That whole grains are protective of weight gain and obesity is increasingly obvious from the growing body of relevant research in the nutritional literature. Traditional diets such as the Mediterranean diet and the Okinawan diet contain high amounts of whole grains, and adherence to such regimens is associated with lower body weights compared to more modern refined diets. The generally accepted nutritional definition of whole grains is any kernel (caryopsis) that contains the endosperm, germ and bran components in their original ratios from the unprocessed plant. The kernel containing these components can be either intact, ground, cracked or flaked. The most common whole grains consumed worldwide are rice, wheat, maize, oats, barley, rye, millet, sorghum and triticale. However, increasingly these grains are refined and eaten in combination with low quality diets. This trend is reflected in increasing rates of Western diseases in developing nations.
The processing of whole grains results in a loss of fibre, antioxidants, phytooestrogesns, phenolic acids, phytic acid and micronutrients as the bran and germ layers of the grain are removed and discarded. This leaves just the starchy endosperm which improves the taste and texture for the consumer, as well as increasing the shelf life for the manufacturer. The loss of fibre is particularly concerning because evidence suggests that this component may be primarily responsible for protection from obesity because of its ability to reduce postprandial glycaemia be delaying glucose absorption. In addition, the fibre can also be fermented to short chain fatty acids (SCFA) in the colon. These SCFA can alter the balance of colonic microbiota and may decrease appetite through the release of peptide-YY and glucagon-like peptide 1 (GLP-1). Further, fibre may also be beneficial at maintaining correct weight because it can decrease the energy density of the food ingested.
This ability of fibre to decrease the energy density of the food consumed is often overlooked. However, it is of great interest because it may explain a significant portion of the weight loss effects of fibre. This is because research suggests that the energy density of the food is one of the determining factors in the amount of calories consumed at a meal. In particular, humans and other mammals tend to eat a consistent weight of food rather than a particular amount of energy. Therefore foods with lower energy densities may involuntarily cause energy restriction. This is different to forced energy restriction because it is naturally regulated and therefore does not lead to metabolic countermeasure to increase appetite. The size of the energy reduction caused by whole grain intakes has been estimated to be around 100 kcal per day for an increase in dietary fibre of 20 to 25 grams. This represents a decrease of around 3 to 4 % of the digestibility of energy in food.
The lower energy densities of whole grain foods relates to the greater water holding capacity of the fibre they contain. However, this effect is variable and is dependent on the type and amount of fibre present in the grain. In particular, the viscous fibre content can have a strong influence of the water carrying capacity and the energy density of the grain when ingested. The grains with higher concentrations of soluble fibre may therefore be superior with regard their effects of body composition. The ability of whole grains to decrease the energy intake of humans appears to be dose dependent. Studies using lower intakes of fibre may therefore explain the inconsistent effects of fibre on energy intake. For example, some research has reported that there is a 36 kcal per day increase in faecal energy content for every 5 grams of fibre, but that at lower fibre intakes this effect is not apparent. The lack of fibre in refined grains may therefore be a major contributor to obesity.
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