Nutrition and Micronutrition : Human Risk Factors or Cancer Prevention

Abstract

Dietary factors have been estimated to be responsible for 30-40% of all cancer. Moreover, epidemilogical studies have revealed many correlations (coincidental, associated or precursor lesions) between the risk of cancer and the prevalence of other diseases. Thus, since all colorectal cancers arise in precursor areas of dysplasia, and since an adenoma is defined histologically as an area of dysplasia, adenomas are precursors of colorectal cancer.

In the case of colon cancer there is considerable evidence that a high proportion of large bowel cancers develop from a poliplïod precursion lesion, the adenoma. Study performed in France showed the role of diet in the occurrence of colorectal adenoma. The risk of colorectal adenomas decreasing linearly with increasing daily consumption of polysaccarides and natural sugar. In contrast sugar, added to food and drinks was observed to have the opposite effect.

In the oropharyngeal cancers the commonest precursor lesion is leukoplakia and the incidence of the cancer is strongly correlated with the intake of alcohol and smoking habit. It is also associated with a low intake of retinoids and carotenoids. In oesophagial cancer, the major histological type of cancer is squamous cell carcinoma (SCC) which accounted for more than 90% of all cases and the most widely recognized precursor of SCC is dysplasia in achalasia. In the large studies carried out in China, the major risk factors for dysplasia were low intakes of retinol, riboflavin, and zinc.

In the aetiology of gastric cancer (GC) It is widely recognized that diet is the important factor. Reduction of nitrates (NO₃) to reactive nitrite (N₂) formed through the action of nitrate-reducing bacteria in the saliva and an hypoacidic stomach can result in the subsequent formation of N-nitroso compounds (NOC) that can act as promoters during the later stage of carcinogenesis. Populations at high GC risk had a high N₃ content in drinking water. Several case-control studies have shown remarkable consistency indicating that heavy use of salt would be compatible with a 50% increase in GC risk. Another factor that is known to have a large impact on GC is the dry heating (pyrolysis) of food rich in aminoacids and proteins which produces highly mutagenic aromatic amine compounds. Thus, the development of GC is multifactorial: excessive salt intake, low intake of fresh fruits and vegetables and Helicobacter pylori infection.

Finally, despite immense research efforts, the causes of breast cancers are still incompletely understood. Carcinogenesis consists of a series of steps which involve endogenous (hormones) as well. as exogenous (e. g. nutrition) factors similar to endometrial cancer in which the triad obesity-hypertension-diabetes and the unopposed action of oestrogens had been identified as potential risk factors.

In conclusion carcinogenesis often involves a multistage sequence of precancerous lesions, each step having its own set of causal agents. The role of diet and micronutrition in individual steps in the multistage carcinogenesis process in likely to provide the best clues to strategies for cancer prevention. Reduced fat intake, increased fiber intake, calcium intake and antioxidant compounds may be important not only in providing antioxidant substances but also in limiting the production of toxic oxidation compounds during food processing.

Since the oyster extract has been shown to be particularly rich in as well as Zn and in antioxidant compound, studies have been undertaken to show its effect on health promoting and as actively protective in cancer.