Dietary fibre is of interest to nutritionists because research suggests that it may have health benefits. Dietary fibre is generally classed into two groups based on its solubility in water. Water insoluble fibres tend to provide roughage to foods and aid gastric transit. However, water soluble fibres have the additional property of affecting blood lipids and may be important in the prevention of certain diseases, most notably cardiovascular disease. Soluble fibres are thought to alter blood lipid levels by binding to bile acids and preventing enterohepatic recirculation. The increased excretion of bile acids may therefore increase the hepatic conversion of cholesterol to bile acids. Water soluble fibre also has the property of being able to increase the viscosity of food mass within the gastrointestinal tract, which may form a barrier on the mucosa and prevent the absorption of glucose and other nutrients, including bile acids.
Oats are a very good source of soluble fibre, and research has shown that frequent consumption will reduce blood cholesterol levels significantly. This reduction tends to be greatest in those men and women with the highest cholesterol levels suggesting that oats have a normalising effect on blood cholesterol levels. The main soluble fibre component of oats is a polysaccharide called β-glucan. β-glucan is a non-starch polysaccharide which is composed of glucose units linked by β-(1-4) bonds separated every two to three units by a single β-(1-3) bond (figure 1). Humans do not have the enzymes necessary to digest the glucose units from these β-glucan chains and so the carbohydrates pass through the gastrointestinal tract undigested. Bacterial hydrolysis of the bonds is possible however, and this dietary fibre is thought to provide a food source for the colonies of microorganisms within the colon.
Figure 1. The structure of β-glucan
As well as soluble fibre, oats also contain tocopherol and tocotrienol isomers of vitamin E. Research suggests that vitamin E is able to protect humans from cardiovascular disease. This makes it difficult for researchers to assess the effects of the soluble fibre component of oats on cardiovascular function. While it is clear that oats do lower blood cholesterol levels, it is still unclear whether the cardio protective effects of oats is derived from the fibre content and its effects on cholesterol, or the vitamin E content of the oats and oat bran. Whatever the mechanism, oats are clearly a food that should be included in a healthy diet if cardio protection is a priority. In addition to the fibre and vitamin E content, oats possess higher levels of protein and fat than other carbohydrate sources which has a stabilising effect on insulin levels.
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