Cancer Prevention in Primary Care: Diet and cancer

Relation between diet and cancer: possibilities for prevention

On the basis of existing evidence, it is not possible to formulate, with any degree of certainty, a precise definition of which cancers are causally related to diet and what proportion of them are due to specific components of the diet. Box 1 shows current dietary hypotheses about the association of cancer risk with dietary factors. Table I shows some of the possible associations between diet and cancer.

Fruit and vegetables

There is strong and consistent evidence that a high intake of fruit and vegetables protects against various cancers. The association is most marked for cancers of the respiratory and digestive tracts. A protective effect has not been found for hormone related cancers.

There is evidence of synergistic relations between smoking or alcohol consumption and malnutrition, particularly a low intake of fruit and vegetables. For oral, pharyngeal, and oesophageal cancers the highest risk is in heavy drinkers who also have a low fruit and vegetable intake. Evidence also suggests that smokers who have a high intake of vegetables sufficient to substantially increase their plasma concentration of (Beta) carotene have a 60% lower risk of developing lung cancer.

Overall, the protective effect of fruit and vegetables seems to be general and found consistently with many different groups of these foods. Although many biological logical mechanisms have been postulated, there is no real evidence that one particular component of the diet is responsible. A fairly consistent picture is emerging about some specific associations between nutrients in fruit and vegetables and specific cancer sites. A reduction in the risks of cancer of the mouth, pharynx, larynx, oesophagus, and stomach is related to the intake of vitamin C and carotenoids. This is consistent with a protective effect for antioxidant substances. Results from a recent study also suggest that dietary (Beta) carotene, raw fruits and vegetables, and vitamin E supplements reduce the risk of lung cancer in non-smoking men and women. Current evidence is not yet strong enough to recommend the specific supplementation of the diet with vitamins A, C, and E, (Beta) carotene, or selenium, despite the suggestive experimental evidence. Some supplements such as selenium can be toxic in large doses and are not advised. Any public health recommendation should therefore relate to vegetables and fruit as a whole rather than to specific food components.

A randomised intervention study in Linxian, China, showed a reduction in mortality from cancer, particularly from stomach cancer, among those receiving supplementation with (Beta) carotene, vitamin E, and selenium. This was the first time it has been shown that dietary supplementation can be used to reduce cancer rates in a study population. This was, however, a special population with high cancer rates and a long history of low dietary intakes of several important micronutrients. The results from this population at a dietary extreme cannot be directly applied to most Western populations, whose diet is generally much richer in essential micronutrients.

Fat and energy

The question of the possible effect of total energy intake compared with the contribution of each energy providing nutrient (protein, carbohydrates, fat, and alcohol) is unresolved. The balance between energy intake and physical activity is also uncertain. Excess energy intake is a risk factor only if it leads to obesity but not when it is associated with high energy expenditure. Obesity is an established risk factor for endometrial and postmenopausal breast cancers, but the association of these cancers with excessive energy intake is less well established. Increased physical activity and control of body weight should contribute to reducing the risk of ischaemic heart disease and strokes and may have an effect on reducing the risk of endometrial and postmenopausal breast cancers.

The association between dietary fat intake and breast cancer is particularly contentious and, at the present time, remains unresolved. There is slightly stronger evidence that high fat consumption may increase the risk of cancers of the prostate, colon, and rectum. It seems fair to conclude that diets high in saturated fats (or in meat, or both) increase the risk of colorectal cancer at least. But, taken overall, the evidence does not justify recommending specific measures to reduce the consumption of fat, whether saturated or not, just to reduce the risk of cancer (although the reduction of saturated fat is amply justified to reduce the risk of ischaemic heart disease).

There is some concern raised by studies showing an increased incidence of or mortality from cancer for some cancers associated with low serum cholesterol concentrations. These observations may, however, be explained by factors that cause these diseases or by the fact that early disease may lower serum cholesterol concentrations.

