be considered as being the last resort—for the obese person who has genuinely tried to lose weight by other means.

Diets that (probably) will not work. There is a never-ending list of diets that are unlikely to be successful. Some are based on misconceptions, others are based on a very limited list of foods, with the idea that you will become bored and eat very little. Such diets (e.g. the cabbage diet, based on eating little apart from cabbage soup; diets based on mango and fresh pineapple; etc.) are obviously unbalanced and nutritionally unsound. Fortunately, most people become bored with the limitations of the diet before they develop severe nutritional deficiencies.

Food combining (sometimes called the Hay diet) is based on the concept of not eating protein and carbohydrate at the same meal—but this ignores the fact that almost all carbohydrate containing foods also contain significant amounts of protein.

The macrobiotic diet is a system of eating associated with Zen Buddhism. It consists of several stages, finally reaching Diet 7, which is restricted to cereals. Cases of severe malnutrition have been reported on this diet. It involves the Chinese concept of yin (female) and yang (male), whereby foods, and even different vitamins (indeed, everything in life), are predominantly one or the other, and must be balanced.

The pH diet is based on balancing the intake of acid forming and base forming foods, with little or no scientific basis, and the zone diet on the unfounded belief that each meal should comprise a fixed proportion of macronutrients: 40 per cent carbohydrate, 30 per cent fat, and 30 per cent protein.

Detox diets are based on the unfounded belief that toxins from food accumulate in the body, slowing metabolism and leading to weight gain. There is no evidence for this. A period of fasting and strict avoidance of such supposed toxins as caffeine and food additives is claimed to be beneficial. Various herbal supplements are often included in ‘detox diets’, with little or no evidence of efficacy. Proponents claim that the sensation of lightheadedness that occurs after a few days of more or less complete fasting is the result of toxins being released, while in fact it is the result of low blood glucose and ketosis—as a result of starvation.

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