Plant foods are good for the health. This is well established in the nutritional literature. What is not well established is the reason for the protective effects of plants in human nutrition. Plant foods comprise of complex tissues that contain a myriad of substances and so it is difficult to isolate the beneficial components that have specific biochemical effects in animals. Of course, it may be that multiple components act synergistically together to produce favourable biochemical changes, and this adds a further layer of complexity to muddy the waters. Plants contain non-digestible carbohydrates, also known as dietary fibre, and fibre certainly contributes to the health benefits of plants. Further, plants are rich sources of essential micronutrients including minerals, vitamins, amino acids and fatty acids, and in this way supply animals with factors required for correct growth and development. However, plants also contain many other non-essential substances, many of which have been shown to be biologically active in humans.
A number of groups of chemicals are known to be synthesised in plants. One of the largest groups of these chemicals are the polyphenols, chemicals with multiple ring structures based on the chemical phenol. An important subgroup of polyphenols are the flavonoids, a group of over 4000 chemicals many of which are present in edible plants and many of which have been shown to be biologically active in human tissues. The flavonoids can be further subdivided into classes, and of these classes the flavonols show promise as potent health promoting phytochemicals in humans. Important dietary flavonols include quercetin, rutin, kaempferol, myricetin, and galangin. These flavonols may exert beneficial effects on human health, and one ways they may do this is through improvements in glycaemic control possibly through improvements to insulin sensitivity or through delaying the absorption of glucose from the gut. These effects would suggest that flavonols may be beneficial to those individuals with insulin resistance, such as those with type 2 diabetes.
Studies have investigated the associations between flavonols and particular health conditions, including type 2 diabetes. For example, in a recent study published in the Journal of Nutrition1, researchers investigated the effects of habitual intakes of flavonols on the risk of developing type 2 diabetes. The results of the study showed that for each 2.5 increase in the intake of flavonols there was a 26 % reduction in the risk of developing type 2 diabetes. In addition, the authors also found that for every 2.5 fold increase in the intake of flavan-3-ols there was an 11 % reduction in the risk of type 2 diabetes. These results suggest that those with high intakes of certain flavonoids may be protected from the development of type 2 diabetes. However, care must be taken when interpreting these results as plant foods containing flavonols and flavan-3-ols also contain other substances that may contribute to this effect. However, despite possible confounding variables, there are clear mechanisms by which flavonoids may benefit blood sugar controls and so the effect may be real.
Dr Robert Barrington’s Nutritional Recommendation: Quercetin in the main dietary flavonol, and it is found in wine, tea and onions. These three foods have all been shown to exert beneficial health effects in humans and this may be explained partly by the presence of high concentrations of quercetin. Flavan-3-ols are flavonoids that include the catechins. Catechins are found mainly in tea and apples, with small amounts in other fruits and vegetables. The inverse association of tea and caffeine intake with body weight is interesting because as body weight increases the risk of type 2 diabetes increases. The flavan-3-ols have also been shown to have beneficial glycaemic effects, which may also explain their protective effects against type 2 diabetes.
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