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

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Patterns of disease and mortality differ around the world, and one of the factors that can be correlated with many of the differences is diet, although there are obviously other environmental (and genetic) factors involved. This chapter is concerned with the ways in which we can gather evidence that diet is, or may be, a factor in the development of chronic non-communicable diseases (especially atherosclerosis, coronary heart disease, hypertension (high blood pressure), and cancer), and how we can use these findings to produce guidelines for a prudent diet and to promote healthy eating.

At one time these chronic non-communicable diseases were known as diseases of affluence, because they were seen mainly in affluent Western countries and mainly among the wealthier sections of society. Increasingly, however, they are becoming major causes of premature death in developing countries, and are more common among the poorer, rather than wealthier, sections of society in developed countries.

The relative amounts of fat, carbohydrate, and protein in the diet are important, as is the mixture of different types of carbohydrate and fat. Consumption of alcohol may have both beneficial and adverse effects on health. A wide variety of compounds in foods (and especially in fruits and vegetables) may also have beneficial effects.

How we gather evidence linking diet and health

Different types of epidemiological study can give information about diet and health. The first clues often come from looking at changes in diet over the past 100 or so years, coupled with changes in the pattern of disease. Such studies suggest that, in parallel with the increase in atherosclerosis, cardiovascular disease, and cancer, there have been increases in the total amount of fat and sugar consumed, with a reduction in starch and dietary fibre. A considerably greater proportion of the fat consumed is animal fat, which is largely saturated. One obvious problem with this comparison is that people now live longer now than they did 100 years ago. This is partly because the diseases associated with under-nutrition are now rare in developed countries, and partly because many infectious diseases that were common killers can now be treated with antibiotics and anti-viral drugs. The provision of clean drinking water and effective management of sewage have also been important factors.

A great deal can be learnt from studying people who have migrated from one country to another, and comparing their patterns of disease, and their diets, with relatives who have remained in the mother country. Similarly, we can look at patterns of disease in different countries—for example, there is a 100-fold difference in the incidence of breast and prostate cancer between West Africa and the USA. Are there any differences in diet that might explain this? Vegetables form a large part of the traditional Japanese and Chinese diets, as does fish in Japan and near the coast in China. The oestrogen-like compounds in soya products may explain the lower incidence of breast and prostate cancer in China and Japan compared with that in Western countries. The traditional Mediterranean diet, rich in fruits and vegetables, with more fish and seafood than meat, and moderate wine consumption is associated with considerably lower incidence of cardiovascular disease than Northern Europe.

Many studies have shown that there is a significant international correlation between fat intake and breast cancer. Such studies assume that the data on food availability in different countries are comparable. The main problem here is that diet is only one of many factors that differ between different countries, and many other lifestyle and environmental factors may also be important. More detailed international correlation studies involve taking blood samples for laboratory studies. The Seven Countries study, which started in the 1950s and has continued to the present, first established that elevated serum cholesterol was a major factor in atherosclerosis and coronary heart disease.

At a more individual level, we can look at people with a given disease and compare them with people who are free from the disease, but matched for age, sex, lifestyle, and as many other factors as possible. This is a case-control study. It gives better information than international correlation studies, and in many studies we also have information not only on diet but also on results from laboratory tests to measure individual nutrients and markers of nutritional status in blood and urine samples.

The problem with case-control studies is that we are looking at people who already have the disease, but the initiation of cancer or cardiovascular disease occurs many years before the disease becomes apparent. What we really need to know is what these people were eating 10 to 20 years ago. Equally, the disease itself may affect the metabolism of nutrients. These problems can be overcome with long-term prospective studies, in which people are followed for many years, with records of diet and other lifestyle factors at the beginning and at frequent intervals throughout the study. Many such studies also include laboratory tests, physical examination, and questionnaires at intervals. The oldest such study is the 1946 birth cohort study in the UK. Every child born in the second week of March 1946 is still being followed, and new cohorts have been added. Other prospective studies include the Framingham study in the USA, which has followed everyone in the town since 1948; the Nurses’ Health Study (again in the USA) which has followed over 100,000 nurses since 1976; the Whitehall Study, in which civil servants in England have been followed since 1967; and the European Prospective Investigation into Cancer and Nutrition (EPIC), which has followed half a million people in ten European countries since the 1990s.

