Expanded commercial availability and new concentrations of this long-trusted ingredient is good news for consumers watching their cholesterol, blood pressure or weight. GREGORY STEPHENS, RD, separates the oats from the chaff
The oat (Avena sativa) is a cereal grain that has been cultivated for thousands of years and is one of the oldest human food staples. Oats have the highest protein quality and concentration of any cereal. The most remarkable constituent of the oat, however, is its unique soluble fibre, called beta-glucan.
A large and steadily expanding body of clinical evidence attests to the extraordinary therapeutic significance of oat soluble fibre, which is a unique mixed linkage beta-glucan comprised of (1-3),(1-4) beta-d-glucan. These primary health benefits include cholesterol reduction and other risk factors for heart disease, also characterized as metabolic syndrome (or Syndrome X). Metabolic syndrome consists of a constellation of metabolic disorders including elevated cholesterol, insulin resistance, hypertension and obesity.
Beta-glucan is the principal constituent of cell walls in the oat seed and, accordingly, it is present in relatively low concentrations (about 2-4 per cent). The outer, branny regions of the oat have the thickest cell walls and represent a richer source of beta-glucan (roughly 6-7 per cent). However, because of the large amounts of whole oats or oat bran that must be consumed, these are generally impractical sources of beta-glucan for long-term consumer acceptance and compliance.
The recent commercial availability of concentrated oat beta-glucan has largely resolved this issue by facilitating compact dosage forms useful as dietary supplements or when formulated into functional beverages and bars. Important to formulators and consumers alike, oat beta-glucan has no side effects, and it is both palatable and well tolerated. Oat soluble fibre imparts a pleasant, creamy mouthfeel when incorporated into foods or beverages. It also tends to mellow tart flavours.
These unique properties arise from the specific molecular conformation associated with the beta-(1-3),(1,4) mixed linkage. This same structure also lends itself to fermentation pathways resulting in enhanced colonic health, as well as showing considerable promise for the prophylaxis and treatment of many infectious diseases. Few nutritional ingredients provide such a wide range of health benefits, combined with the broad consumer acceptance that oats have achieved.
Daily ingestion of modest quantities of oat beta-glucan can cause a significant reduction in serum cholesterol, notably harmful LDL cholesterol.1 For carefully controlled studies utilising prepared diets, clinically observed outcomes for LDL reduction arising from oat beta-glucan can exceed 20 per cent.2,3 Studies relying upon less stringent dietary control have also demonstrated significant reductions in LDL, mostly in the 5-12 per cent range.
Clinical researchers are also studying oat beta-glucan and other cholesterol-lowering ingredients in comparison to therapeutic drugs, particularly those in the number one-prescribed statin class. The Journal of the American Medical Association recently reported findings that the combination of viscous fibres, including oat beta-glucan and other ingredients (such as plant sterols and soy protein) in a ?dietary portfolio? of cholesterol-lowering foods, resulted in decreases in mean LDL cholesterol of 28 per cent. A comparison of these results with the cholesterol reduction achieved by statins showed no significant differences in efficacy between this dietary portfolio and the statin drug treatment.4
Cholesterol reduction with oat beta-glucan is dose-responsive, and this has been affirmed by a meta-analysis, including the finding that the response is greater for subjects with higher initial levels of cholesterol.5 In 1997, on the basis of these and dozens of other studies, the Food and Drug Administration enacted a health claim addressing the cardiovascular benefits of oat beta-glucan.6 The claim provides for a minimum effective daily dose of 3g, which may be divided into sub-doses as small as 0.75g.
The marketing alternative to a health claim is provided under the Dietary Supplement Health & Education Act, wherein the FDA model structure-function claim does not permit any reference to cholesterol reduction or heart disease.
