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close this bookWIT's World Ecology Report - Vol. 08, No. 3 - Critical Issues in Health and the Environment (WIT, 1996, 16 pages)
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The Relationship Between Diet and Chronic Diseases

Sushma Palmer, D.Sc., Central European Center for Health and the Environment, Berlin, Germany.

The agricultural and the industrial revolutions introduced radical changes in food cultivation, distribution, and availability worldwide. Recent technological advances in food processing and increases in per capita income, especially in industrialized nations, have led to profound changes in dietary preferences. One notable change since the early part of this century is the dramatic and rapid increase in consumption of fats and refined sugars - the affluent diet - coupled with a substantial drop in complex carbohydrate intake. The improved food supply and food security practically eliminated starvation worldwide, guarded against micronutrient deficiencies, and led to improved nutritional status and increased life expectancy. The long-term adverse health effects of the high fat affluent diet, however, have become apparent in the last few decades through the emergence in industrialized countries of chronic diseases such as coronary heart disease, cerebrovascular disease, various cancers, and obesity.

CORONARY HEART DISEASE (CHD)

There is convincing evidence from scores of epidemiological, clinical, and laboratory studies that dietary factors play a decisive role in the etiology and prevention of coronary heart disease.

Ancel Keys in a 1950s study showed that saturated fat intake varied sevenfold, i.e. between 3% and 22% of total energy in Japan, and Eastern Finland - regions that showed a fivefold difference in coronary heart disease rates. He also suggested that, at a population level, serum cholesterol was strongly related to the incidence of coronary heart disease (CHD) and that there was a strong correlation between the intake of saturated fat and serum cholesterol.

It is now well established that on population basis, the risk of CHD rises progressively with increases in serum cholesterol. Rural China, where the serum cholesterol level averages 125 mg/dl, has one tenth the incidence of CHD observed in the U.S. For many Western countries, in contrast, with serum cholesterol averaging 200 mg/dl or higher, the whole population may be described as high risk, and the concept of a "normal" serum cholesterol, therefore, may have little meaning.

Epidemiological studies in middle-aged men provide clear evidence that the risk of CHD in individuals is increased by three major factors: high serum cholesterol, high blood pressure, and cigarette smoking. The fundamental importance of diet in the development of coronary heart disease is mediated through its effects on the development of hyperchoesterolaemia and hypertension; and the type and amount of fat are the most significant dietary factors.

It is now recognized that most saturated fatty acids elevate serum cholesterol while polyunsaturated fatty acids reduce the level, and monounsaturated fatty acids tend to have little direct effect. Dietary cholesterol independently contributes to CHD risk, and a population average intake of less than 300 mg/day has been recommended by most international committees.

Population subgroups consuming diets rich in plant foods and dietary fiber exhibit lower CHD rates than the general population. For example, Seventh-Day Adventists in the Netherlands and Norway have CHD rates that are one-third to one-half of those in other subgroups in those populations. Serum cholesterol levels among vegetarians are significantly lower than among lacto-ovo-vegetarians and non-vegetarians. Controlled trials in human beings using diet or drugs to reduce serum cholesterol show a reduction in the incidence and progression of CHD.

From the standpoint of public health policy, a low intake of saturated fatty acids is the preferred option for preventing coronary heart disease and is the strategy that is still recommended by most international committees. In most industrialized countries, a high total fat intake coincides with a high saturated fat intake. Diets with 40% of energy from total fat-typical of many European countries and North America - often provide 15-20% of the energy from saturated fat. Reducing total fat intake to 30% of energy will therefore have a substantial effect on saturated fatty acid intake to 30% of energy in those populations, while still allowing the different unsaturated fatty acids to contribute up to 20% of energy.

Diseases/Infections

Important Reservoir/Carrier

Transmission

Multiplication in Food

Examples of some Incriminated Foods



Person to Person

Water-borne

Food-borne



Ascariasis

Man

-

+

+

-

Soil-contaminated food

Brucellosis

Cattle, goats, sheep

-

-

+

+

Raw milk, dairy products

Cholera

Man, marine life

+/-

+

+

+

Salad, shellfish

E. coli infections

Man, cattle, poultry, sheep

+a

+

+

+

Salad, raw vegetables, milk, cheese, undercooked meat

Giardiasis

Man, animals

+

+

+/-

-

Vegetables, fruit

Hepatitis A, viral

Man

+

+

+

-

Shellfish, raw fruit and vegetables

Listeriosis

Environment

-b

+

+

+

Cheese, raw milk, coleslaw, meat products

Salmonellosis (other than typhoid)

Man, animals

+/-

+/-

+

+

Meat, poultry, eggs, dairy products, chocolate

Shigellosis

Man

+

+

+

+

Potato/egg salads

Trematode infections

Freshwater fish and crabs, cattle, goats

-

-c

+

-

Undercooked/raw fish and crabs, watercress

Trichuriasis

Man

-

0

+

-

Soil-contaminated food

Typhoid and paratyphoid

Man

+/-

+

+

+

Dairy products, meat products, shellfish, vegetable salads

+ = Yes; +/- = Rare; - = No; 0 = No information.

a No information for infections due to enteroinvasive E. coli. b Transmission from pregnant woman to fetus(es) occurs frequently. c +/- for foodborne trematode infections due to Fasciola hepatica.

