Many studies have looked at the role of dietary factors in increasing or reducing the risk of cancer. Most studies have shown that dietary factors are only associated with a change in cancer risk, but not completely responsible for it. To prove association of a dietary factor and cancer is challenging as there are numerous dietary constituents and numerous cancers. Many dietary factors may not act in isolation and it may be their interaction with other dietary, lifestyle and/or genetic factors.
Dietary factors have been thought to contribute to about 30% of cancers in Western countries, making diet second only to tobacco as a preventable cause of cancer. The contribution of diet to cancer risk in developing countries has been considered to be lower, perhaps around 20%.
Although fruits and vegetables supply less than 5% of total energy intake, the concentration of micronutrients in these foods is greater than in most others. They are rich in Antioxidants, Minerals, Fiber, Potassium, Carotenoids, Vitamin C, Folate and other vitamins.
Consumption of fruits and vegetables appears to be less important in cancer prevention than previously assumed. With fortification of breakfast cereal, flour, and other staple foods, the frequent consumption of fruits and vegetables has become less essential for cancer prevention. Though, high consumption of fruits and vegetables during childhood and adolescence could be more effective in reducing cancer risk than consumption in adult life due to the long latency of cancer manifestation.
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What is the protective role of carotenoids and lycopene?
Carotenoids are antioxidants prevalent in fruits and vegetables. They enhance cell-to-cell communication, promote cell differentiation, and modulate immune response. But studies have not revealed evidence of a protective effect of beta-carotene.
Lycopene is a carotenoid mainly found in tomatoes and has antioxidant properties. Frequent consumption of tomato-based products may be associated with a decreased risk of prostate, lung, and stomach cancers.
How effective are soy products and nuts in cancer prevention?
Soybeans contain isoflavones, phyto¬estrogens that compete with estrogen for the estrogen receptor. In Asian countries, which have a high consumption of soy foods, breast cancer rates have been low. Soy consumption may affect estrogen concentrations differently depending on the endogenous baseline level.
Studies have shown that there is a modest decrease in the risk of breast cancer with soy consumption, with childhood consumption being more effective. In a recent meta-analysis of 18 epidemiologic studies, including over 9,000 breast cancer cases, frequent soy intake was associated with a modest decrease in risk (odds ratio = 0.86; 95% CI, 0.75 to 0.99). Wu et al. observed that childhood intake of soy was more relevant to breast cancer prevention than adult consumption.
Nuts are rich in unsaturated fatty acids, high-quality protein, fiber, vitamins (e.g., folate, niacin, and vitamin E), minerals (e.g., potassium, calcium, and magnesium), and phytochemicals (e.g., carotenoids, flavonoids, and phytosterols). They have anticarcinogenic, anti-inflammatory, and antioxidant properties.
Studies have shown their consumption to reduce the mortality with breast and pancreatic cancer. In two large, independent cohorts of nurses and other health professionals, the frequency of nut consumption was inversely associated with total and cause-specific mortality, independently of other predictors of death for breast cancer and pancreatic cancer.
What is the role of dietary fiber?
Dietary fiber contains plant polysaccharides and lignin which are resistant to hydrolysis by the digestive enzymes. They have the following effects-
- “Bulking” effect, which reduces colonic transit time
- Binding of potentially carcinogenic luminal chemicals.
- Fiber may also aid in producing short-chain fatty acids that may be directly anticarcinogenic
- Fiber may induce apoptosis.
But studies till date do not indicate an important role for dietary fiber in the prevention of cancer.
Role of Vitamin D
In cancers of the oral cavity, pharynx, and esophagus, dietary factors that may be protective are fruits and vegetables, micronutrients like riboflavin, folate, vitamin C and zinc. Consuming drinks and beverages at high temperatures may be causative.
Nasopharyngeal cancer particularly common in Southeast Asia is associated with a high intake of Chinese style salted fish, especially during early childhood. Chinese-style salted fish is a special product which is usually softened by partial decomposition before or during salting.
Ingestion of foods contaminated with the mycotoxin aflatoxin is an important risk factor for Hepatocellular carcinoma.