Experimental studies can also give useful information. Once it had been established that elevated serum cholesterol was a risk factor for atherosclerosis and coronary heart disease, and epidemiological studies suggested that different types of fat may affect serum cholesterol differently, a series of experiments was conducted in the 1960s. People were fed standardized diets in which the total amount of fat remained constant, but where mono-unsaturated fat was replaced with either saturated or polyunsaturated fat (see ). The amount of cholesterol in the diets was also varied in separate experiments. The results showed that compared with mono-unsaturated fat, saturated fats increase serum cholesterol proportionally to twice the intake, while polyunsaturated fats decrease it proportionally with intake. Dietary cholesterol also increases serum cholesterol, but only proportionally to the square root of intake.

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1. Saturated and unsaturated fatty acids, and the cis- and trans-arrangements around a double bond

Other studies have investigated the effects of changes in diet on blood pressure, and have identified a high salt intake and excessive alcohol consumption as significant risk factors. There have also been trials of supplements of individual nutrients to reduce the risk of developing cancer.

Guidelines for a prudent diet

These epidemiological and experimental studies have provided evidence for a dietary pattern that is associated with reduced risk of developing cancer, atherosclerosis and cardiovascular disease, hypertension and stroke, and type II diabetes. As discussed earlier, such a diet provides 30 per cent of energy from fat (with only about 30 per cent of this fat being saturated and 6 to 10 per cent being polyunsaturated fat; see for the structures of the different types of fatty acids in dietary fats); 55 per cent of energy from carbohydrates (mainly starch, with only 10 per cent of energy from sugars); and 15 per cent of energy from protein.

compares this prudent diet with the average Western diet—where there is a need to reduce total fat and sugar consumption, and to increase starch consumption. At the same time, the proportion of fat that is saturated needs to be reduced; average intakes of mono-and polyunsaturated fats are about right, so that all of the reduction in fat intake should be achieved by reducing saturated fat intake. Average salt intake is considerably in excess of requirements, and should be reduced to minimize the risk of hypertension. Average intakes of alcohol are also greater than is considered desirable for good health.

Table 5. Guidelines for percentage of energy from fats, carbohydrates, and proteins in a prudent diet compared with average Western diets

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Polyunsaturated fatty acids. Polyunsaturated fatty acids are those with two or more double bonds in the molecule (see ). They are important in the structure of cell membranes, and are the precursors of prostaglandins and related compounds that have hormone-like actions. Enzymes in the body are capable of increasing the chain length of polyunsaturated fatty acids up to the 20 carbons that are required for prostaglandin synthesis, and of introducing additional double bonds between an existing double bond and the carboxyl group of the fatty acid. However, there are no human enzymes that are capable of introducing additional double bonds between the terminal methyl group and the first existing double bond. As shown in , there are two families of polyunsaturated fatty acids, ω3 (omega-3) and ω6 (omega-6), that give rise to different families of prostaglandins and related compounds, with different actions in the body. Since it is not possible to convert a ω6 fatty acid to a ω3 fatty acid, there is a need for both in the diet. The ratio of ω3 to ω6 in the diet is important, because the same enzymes are involved in metabolizing both families of polyunsaturated fatty acids, and they compete with each other. The major dietary sources of ω6 polyunsaturated fatty acids are plant oils, and the major sources of ω3 fatty acids are oily fish such as salmon, trout, and herring, as well as fish liver oils. Fish oils provide ready-formed long-chain ω3 fatty acids and there is good evidence that eating one or two servings of oily fish a week has health benefits and reduces the chances of cardiovascular disease.

Trans-fats. Most of the naturally occurring unsaturated fatty acids have the cis-conformation—as shown in , the chain of carbon atoms continues on the same side of the double bond. When liquid oils are treated to convert some of the unsaturated fatty acids to saturated fatty acids for the manufacture of spreads and other solid fats that can be used for food manufacture, some of the unsaturated fatty acids undergo a change from the cis-conformation to the trans-conformation. Evidence has accumulated over the past few years that not only do these trans-fatty acids not have the same beneficial effects as cis-unsaturated fatty acids, but they are actually more hazardous to health than saturated fatty acids. Trans-fatty acids should provide no more than 1 per cent of energy intake. Many food manufacturers have eliminated trans-fatty acids from their products, by using alternative, but more expensive, methods of preparing solid fats from liquid oils.

