Health effects of whole grain
Prospective studies have found that the intake of whole grain is associated with a number a of health benefits. A high whole grain intake is associated with a lower risk of certain chronic diseases including type 2 diabetes, cardio-vascular disease, obesity/weight management and certain types of cancer. The evidence on type 2 diabetes and cardio-vascular disease is rather well documented, but especially with regard to cancer, more research needed to elucidate the role of whole grains in disease prevention.
Type 2 diabetes
Type 2 diabetes (or non-insulin-dependent diabetes mellitus (NIDDM)), is a metabolic disease resulting in a combination of an insufficient production of insulin and/or reduced insulin sensitivity. Genetic predisposition as well as overweight and obesity are risk factors of developing type 2 diabetes.
A high consumption of food with a high glycemic index (foods with a high content of carbohydrate that are quickly digested and releases glucose rapidly to the blood stream) is believed to cause hyperglycemia (high blood sugar) and insulin resistance and possibly thereby increase the risk of developing type 2 diabetes. Whole grain products have a lower glycemic index compared to refined grain products and thereby lead to a smaller postprandial glucose and insulin response. Whole grains are furthermore believed to decrease the fasting glucose levels of the blood. It is therefore believed that whole grain foods can reduce insulin resistance and in addition improve glucose sensitivity and consequently be preventive towards the development of type 2 diabetes.
The evidence from prospective studies has rather consistently found a protective effect of whole grains on the development of type 2 diabetes. A Cochrane Review has been published on whole grain and risk of type 2 diabetes. The conclusion of the review was that more research is needed to elucidate the role of whole grain in the prevention of type 2 diabetes.
Cardio-vascular disease
Cardio-vascular disease is one of the major causes of death in the Scandinavian countries. Back in 1977 Morris et al found that the cereal fiber had a protective effect on the development of cardio-vascular disease. Since that time a lot of research has been conducted on whole grain and cardio-vascular disease. Research from the USA and Finland suggests that the effects of fiber from whole grain on cardio-vascular disease exceed the effects of fiber from fruit and vegetables. The effects of fiber from whole grain seems more consistently and strongly associated with lower risk of cardio-vascular disease. Furthermore studies have shown that fiber alone cannot explain the effects of whole grain on risk of cardio-vascular disease.
The mechanisms of the protective role of whole grain towards development of cardio vascular disease are still unclear. Possible mechanisms include total cholesterol and LDL cholesterol lowering properties of soluble fiber and oleic and linoeic acids. Furthermore, the phytoestrogens from especially rye are believed to play a role as well, but the exact mechanism is still to be clarified. Also, the antioxidants of whole grain are believed to suppress LDL oxidation and thereby protect against cardio-vascular disease.
Prospective studies have rather consistently found a protective role of whole grains in the prevention of cardio-vascular disease. More research is, however, needed to clarify the exact role of whole grains in the prevention of cardio-vascular disease.
Obesity/weight management
Prospective studies have suggested that whole grain intake is inversely associated with a BMI and waist circumference. Other studies have, however, not been able to find an association.
The mechanisms behind the possible obesity-preventive properties of whole grains are not fully elucidated. One of the possible mechanisms is that the beneficial effects of whole grain on insulin sensitivity and fasting glucose have an indirect effect on the weight. Another mechanism is that whole grain food because of the low energy density and high fiber content induce satiety resulting in a lower energy intake. Also, the high content of soluble fibers, especially found in barley and oats, prolongs the gastric emptying and slow the uptake of nutrients in the small intestine resulting in a lower energy uptake. Furthermore fibers from whole grain have shown to enhance the secretion of gut hormones e.g. cholocyctokinin known to cause satiety.
More prospective studies are needed to reveal the association between whole grain and weight management.
Cancer
The World Cancer Research Fund / American Institute for Cancer Research (WCRF/AICR) have estimated that about one third of all cancer cases can be prevented by appropriate food and nutrition, regular physical activity and avoidance of obesity. With cancer being one of the diseases causing most deaths in the Western world, knowledge of the possible cancer protective role of foods is very important in reducing the incidence of cancer.
The WCRF/AICR report on Food, Nutrition, Physical Activity, and the Prevention of Cancer concludes that it is possible that foods containing fiber are protective of colorectal cancer, also the report conclude that foods containing fiber have a suggestive protection of cancer in the oesophagus, but the evidence remains limited. The report does not assess the association between whole grain foods and cancer directly. The effect of the diet on the development of cancer varies very much from cancer site to cancer site – diet is for instance believed to be responsible for about 70 % of the cases of colorectal cancer in the US, but only 20% of the cases of lung and bladder cancer.
Several mechanisms of the cancer protective role of whole grain have been suggested. Firstly, the short-chain fatty acids produced by the fermentation of fibers are believed to have tumor suppressive effects. Also, the phytoestrogens and antioxidants of the whole grains are believed to be cancer protective. Finally the beneficial effects of whole grain on glucose responses are believed to protect against colon and breast cancer.
Prospective studies have most consistently found a protective role of whole grain and colorectal cancer. Other cancer types have also been mentioned, but the evidence from prospective studies is still very sparse.