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Scientific Basis for Olive Oil, Mediterranean Diet and Cancer Prevention

Author : Eurosciences Communication in co-operation with the Institute of Arteriosclerosis Research at the University of Münster, Germany

1. Introduction

Cancer accounts for about 20% of all deaths in Europe. However, cancer mortality rates are generally highest in the northern and eastern European countries and lowest in the Mediterranean countries. It is estimated that about 35% (range of 10-70%) of all cancer deaths may be attributed to dietary factors.

Epidemiological studies provide most of the evidence on diet and cancer. Good dietary intervention studies of the risk of cancer, however, need to be extensive and long-term, and so are rare.

2. Cancer and the Mediterranean diet

2.1 Role of body weight

Obesity is a clear risk factor for postmenopausal breast cancer and cancer of the prostate, endometrium and gall bladder. It is also probably a risk factor for renal-cell carcinoma and cervical cancer. The public health message is "avoid obesity and being overweight". This is reinforced by the link between obesity and diseases such as heart disease, diabetes, gallstones, etc. As noted in previous Fact Sheets , the Mediterranean diet is suitable for the prevention of obesity, and therefore for the prevention of obesity-related cancer.

2.2 Role of dietary fat

Population studies have shown that total fat intake is associated with cancer at a number of sites, particularly the colon, breast, endometrium, ovary and prostate gland. All these cancers are related to a Western-type diet and to excess energy intake. However, the public health message is not clear, because prospective studies with breast cancer have failed to show any relation to total fat intake, and one study on gastric cancer has suggested a protective effect of fat intake derived from meat and dairy products, against gastric cancers.

International correlation studies suggest that the type of dietary fat is important in the aetiology of fat related cancers. Animal fat consumption per capita is positively correlated with colon, prostate, breast and ovary cancer mortality rates. The case for the relationship between animal fat intake and the risk of colorectal cancer is particularly strong. In contrast, mortality rates of colon cancer are relatively low in Greece, Spain and Southern Italy, where the intake of animal fat is low and olive oil is the most common type of fat consumed.

Until recently, vegetable fats/oils were considered neutral with respect to the risk of cancer. However, recent analyses suggest that olive oil may have a protective effect against cancer at certain sites, particularly breast cancer. PUFAs of the n-series in human studies show some protective properties against cancer, whereas they appear to be neutral with respect to the risk of cancer. However, the role of n-6 PUFA becomes less clear when evaluating animal studies. Investigations have been carried out on laboratory animals, indicating that n-6 PUFAs are more likely to increase the risk of cancer than other types of fatty acids.

2.3 Role of protein

There is no evidence showing that there is an independent relationship between protein consumption and cancer risk.

2.4 Role of complex carbohydrates and dietary fibre

It is not clear which component of the total complex carbohydrates provides the protective effect which is seen. Cereals appear highly protective against cancers of the colon, breast, endometrium and prostate. A prospective study has confirmed the protective effect of high-fibre foods against colorectal cancer.

2.5 Role of fruit and vegetables

Epidemiological evidence shows that a high intake of fruit and vegetables, particularly raw vegetables, protects against cancers at different sites, especially those of the digestive and respiratory tracts and the hormone-related cancers. They have an anti-carcinogenic action at a wide range of sites and there is no positive correlation between fruit and vegetable intake and cancer. They contain a variety of anti-carcinogenic agents, e.g., carotenoids, vitamins C and E, dietary fibre, selenium, glucosinolate, indoles, flavenoids, protease inhibitors, and plant sterols. Only the actions of anti-oxidant vitamins and provitamins have been supported by human epidemiology, at present. However, it is likely that no single agent functions as a key protective factor in isolation, but that all of them have some protective role under some circumstances.

3. Role of olive oil in cancer

Various epidemiological studies indicate that the regular consumption of olive oil is inversely related to cancer at different sites. Most of the studies address the relationship between olive oil and breast cancer or gastric cancer. While more research work is needed, the existing evidence consistently, although not conclusively, supports a protective role of olive oil in breast cancer prevention. A protective effect of olive oil in gastric cancer is less clear. The only conclusion that can be drawn at present for gastric cancer prevention is that an increased fruit and vegetable intake seems to be helpful.

Although there are also findings which suggest protective effects of olive oil against cancer at other sites, e.g., colon, endometrium and ovary, the evidence is limited, because the number of studies is small, and their results cannot be more than an indication of a possible effect. However, no study supports a tumor-promoting effect of olive oil.

4. International recommendations for cancer prevention

There is a general consensus that diet is an important component in the aetiology of cancer. Scientific evidence is primarily derived from epidemiological studies as well as animal and in vitro experiments. In the former, foods or food groups are more strongly associated with the risk of cancer than nutrients, and for many foods the results are neither persuasive nor consistent. Well-designed, strictly controlled intervention studies in humans which could support the role of single foods or nutrients in cancer prevention with sufficient strength are missing. Thus, the scientific evidence for detailed recommendations with respect to cancer prevention is limited.

Several health authorities have made dietary recommendations for cancer prevention. The recent guidelines of the American Cancer Society consist of 6 points:

• Avoid obesity.
• Cut down on total fat intake.
• Include a variety of vegetables and fruits in the daily diet.
• Eat more high-fibre foods, such as whole grain cereals, vegetables and fruits.
• Limit consumption of alcoholic beverages, if you drink at all.
• Limit consumption of smoked, salt-cured and nitrate-cured foods.

Similar recommendations are given by the National Cancer Institute, but their guidelines differ from those of the American Cancer Society by specifying levels of nutrient intake for the general population (no more than 30% of total calories from fat and 20-30 grams of dietary fibre daily).

The United States has started to implement these guidelines with a nation-wide programme, called the "5-a-day for better health programme", disseminated via supermarkets, restaurants, the media, the public and research. The recommendations are to consume mainly vegetable foods, to eat five or more portions of fruit and vegetables per day, and, in addition, to eat six or more portions of bread, cereals or grain per day.

"Europe against Cancer", the programme by the European Commission, has some recommendations on nutrition and diet:

• Increase the daily intake of fresh fruits and vegetables, as well as high fibre grain products
• Avoid obesity, increase regular physical activity, and limit the intake of high-fat foods
• Reduce alcohol consumption

The results of the November 1996 WHO conference "Nutrition in prevention and therapy of cancer" will be published as consensus statements in the course of 1997. As a general policy statement for reducing the risk of cancer, it was said that fruits, vegetables and whole-meal cereals should be the main components of the daily diet. Furthermore, avoidance of obesity and high alcohol intake, coupled with regular physical activity, can contribute to a reduction of the risk of cancer. In addition, it was stated that there is no kind of diet which could treat colon, gastric, breast or lung cancers.

Source: http://europa.eu.int/comm/agriculture/prom/olive/medinfo/index.htm