Observational studies show that those with the highest intake of plant foods are the least likely to suffer from Western lifestyle disease. In this respect plant foods are protective of cancer, cardiovascular disease, obesity, type 2 diabetes and arthritis. Despite voluminous literature investigating the benefits of plant based diets, it is still not clear exactly why plant foods are protective of disease. The reason for the ambiguity is likely that there is not a single reason for the benefits of plant foods, but likely a number of factors come together to provide synergistic benefits to the health. Of course, the protective effects of plants may be due to the fact that those that eat fewer plant foods tend to eat the typical Western diet, the latter being associated with an increased risk of Western disease due to the metabolic poisons it contains. Therefore the benefits of plant foods could come from positive factors in the plants in combination with the lack of negative factors in the typical Western diet.
Plants are a good source of fibre and the typical Western diet is absent of this fibre. Fibre is beneficial to the health because it slows the absorption of glucose from the gut to the blood, and this is necessary for the correct metabolism and oxidation of fuels. The low fibre content of the typical Western diet leads to rapid elevations in blood sugar and this causes insulin resistance, weight gain, and a cluster of metabolic changes termed the metabolic syndrome, the later which increases the risk of type 2 diabetes considerably. Plants are also a rich source of chemicals that are bioavailable in humans and which may have antioxidant properties. These chemicals include polyphenols and carotenoids, and evidence suggests that their ability to reduce oxidative stress in humans is the reason for their protective effects against disease. Plants are also good sources of vitamins that protect from Western diseases and may contain high levels of minerals if the soils they are grown on are rich in those minerals.