Fat currently forms about 40% of total energy in the British diet. Current recommendations for the prevention of ischaemic heart disease are that people should reduce their total and saturated fat intake to no more than 35% and 11% of food energy respectively. On a population basis, a reduction in total dietary fat from 40% to 35% of total energy intake, or a reduction in saturated fat from 17% to 11%, would lead to a significant reduction in serum cholesterol concentrations. It has been estimated that a 10% reduction in the average serum cholesterol concentration of the population might in time result in a 20% to 30% reduction in mortality from ischaemic heart disease. Significant reductions in serum cholesterol concentrations have been achieved in entire Nordic and American communities through dietary change over a few years. The effect on the prevention of cancer - for example, colorectal cancer - of such reductions in total and saturated fat by dietary changes is a much more complex issue and must, at the present time, be based on probabilities rather than certainties. Whereas this benefit could be substantial for those people who would benefit, current knowledge is insufficient to identify such people.

Fibre

In the case of fibre, the evidence has been confused by uncertainty about which compounds exert the beneficial effect. There is some evidence, as yet inconclusive, that dietary fibre protects against large bowel cancer. The term fibre encompasses many components, each of which has specific physiological functions. Most studies of colorectal cancer have found no protective effect of fibre from cereals but have found a protective effect of fibre from vegetables and, perhaps, fruit. This could conceivably reflect an association with other components of fruit and vegetables, with fibre intake acting merely as an indicator of consumption. Classic fibre, however, is an important component of the undigested carbohydrate that reaches the large bowel, and from the point of view of public education it is still appropriate to recommend and increased consumption of unrefined cereals and green vegetables, in which it principally occurs.

Summary of the evidence

Although several lines of evidence indicate that dietary modifications may reduce the risk of cancer, the contribution of diet to the total incidence of and mortality from cancer cannot be measured on the basis of present knowledge. Practical dietary interventions that reduce the risk of cancer are difficult to formulate as, in general, the evidence is theoretical or contradictory and too weak to justify specific intervention. Any recommendations relating to alterations in diet will need to be integrated with more general advice about diet and preventing chronic disease (particularly heart disease) rather than focusing specifically on the possible effects in reducing the risk of cancer.

Overall, the accumulating evidence is sufficiently strong to justify public education about the importance of increasing the consumption of fruit and vegetables, especially raw and lightly cooked vegetables and salads (box 2). Reductions in the intake of total and unsaturated fat should relate to prevention of coronary heart disease and not specifically to cancer, although a reduction in the intake of fat could conceivably lead to a reduction in the incidence of colorectal cancer.

The evidence for the importance of starchy, fibre rich foods in limiting the risk of large bowel cancer is increasing. There is insufficient evidence to support a recommendation to increase wholegrain cereal on the grounds of exerting a protective effect against cancer, but an increase in these products may be desirable to compensate for the reduction in dietary fat.

There is no conclusive evidence that vitamin or mineral supplements protect against cancer. Moreover, their use may occasionally result in overdosage or false reassurance resulting in a tendency to ignore the wider recommendations about fruit and vegetables.

There is no evidence that food additives such as preservatives, flavourings, colouring, or artificial sweeteners cause cancer in humans.

It has been suggested for a long time that salt might be a risk factor for gastric cancer and that its consumption should therefore be reduced. There are other reasons why reducing salt intake may also be beneficial - for example, lowering blood pressure.

Other recommendations, based on existing evidence, suggest that the consumption of smoked, cured, and pickled foods should not be increased above current levels and that food which has become mouldy should be avoided. All the adverse evidence is from developing countries where there is heavy dependence on such foods. The extent to which the above items are consumed in the United Kingdom does not constitute a hazard.

Other food substances currently being studied for their potential protective effect are fish oils (-3 fatty acids), plant derived oestrogenic compounds (phytooestrogens), and calcium.

