There are many factors causing cancer but unfortunately, we have not been able to clearly identify them. However, the United States Surgeon General published a report in 1988 "that diet could be responsible for up to 70% of all cancers - with a (conservative) 'best estimate' suggesting that diet was responsible for 35% of all cancers." (Coleman, 1995). Agents or substances that promote cancer, called carcinogens, cause cancer through alteration of metabolism in the cells or directly damage the DNA structure in cells. Most of these carcinogens are obtained from food like stored grains, nuts and peanut butter, and numerous potent carcinogens, which are similar to those found in cigarettes, may also be produced in cooking protein-rich food at high temperatures like broiling or barbecuing meat.
Evidences show that fruit and vegetable rich diet lowers the risk of heart disease and stroke. One study done as part of the Harvard-based Nurses' Health Study and Health Professionals Follow-up Study, the longest and largest study about the subject in history in which dietary habits of roughly 110,000 men and women were monitored for 14 years. In this study, it is clear that the higher the average daily intake of fruits and vegetables, the lower the risk of acquiring cardiovascular illnesses, as compared to those with less fruits and vegetables intake, were 30% less likely to have a heart attack or stroke.
According to the Harvard School of Public Health, early studies revealed that a fruit and vegetable diet is likely to be a protection against certain cancers. A research on fruits, vegetables and cancer by the International Agency for Research on Cancer, which is part of the World Health Organization, states that:
"There is limited evidence for a cancer-preventive effect of consumption of fruit and of vegetables for cancers of the mouth and pharynx, esophagus, stomach, colon-rectum, larynx, lung, ovary (vegetables only), bladder (fruit only), and kidney. There is inadequate evidence for a cancer-preventive effect of consumption of fruit and of vegetables for all other sites." However, considering all evidence from human epidemiological, animal, and other types of studies, it appears that eating more fruit "probably lowers the risk of cancers of the esophagus, stomach and lung" and "possibly reduces the risk of cancers of the mouth, pharynx, colon-rectum, larynx, kidney, and urinary bladder." Eating more vegetables "probably lowers the risk of cancers of the esophagus and colon-rectum" and "possibly reduces the risk of cancers of the mouth, pharynx, stomach, larynx, lung, ovary and kidney."
All fruits and vegetables contribute to minimize the risk of heart diseases, but green leafy and cruciferous vegetables, and citrus fruits and juice are the biggest factors in this field. Two studies in Harvard show that adding fruit and vegetable intake by one serving a day can surely reduce risks of coronary diseases by 4%.
For ten years, researchers at Johns Hopkins in the US and Yonsei Univerity in Korea monitored about