The association of diet changes with new diseases was not easily made. Nutrition is a relatively new science. Even something as basic as the discovery of vitamins didn't take place until well into the 20th Century. In many ways World War II marked the beginning of modern nutrition. Vast amounts of data on the health of draftees created statistical baselines. The Dutch famine imposed by a German blockade was especially illuminating because reliable records were kept relating health to dietary intake within a previously healthy population.
One of the nutritional surprises from World War II was that the incidence of heart attacks and several related ailments declined sharply during the Dutch famine. The most plausible explanation seemed to be that the marked decline in the availability of meat and the increased dependence on vegetable foods benefited the cardiovascular system in some way. This idea was reinforced by similar findings in other places forced to reduce meat consumption during the war, including New York City where a meatless Tuesday campaign had been initiated. Nutritionists began looking at populations from all over the world to see if this pattern held. It did. Some of the most convincing studies showed that Asian immigrants to the United States began to have the same increased rates of cancer and heart disease as Americans while their relatives who remained in Asia didn't. This seemed to show that genetics was not playing a big role and the finger was pointing more directly at diet.
A fascinating exception to this pattern was found in France. The French diet is typically very high in animal fats and cholesterol, which have been closely associated with heart disease and cancer. Despite this fact, the prevalence of these diseases in France is far lower than in the United States or England. Nutritional detectives investigating this so-called "French Paradox" looked for differences in diet that might explain the differences in disease rates. They found that along with meat, cheese and butter, the French people are very fond of vegetables, garlic and red wine. Their suspicions were that compounds, perhaps the abundant range of antioxidants in these foods were primarily responsible for the favorable disease rates.
By the 1990s it had become quite clear that fruits and vegetables offered a significant protection against many types of cancer and cardiovascular disease. Over 200 well-designed scientific studies have now shown this relationship. With almost no exceptions, the studies agree that the more fruits and vegetables people eat, the lower their risk of cancer.