Evidence suggests that consumption of whole grain foods is associated with protection from Western lifestyle diseases. In contrast, consumption of refined grains increases the risk of a number of major Western diseases including obesity, cardiovascular disease, type 2 diabetes and cancer. Because of the different health outcomes between whole grains and refined grains, it is worth considering the nutritional differences between the two forms of cereal grain in order to assess the reasons for the discrepancy in health outcomes. Generally whole grains retain the original composition of the cereal grain while refined grains have certain components removed during processing. Grains possess an outer bran layer, an inner endosperm later and a germ layer. The bran is a layer between the outer seed coat and the aleurone layer of the endosperm. The endosperm is a starchy layer within the aleurone layer and within the endosperm in the germ later. The germ is the embryo for sprouting a new plant and the endosperm is a source of energy for this growth until photosynthesis can proceed.
Processing the grain tends to remove the outer seed coat, the bran and the germ layers. This is done for taste reasons, as white refined flour from cereal grains is prefered by the Western palate. In addition, white flour has a longer shelf life compared to its whole grain equivalent, and this is beneficial for the food industry. This leaves the starchy endosperm as the main component of white refined flour. The bran, germ and aleurone layers contains most of the fibre in the grain and also many of the vitamins and minerals and other nutrients. Removing the fibre is problematic because it may increase the rate of digestion of the starchy endosperm layer, and this can increase the absorption rate of the subsequent glucose considerably. Refined grains therefore tend to produce greater rises in blood sugar when compared to their whole grain alternatives. Over time the exaggerated blood sugar responses caused by fibreless refined grains can cause insulin resistance and this contributes significantly to disease.
However, the refining of the micronutrients from cereal grains can also cause health problems. The mineral chromium for example is required for correct function of the insulin receptor. Without chromium as a cofactor, the insulin receptor becomes insensitive to the hormone insulin and this causes increases in blood sugar levels and metabolic dysfunction. Grains are also a good source of antioxidants, and refining the bran, germ and aleurone layers from grains removes most of their antioxidants. However, whole grains cereals might also contribute to ill health if their processing results in a flour that is too fine. This is because the outer layers of fibre and intact cell walls might be required to provide the beneficial glycaemic effects of whole grains. This certainly explains the beneficial glycaemic effects of rolled oats, grains that retain not only their original composition, but also much of their structure. As will all plants foods eating them in an as unprocessed form as possible may therefore be the most advantageous situation for health.