Dietary fibre. Plant cell walls contain a variety of compounds that are not digested, but nevertheless have an important role in the diet. Collectively these compounds are referred to as dietary fibre or non-starch polysaccharides. The term ‘dietary fibre’ is misleading in that few of the compounds involved actually form fibres, and indeed some (the plant gums) are soluble, but undigested, carbohydrates. It is more correct to call them non-starch polysaccharides. All plant foods contain dietary fibre in varying amounts. Whole grain cereals are an especially rich source, but much is lost when wheat is refined to produce white flour.

At the simplest level, dietary fibre provides bulk in the diet. This permits easier passage of food through the gut and reduces the risk of either constipation or diarrhoea. A low fibre diet is associated with increased risk of diverticular disease of the colon and haemorrhoids, and increasing fibre intake helps to relieve both conditions.

Dietary fibre also physically adsorbs a number of compounds in the diet that are potentially liable to cause cancer, so preventing their absorption. It also adsorbs bile salts, which are secreted from the gall bladder into the intestine for fat digestion and absorption.

This means that less of the secreted bile salts is re-absorbed, so that there has to be more synthesis in the liver. The precursor for bile salt synthesis is cholesterol, so an increased intake of dietary fibre helps to lower serum cholesterol.

The plant gums and other compounds that make up soluble dietary fibre slow the rate of absorption of nutrients from the gut. This is beneficial for people with diabetes, since the main cause of problems of controlling blood glucose arise when there is a rapid increase in glucose becoming available after a meal. A number of studies have shown that consuming a supplement of soluble dietary fibre before a meal improves control of blood glucose.

Although we cannot digest the various compounds that make up dietary fibre, they do provide a substrate for intestinal bacteria, which can ferment them. This helps to maintain a healthy population of intestinal bacteria, which can compete with pathogenic bacteria that cause food poisoning. Some of the products of this bacterial fermentation are also available to the cells lining the intestinal tract. They provide a major metabolic fuel for these cells, and there is some evidence that they protect against the development of cancer of the colon.

Fruits and vegetables—five servings a day. All of the epidemiological evidence suggests that people who consume an average of five servings of fruit and vegetables a day are less at risk of obesity, cardiovascular disease, and cancer. It does not matter if these are fresh, frozen, or canned, and fruit juices are also beneficial, although they may provide undesirably high levels of soluble sugars that can cause dental decay. The benefits of fruit and vegetable consumption are partly due to the beneficial effects of dietary fibre discussed above, and partly to the fact that a diet that is rich in fruit and vegetables will be relatively low in fat, and especially saturated fat. A relatively high fibre diet is beneficial for weight reduction because it helps to prevent feelings of emptiness and hunger while providing relatively few calories. In addition, fruits and vegetables provide a good source of vitamins and minerals, as well as a large number of other compounds that, while not dietary essentials, have a protective effect against cancer and cardiovascular disease.

Alcohol. Most countries have guidelines on prudent levels of alcohol consumption. In the UK, the Royal College of Physicians first developed guidelines for alcohol consumption in the 1990s. They defined a unit of alcohol as being equivalent to 8g of pure alcohol—this is the amount in half pint of beer, 100 ml of wine (a small glass), or a single 25 ml measure of spirits. The prudent upper level of consumption is 21 units per week for men and 14 units per week for women. The sex difference is because the liver, which is where alcohol is metabolized, is smaller in women than in men, so that women are more at risk from the adverse effects of excessive alcohol consumption than are men. The UK Department of Health modified these guidelines in 1995, setting the prudent upper limit as 4 units per day for men and 3 units per day for women. This was not intended to increase the upper level of alcohol intake, but rather to emphasize the point that binge drinking (consuming the whole week’s ‘allowance’ in one evening) is more hazardous to health than regular consumption of moderate amounts.

