Fruit and vegetable consumption and risk of overall mortality and gastrointestinal cancer

Abstract

Fruits and vegetables are considered a major part of a healthy diet, because consumption increases satiety and reduces total energy intake by replacing more energy dense foods. In addition, they contain dietary fiber, vitamins, minerals and thousands of naturally occurring biologically active compounds (phytochemicals). A minimum consumption of 400 grams of fruits and vegetables per day is recommended to prevent noncommunicable diseases, such as cardiovascular disease, certain types of cancer and diabetes, and at the same time prevents nutritional deficiencies. This thesis studies the association between fruit and vegetable consumption and the risk of overall mortality and gastrointestinal cancers. This thesis shows that people with high consumption of fruit and vegetable have a 10% lower risk of overall mortality and achieved a similar risk 1.1 years later when compared with people with a low consumption. This risk reduction was based on reduced risks of death from diseases of the circulatory, respiratory and digestive systems when comparing people with high versus people with low consumption. In contrast, an increased risk of death from diseases of the nervous system was seen in these people. Associations were more notable for (raw) vegetable consumption when compared to (cooked vegetables or) fruit consumption. When focusing on gastrointestinal cancers, we observed a slightly lower risk (7%) of colon cancer for every increase in cabbage consumption of 40 grams per day, but a slightly increased risk (5%) of colon cancer for every increase in mushroom consumption of 10 grams per day. Additionally, a high variation in consumption of fruits or vegetables does not seem associated with the risk of colon or rectal cancer, although people with a high variety in fruit consumption showed a higher risk of rectal cancer when compared with people with a low variety. This thesis also includes a study on circulating levels of antioxidants and the risk of colon or rectal cancer. People with high levels of plasma retinol (vitamin A) had a 37% lower risk of colon cancer and a 54% lower risk of proximal colon cancer when compared with people with low levels of plasma retinol. No other compounds with antioxidant characteristics seemed associated with colon or rectal cancer. The role of one-carbon metabolism, i.e., the metabolism around folate (vitamin B9), was studied by examining the association between related genetic variants and pancreatic cancer. Suggestive evidence for genetic associations were observed in the first part of the study (cohort studies), but were not replicated in the second part of the study (case-control studies). Lastly, the reliability of a single measure of circulating compounds with antioxidant characteristics or compounds related to one-carbon metabolism was studied. Measures for homocysteine and vitamin B12 were highly reliable, but all other compounds showed a (moderately) low reliability. This means using a single measure in studies can lead to a serious attenuation of the risk estimates. The studies included in this thesis result in a better description of the impact of fruit and vegetable consumption on general (and gastrointestinal) health and therefore result in better policies in the context of public health.