Need for further research about diet and cancer

A sound prevention policy depends on an understanding of the true relation between diet and cancer. Apart from the reasonably consistent data on the protective effect of fruit and vegetables, good epidemiological data on diet and cancer are scarce and inconsistent. A major problem in conducting dietary studies has been the difficulty in assessing the content of any diet (current or past). The results provided by retrospective case- control studies remain controversial on many important issues. These studies rely on self reporting of past dietary habits, which can be notoriously inaccurate. There may be a bias between cases and controls. Patients with cancer may recall diet in a different way from healthy controls. Randomised intervention studies are the ideal means of providing sound conclusions but are difficult to implement as human nature and appetite are hard to control. Long term prospective cohort studies are not an ideal solution because the time at which carcinogenesis begins cannot be determined. They do, however, offer the advantage that data on current diet can be collected well before the onset of disease and there is no case- control bias.

The European prospective investigation of cancer (EPIC) is the largest detailed study of diet and cancer yet undertaken. It has been designed to test the dietary hypotheses outlined in box 1. > 4000 000 healthy middle aged men and women from seven European countries will be asked to provide a blood sample to obtain objective measurements of their nutritional status, together with comprehensive dietary, medical, and lifestyle information. All seven European countries in the study will be using the same standardised core protocol so that the results can be pooled, thereby greatly increasing the power of the study. The collaboration also enables the study to investigate the effects of a wide range of foods, as eating habits are quite different in the southern and northern European countries. Follow up for incidence of and mortality from cancer will be for at least 10 years.

In Britain 75 000 patients are being recruited for the study through general practices throughout the country.

Strategies for prevention Population versus individual based approach

Box 3 shows some of the problems in implementing a prevention policy in relation to diet and cancer. A population approach carries with it the potential for major reductions in cancer incidence and mortality. On the basis of the present evidence, with the exception of advice on fruit and vegetable consumption, such an approach cannot be advocated for cancer alone. An additional but important consideration is that measures that are beneficial to the population as a whole are not necessarily beneficial to each individual. Moreover, it is not possible to predict with any degree of certainty which people will benefit. The probabilities of benefit to people from recommendations relating to dietary change are much higher for preventing cardiovascular disease than for preventing cancer. It is important, therfore, that any dietary guidelines relating to cancer prevention should provide sufficient evidence to the public of the benefits and disadvantages (or risks) to enable them to make informed decisions about dietary changes. This includes the need for a presentation of the current limits of knowledge about diet and cancer.

An individual based approach would result in a small reduction in overall cancer risk in the population. None the less, dietary interventions demand to some extent a strategyfocused on individual people. Such a strategy could promote individual health education in primary care and would be complementary to a population strategy. The success of an individual based strategy will depend to a considerable extent on the provision of adequate support and training for the relevant members of the primary care team. A high risk approach specifically related to preventing cancer is not currently possible, with the exception of appropriate dietary advice offered to obese patients.

“Health of the nation”

The targets in the government's health strategy Health of the Nation for diet and nutrition relate specifically to cardiovascular risk prevention (box 4). There is clearly a conflict between government health and agricultural policies. Current government agricultural grants and subsidies are heavily weighted towards a range of meat as well as milk products and to crops grown largely for feeding farm animals (table II). When the high fat content of these products is taken into account the Health of the Nation targets seem less likely to be achieved. It should be a priority, therfore, to implement national (and international) policies on food subsidies that are linked to health priorities.

The Health of the Nation strategy emphasises the importance of public health information that encourages healthy eating, and it emphasises the need for public education resources, as well as adequate education and training of all appropriate health professionals. It also advocates the provision of “plenty and accessible supplies of starchy staples, vegetables, and fruit.” While the importance of education and accessibility cannot be denied, other factors such as culture and economic considerations may also be significant. Figure 1 shows the consumption of fruit and vegetables in the seven European countries participating in the European prospective investigation of cancer study. Although some social and cultural factors influence this distribution, economic factors related to the cost of fruit and vegetables may also play a part. There is evidence indicating that households with lower incomes are more likely to consume foods of a lower nutritional quality than those with higher incomes. Figure 2 shows the relation between family income and fruit consumption in the United Kingdom. If an increasing quantity of fruit and vegetables is to be a realistic option in the nation's diet consideration needs to be given to ways of making them more affordable.

