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International comparisons indicate a high correlation between dietary fat intake and the occurrence of cancer of the breast, prostate, uterus (body), ovary, and colon. These data parallel the results of animal experiments, as well as a number of epidemiological studies, though recently some cohort studies have failed to confirm the associations noted in previous case-con- trol studies, especially for breast cancer (Hunter et al., 1996), thus casting doubt on the causal nature of the associations. Part of the difficulty could be in accurate quantification of dietary fat consumption at the relevant time in the natural history of many cancers, so that cohort studies may simply be failing to find an association due to a lack of precision in the measurement. In animal studies, polyunsaturated fat appears to increase risk; however the epidemiological studies do not support that in humans.

Meat

There is some evidence that consumption of red meat and, perhaps in particular processed meat, increases the risk of colon cancer (Key et al in preparation). Whether this is a direct effect of substances in red meat, such as saturated fat, or the effect of cooking or food processing methods is uncertain. In nearly every cancer site for which the effect of white meat (for example, chicken) or fish consumption has been evaluated, no increase in risk has been found.

Prevention

Nitrites and salt

Studies of the declining incidence of stomach cancer have suggested that this trend is related to changes in dietary patterns, particularly the decrease in salting and pickling for food preservation, the increasing use of refrigera- tion and the associated increase in the availability of fruits and vegetables (and thus vitamin C) year round. Salting and pickling involve certain chemi- cals that are known to combine with amines in the stomach to produce nitrosamines—powerful carcinogenic agents. This mechanism may account for the high incidence of stomach cancer in some areas of Japan and certain other parts of the world, such as Chile and Costa Rica; the hypothesis is supported by epidemiological studies in North America and Europe (World Cancer Research Fund, American Institute for Cancer Research, 1997). Nasopharyngeal cancer, common in South Asia, has been consistently asso- ciated with a high intake of Chinese-style salted fish, a special product which is usually softened by partial decomposition before or during salting (Key et al in preparation).

Further prospective data are needed, in particular to examine whether some of the dietary associations may be partly confounded by Helicobacter

pylori infection and whether dietary factors may modify the association of Helicobacter pylori with risk.

Contaminants

Certain substances naturally present in food and other substances generated during its preparation have carcinogenic potential. Additionally, food may become contaminated with chemicals capable of causing cancer. In Africa and some parts of Asia, for instance, the growth of mould on nuts and other foods under particular conditions of storage produces aflatoxin, a highly potent carcinogen strongly implicated in the high incidence of liver cancer (IARC, 1993). Generally, however, food contaminants are responsible for only a small amount of diet-induced cancer (Tomatis et al., 1990).

Additives

Substances added to food as preservatives or to enhance colour may also be carcinogenic. Since 1956, the Food and Agriculture Organization/World Health Organization (FAO/WHO) Food Standards Programme has set max- imum levels for additives, contaminants, and pesticide residues to minimize this possibility. These standards are implemented by the Codex Alimentarius Commission. It seems unlikely that currently permitted additives have any significant effect in increasing the risk of cancer (Tomatis et al., 1990).

Prevention

Promoting dietary modification

Prevention of cancer by dietary means can be encouraged by observing the following public health goal (World Cancer Research Fund, American Institute for Cancer Research, 1997; Joint WHO/FAO expert consultation on diet, nutrition and the prevention of chronic diseases, in preparation): populations should consume nutritionally adequate and varied diets, based primarily on foods of plant origin.

In addition, the following measures should be advocated for individuals: 1. Maintain body mass index (BMI) in range of 18.5 to 25 kg/m2_{, and avoid}

weight gain in adulthood.

2. Engage in regular physical activity.

3. Consumption of alcoholic beverages is not recommended: if consumed, do not exceed 2 units per day (1 unit is equivalent to approximately 10 g of alcohol and is provided by one glass of beer, wine or spirits).

4. Minimize exposure to aflatoxin in foods.

5. Chinese-style salted fish should only be eaten in moderation, especially during childhood. Overall consumption of salt-preserved foods and salt should be moderate.

6. Have a diet which includes at least 400 g/day of fruit and vegetables. 7. Meat: those who are not vegetarian are advised to moderate consump-

tion of preserved meat (e.g. sausages, salami, bacon, ham etc.) and red meat (e.g. beef, pork, lamb). Poultry and fish (except Chinese-style salted fish, see 5. above) have been studied and found not to be associated with increased cancer risk.

8. Do not consume very hot foods or drinks.

Countries in which the traditional diet results in a low incidence of diet- associated cancers should take action to ensure that their patterns of food consumption do not change to those of North America and western Europe.

A national cancer control programme offers an opportunity to implement the recommendations of the WHO Study Group on diet, nutrition, and the prevention of chronic disease (WHO, 1990a), the World Cancer Research Fund and American Institute for Cancer Research (1997), and the recent WHO/FAO expert consultation (Joint WHO/FAO expert consultation on diet, nutrition and the prevention of chronic diseases, in preparation).

Among the measures to be considered to promote dietary modification are the following:

• government recognition of dietary factors in cancer etiology and consid- eration of the implications of those factors for the relevant ministries

Prevention (especially health and agriculture); Prevention

• appropriate education on diet in schools;

• public education campaigns about diet for adults;

• collaboration with representatives of the food industry (both production and service sectors) to ensure compliance with the nutritional objectives of the programme.

An international strategy comparable to that currently underway for tobacco can be envisaged.

ALCOHOL