Although studied extensively, the mechanism by which oat beta-glucan reduces cholesterol is unclear. However, it has been demonstrated that development of viscosity in the gut, which ensues from the solubilisation of beta-glucan, is an essential precursor.7
Because beta-glucan is a hydrocolloid, it has the capacity to form highly viscous solutions at low concentrations in vivo and this can lead to binding and increased excretion of bile acids, reduced absorption of fat and cholesterol, and a reduced absorption rate of available carbohydrates. Any or all of these events can result in attenuated cholesterol levels. This also suggests that beta-glucan is likely to be more effective when taken with meals.
Metabolic syndrome & carbs
Metabolic syndrome has been characterised as a constellation of disorders, each representing a significant risk factor for cardiovascular disease.8 This includes elevated LDL cholesterol, insulin resistance, hypertension and obesity. There is recent epidemiologic evidence to suggest that all of these conditions may be significantly improved by reducing serum insulin levels.9
As discussed below, oat beta-glucan provides cardiovascular and related health benefits, each of which can be linked to reduced insulin levels. This, in turn, is driven by the solution properties of oat beta-glucan and, especially, its ability to form highly viscous solutions at low concentrations in the gut.
Various studies have shown that oat beta-glucan blunts the glycaemic response that occurs subsequent to ingestion of carbohydrates.10,11 Just as is the case with serum lipids, this phenomenon is related to the viscosity of beta-glucan, which can increase the duration of intestinal transit and result in delayed digestion of starches and other available carbohydrates, hence leading to reduction of the glycaemic response.
In one study, 11g oat beta-glucan added to a 50g carbohydrate (glucose) meal attenuated the post-prandial blood glucose response. Researchers measured a 60 per cent reduction in the glycaemic index of glucose Ã¢ÂÂ with pure glucose, by definition, having a GI of 100. Significant blunting of the glycaemic response naturally leads to significant blunting of the insulinaemic response. This has been confirmed in many similar studies, including studies utilising subjects with type 2 diabetes.12,13 Thus, oat beta-glucan is emerging as a tool for diabetes and serum glucose management.
Because the glycaemic and insulinaemic responses for beta-glucan are driven by viscosity, which is concentration-dependent, there is an inherent dose relationship. Significant glycaemic and insulinaemic responses have been observed for doses as low as 1.4g beta-glucan,14 and the threshold dose for an effect is probably considerably lower.
It is noteworthy that biological response appears to plateau above certain concentrations, which suggests that optimal dosing may be achieved by a series of smaller divided doses, rather than a larger bolus dose.14
Hypertension and weight loss
There is mounting evidence that dietary intervention with oats can reduce blood pressure. Reductions in systolic and diastolic pressure of 7.5mm Hg and 5.5mm Hg, respectively, have been reported.15 The results of another study indicate that oats may allow hypertensive individuals to reduce or even eliminate their antihypertensive medications.16
Epidemiologic evidence suggests there is an inverse relationship between oat consumption and blood pressure.17 There is also preliminary evidence that reduced blood cholesterol may contribute to improved blood pressure.18 Blunting of the postprandial glucose and insulin responses has been linked to reductions in the rate of return of hunger, and, also, reductions in subsequent energy intake.19 The increased viscosity of the alimentary bolus caused by solubilised beta-glucan can delay nutrient absorption and result in reduced hunger. It is well known that oats and, particularly, the soluble fibre in oats (beta-glucan) induce high levels of satiety.
In a study that quantified satiety, oats were found to have the third-highest satiety rating among 38 foods in six categories tested.20 This phenomenon may be mediated by increased secretion of gut hormones, such as cholecystokinin, which enhances satiety.21 Satiety can lead to weight loss, and, although relatively few studies are available, significant weight loss has been demonstrated for diets containing oat beta-glucan.22,23,24,25,26
Gregory Stephens, RD, is vice president of sales and marketing for Pennsylvania-based Nurture Inc, supplier of OatVantage oat bran concentrate, which contains 54 per cent beta-glucan. www.nurture-inc.com/
Respond: [email protected]
All correspondence will be forwarded to the author.
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