Fig 1

Consumption of fruit and vegetables (excluding potatoes) inseven European countries

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Fig 2

Consumption of fruit by family income in United Kingdom, 1952 and 1990.5A is high income families and D low income families; 1 ounce=28g

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How effective are dietary interventions in primary care?

The requirements of bands 2 and 3 of the new general practitioner health promotion programme include the need for dietary management for a number of conditions and the need to monitor diet and offer interventions when appropriate for all patients aged 15 to 74.

Authoritative guidelines on dietary management in primary care are conspicously absent because of lack of research. When data are available they show that giving dietary advice in primary care does not seem to produce long lasting dietary changes.

There are many barriers to providing effective nutritional interventions in primary care. Several studies have shown that the dietary knowledge of general practitioners and practice nurses is incomplete and occasionally inaccurate. Appreciable deficiencies in their ability to give practical and appropriate dietary advice have also been identified. These studies point to the need for improved nutritional education and training for members of primary care teams.

Lack of incentive to carry out nutritional interventions may also be a factor, particularly if the success rate of such interventions is low. A randomised study in primary care found that knowledge of cholesterol level had little effect in motivating patients to comply with advice to reduce fat in their diet. Another study found that one third of a group of men who had recently had a myocardial infarction were unable or unwilling to follow dietary advice. Thus, even in those at established high risk the endeavours of primary care teams to offer dietary advice may often be met with resistance, if not active resentment. Motivation will be affected by such encounters.

Little information is available on the best methods of making dietary assessments and implementing nutritional interventions in primary care. A randomised study is currently under way in patients with moderately raised cholesterol concentrations to compare the effectiveness of nutritional advice provided by nurses in achieving lowered blood cholesterol concentrations with the effectiveness of advice provided by a dietitian. In other areas of health promotion in primary care, studies have shown that using a nurse without the involvement of a general practitioner is not effective. For both smoking cessation and reducing alcohol consumption brief advice from a general practitioner is an essential component of the intervention. Follow up can be provided by practice nurses, who may give greater priority than do general practitioners to offering dietary advice. Training for practice nurses has been shown to enhance both their knowledge and their perception of the effectiveness of dietary interventions. Although the one to one approach is time consuming, it has been shown to be effective in other interventions to change lifestyle. There is little evidence supporting the long term efficacy of group approaches.

The effectiveness of much of the wide range of nutritional health education materials currently available to primary care teams needs to be evaluated. Current evidence on their use is contradictory. Whereas one study found that the use of health education leaflets in primary care was effective, by reinforcing the message, another found little evidence to support the use of nutrition leaflets to effect change.

Despite the scarcity of published work showing the efficacy of nutritional interventions in primary care, the demands of the general practitioner contract mean that a range of dietary interventions are currently being used. Box 5 shows the key issues for primary care teams. Further research is needed to determine the best methods of implementing dietary advice in primary care.

Conclusion

A sound prevention policy depends on a better understanding of the relation between diet and cancer. With the exception of the protective effect of fruit and vegetables, the evidence linking diet and cancer is inconclusive and contradictory on many issues. The precise mechanism of action of the many dietary factors that may increase or reduce the risk of cancer remains unclear.

Research is difficult to undertake because the human diet does not consist of isolated food components. Furthermore, dietary patterns strongly correlate with other factors such as smoking and drinking, which independently affect the risk of cancer. Therefore, any attempt to isolate one single factor in carcinogenesis may be futile. Despite this, it is important to follow up promising leads concerning diet and cancer by undertaking prospective cohort and intervention studies.

Knowledge is required of the best methods of implementing dietary recommendations if they are to achieve their full impact. A population approach has a much greater potential for achieving major reductions in the incidence of and mortality from cancer than has an individual based approach. However, on the basis of the present evidence, with the exception of advice on fruit and vegetables, a popular approach cannot be advocated for cancer alone. Dietary intervention in primary care requires advice for individual people. The success of an individual based strategy will depend on adequate education, training, and support being made available for the relevant members of primary care teams. Further research is required on means of implementing dietary changes in primary care.