Free radicals and antioxidants. One of the theories to explain the underlying cause of cancer and atherosclerosis is that tissues are damaged by free radicals. Free radicals are highly reactive molecules that have an unpaired electron. When a radical collides with another molecule, it becomes stable by removing or donating a single electron, but in the process it generates a new radical. Radicals can cause damage to DNA, leading to possible mutations and the development of cancer; and to fats, leading to the development of atherosclerosis; as well as to proteins, leading to the development of auto-immune diseases, including rheumatoid arthritis. The radicals that cause the most damage are oxygen radicals, and compounds that provide protection against radical damage are generally referred to as antioxidants.

Oxygen radicals are formed in the body as part of normal metabolism and protection against infection; it is estimated that about 5 per cent of the oxygen we consume each day forms potentially damaging radicals. Many epidemiological studies have shown that relatively high blood concentrations of antioxidants such as β-carotene and vitamins C and E are associated with lower risk of cancer and cardiovascular disease. However, intervention trials with high doses of antioxidants have not shown the expected protection, and many of the trials of β-carotene and vitamin E have shown increased mortality among those taking the supposedly protective supplements.

There are three possible explanations for this adverse effect of high dose antioxidant supplements. Antioxidants act by forming stable radicals that persist long enough to penetrate deeper into tissues, so causing more damage. Some antioxidants, especially β-carotene, may act as antioxidants in tissues where there is little free oxygen, but in tissues such as the lung they become pro-oxidants, generating damaging oxygen radicals. The Food Standards Agency in UK has warned smokers not to take β-carotene supplements because of the increased risk of lung cancer. Perhaps most importantly, antioxidants may also have an adverse effect because much of the signalling for damaged cells that might go on to develop into cancers to undergo programmed cell death depends on oxygen radicals. High concentrations of antioxidants will quench these important signalling radicals, leading to survival of potentially cancerous cells.

Publicizing healthy eating

There are two main approaches to publicizing the key messages about healthy eating and a prudent diet: nutritional labelling of foods and public health campaigns that translate the nutritional guidelines in into foods.

Nutritional labelling of packaged and manufactured foods provides a great deal of information: the energy yield, fat (and saturated fat), carbohydrate (and sugars), and vitamin and mineral content, both as amount per serving or per 100g and also as percentage of the reference intake, daily value, or guideline daily amount. Increasingly, in addition to nutritional labelling, manufacturers are being encouraged (and in some countries required) to have ‘front of package’ labelling that highlights the fat, saturated fat, sugar, and salt content of the food. This may take the form of a ‘traffic light’ colour coded label—green for low fat, saturated fat, sugar, and salt; amber for medium; and red for high. Alternatively, labels show the energy, fat, saturated fat, sugar, and salt content as the amount and as a percentage of the guideline daily amount or daily value—the amount called for by the prudent diet discussed above. The most successful front of package labels use both schemes—traffic lights plus the amount and percentage of guideline daily amount.

While such labelling helps the discriminating consumer to make healthy choices, it is relatively little help in converting nutritional guidelines into foods. The food pyramid (see ) has the starchy foods that should provide the main part of the diet at the base, with fruit and vegetables in the next row, then dairy produce, meat, and fish, and with foods that should be eaten sparingly (fats, oils, and sweets) at the apex. Apart from fats, oils, and sweets, there is a suggestion of how many servings of each group of foods should be consumed each day. The food pyramid has been criticized for the fact that the foods that should be eaten in smallest amount are shown at the top of the pyramid, which implies to some people that they are the most important foods.

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2. The food pyramid

In the UK, the Foods Standards Agency has developed the ‘eatwell plate’ (formerly called the balanced plate) shown in . Here the five groups of foods (fruit and vegetables, bread, cereals and potatoes, meat, fish and alternatives, milk and dairy foods, and foods and drinks containing fat and sugar), are shown as sectors of the plate representing the relative amounts that should be eaten each day. The USA has abandoned the food pyramid and now uses a simple four sector plate (‘choose my plate’ in ) for fruit, vegetables (half the plate is fruit and vegetables), grains (more than a quarter of the plate), and protein—with a separate container for dairy produce. There is no mention of fats, oils, and sugars that should be consumed in limited amounts, since the aim is to emphasize a positive message.

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3. a) The ‘eatwell plate’ and b) ‘choose my plate’

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