Whole grains are defined as grains with the same ratio of bran, germ and endosperm as in the original plant. Grains contain a tough outer bran layer that serves to protect the inner layers from disease, water and sunlight. The inner germ layer is the embryonic part that will form the new plant and between these inner and outer layers is the endosperm, which provides the energy for the growing germ. Research suggests that whole grains have health benefits such as reducing the risk of death from cardiovascular disease, as well as decreasing risk of obesity and diabetes. These benefits are attributed to the numerous phytonutrients contained within whole grains. The majority of the beneficial phytonutrients in whole grains tend to be in the outer protective bran layer and the inner embryonic germ layer, but the germ contains a not inconsiderable quantity.
The outer bran coating that protects both the inner germ and endosperm contains a number of phenolic compounds, vitamins and minerals, as well as high amounts of fibre. The endosperm is rich in carbohydrate, protein vitamins and minerals and makes up the largest component of the grain. The germ contains vitamins, minerals protein and fat. Generally, grains contain a number of phenolic compounds including derivatives of benzoic acid and cinnamic acids such as ferulic acid, vanillic acid, caffeic acid, syringic acid, p-courmaric acid, anthocyanidins, quinines, flavonols, chalcones, flavones, flavanones and amino phenolic compounds. In the plants these phenolic compounds provide antioxidant protection to the grain, produce astringency that deters consumption by predators, and generates physical and chemical barriers by cross linking with carbohydrate molecules.
When consumed by humans that same phenolic compounds are able to provide protection from free radicals by donation of hydrogen atoms in chain breaking reactions. The antioxidant content of whole grains is directly related to their phenolic compound, with corn having the highest phenolic content followed by wheat, oats and rice. The antioxidant content of whole grains compares favourable with fruits and vegetables, with an average 41 gram serving of whole grain breakfast cereal containing similar antioxidant capacity as an 85 gram serving of fruit and vegetables. As well as phenolic acids, the antioxidant effects of whole grains are boosted by the presence of carotenoids with leutein, zeathanthin, β-cryptoxanthin, α-carotene and β-carotene being the most common. Whole grains also contain high amounts of vitamin E, with β-tocopherol being the most common form of vitamin E in wheat.
Research is interested in the sterol and stanol content of whole grains because evidence suggests that they may be involved in the inhibition of cholesterol production in humans. Sterols and stanols are also thought to increase the excretion of cholesterol, further producing a lipid lowering effects. Other substances that may be involved in the lowering of plasma cholesterol in whole grains include the dietary fibre, oligosaccharides and the resistant starch component. In addition to reducing the plasma cholesterol concentration in humans, the fibre component of whole grains is associated with normalisation of insulin and glucose levels. This glycaemic stabilising effect is thought to provide protection against the development of diabetes and metabolic syndrome. Cancer protection may be provided in whole grains from intestinal products of lignin digestion in the colon, as well as from protease inhibitors, lectins, phytic acid, amylase inhibitors, saponins and phenolic acids.
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