SOURCE: The World Health Report, 1996.

CANCER

The relationship between specific dietary components and cancer are much less well established than those between diet and cardiovascular diseases. However, the overall impact of diet on cancer rates throughout the world appears to be significant. For population in industrialized countries, where cancer rates are highest and account for approximately one-quarter of all deaths, some epidemiologists estimate that 30-40% of cancers in men and up to 60% of cancers in women are attributable to diet.

The evidence for the influence of diet on cancer risk is derived from several sources. Correlations between national and regional food consumption data and cancer rates, and studies of the changing rates of cancer in populations as they migrate from a region or country of one dietary culture to another, have led to many important hypotheses. Case-control studies of dietary habits, provide stronger evidence for the effects of diet in relation to major cancers. Many of these observations from human populations have been supported by animal experimental data.

Cancers of the oral cavity, pharynx, esophagus, stomach, large bowel, liver, pancreas, lung, and endomentrium are among those that have been linked repeatedly to dietary factors in different populations. In general, a high intake of total fat - and in some case-control studies also saturated fat - is associated with an increased risk of cancers of the colon, prostate, and breast. The evidence is strongest for cancer of the colon and weakest for breast cancer. The epidemiological evidence is not totally consistent but is generally supported by laboratory data from studies in animals. The experimental data, however, also point to an adverse effect of very high intakes of polyunsaturated fats, at levels that are considerably above current intakes in human populations.

Diets high in plant foods, especially green and yellow vegetables and citrus fruits, are associated with a lower occurrence of cancers of the lung, colon, esophagus, and stomach. Although the mechanisms underlying these effects are not fully understood, such diets are usually low in saturated fat and high in starches and fiber and several vitamins and minerals, including beta-carotene and vitamin A. There is no conclusive evidence that these beneficial effects are due to the high fiber content of such foods.

Sustained heavy alcohol consumption appears to be causally linked to cancer of the upper alimentary tract and liver. Excessive body weight is clearly a risk factor for endomentrial and post-menopausal breast cancers, but the association of these cancers with excessive energy intake per se is less well established. A reduction in risk is also likely when fat intake is reduced towards 30%, especially if this dietary change is combined with a change in other dietary components.

OBESITY

Development of obesity in individuals reflects the interaction of dietary and other environmental factors with genetic susceptibility. However, since there is no convincing evidence that genetic susceptibility accounts for differences in obesity among populations, the differences in prevalence of obesity are largely attributable to "environmental" factors (especially diet and physical activity). Within a single population, those who become obese usually come from overweight families and there is evidence of heredity for obesity. Thus, from a public health point of view, the challenge is to modify the population's environmental circumstances so that the susceptible individual members of the population are less liable to become obese.

There is substantial evidence that dietary factors not only predispose people to chronic diseases, but also that modest change in diet can substantially reduce risk of the major causes of death and disability, i.e., CHD, cancer, and others in the population. An upper limit of fat intake for the population was set by WHO at 30% of calories with the recognition that as additional, more definitive evidence accumulated for cancer, it may be necessary to lower the upper limit for total fat intake to 20-25% of energy. There is also suggestive evidence that obesity may be associated with high total fat intake, but again there is no specific level that marks the lower limit.

The lower limit of total fat intake was set at 15% of calories to take into account the low energy density of diets in developing countries if they are based heavily on cereals and other foods low in energy density. This level of intake would be adequate to provide for essential fatty acid needs.

Epidemiological data suggest that as intake of saturated fatty acids decreases, there is a progressive fall in mortality from cardiovascular disease, but not necessarily below about 10% of SFA energy. Thus 10% was set as the upper limit for the population nutrient goals for saturated fatty acids. The lower and upper population nutrient goals for dietary fiber were set at 16 g and 24 g of nonstarch polysaccharide, respectively. They are consistent with estimates of about 27 g and 40 g of total fiber. The recommendations for total fat and protein account for 25-45% of total calories. Excluding alcohol, the remaining energy intake should be provided by carbohydrate, for which the lower and upper limits for the population were set at 55% and 75%, respectively. As mentioned earlier, there are specific advantages to the intake of complex carbohydrates and thus a lower limit of 50% of calories was set for this class.

CONCLUSIONS

In summary, dietary factors are now known to influence the development of a wide range of chronic diseases, including coronary heart disease, various cancers, hypertension, cerebrovascular disease, and diabetes. These conditions are the commonest cause of premature death in industrialized countries. On current projections, cardiovascular disease and cancer will emerge, or be established, as substantial health problems in virtually every country in the world by the year 2000.