Scientific Name(s): Avena sativa L.
Common Name(s): Avena (Spanish), Hafer (German), Ma-karasu-mugi (Japanese), Oats
Oats grow as hardy annual grasses able to withstand poor soil conditions in which other crops are unable to thrive and are best adapted to areas with a cool, moist climate; Russia, the United States, Finland, and Poland are the world's major oat-producing countries. The plant grows to approximately 61 to 91 cm in height and has straight, hollow, blade-like leaves. The flowers contain 2 or 3 florets and are clustered at the top of the plant. Oat grain grows enclosed in 2 hulls that protect it during development. It contains 3 main structures: the bran, endosperm, and the germ containing embryonic structures that can grow into a new plant.1, 2, 3
Derived from wild grasses, the oat evolved into today's cultivated plant. The oldest known oat grains were found in Egyptian remains dating from approximately 2,000 BC. Scottish settlers introduced oats into North America in the early 17th century. Before being used as a food for humans, oats were used as a livestock feed in the form of grain, pasture, hay, or silage. Traditional medicinal uses of oats include the treatment of rheumatism, depression, chronic neurological pain, atonia of the bladder, and, externally, as a skin cleanser and emollient.2, 3
The dietary value of oats is very high compared with that of other cereals in the Gramineae family. Oats are a good source of soluble and insoluble fiber, manganese, selenium, phosphorous, tryptophan, thiamine, and vitamin E (mainly as alpha-tocopherol). The protein content is 15% to 20% higher than that of other cereal grains3 with approximately 10% consisting of storage proteins known as avenins.4 These proteins belong to the prolamin group and are related to the gluten found in wheat.4
Oat bran contains the soluble dietary fiber beta-glucan, a highly viscous soluble polysaccharide with a linear, unbranched structure composed of 4-O- and 3-O-linked beta-D-glucopyranosyl units. Other polysaccharides in oat include starch, araban, and xylan gums. Lipid content is high, especially in unsaturated triglycerides. Lipase, lipoxygenase, and superoxide dismutase are enzymes present in oats.3
Phenolic esters, including avenacins, phenols (hydroxycinnamic, ferulic, p-coumaric, and caffeic acids), and other phenolic compounds (benzoic and cinnamic acids, quinones, flavones, flavonols, chalcones, flavanones, anthrocyanidines, aminophenolics, avenanthramides) have been identified.3, 5, 6, 7
Uses and Pharmacology
The widespread use of oats and oatmeal preparations makes the findings of animal experiments largely redundant.
An extract of oats is used in traditional Ayurvedic medicine to cure opium addiction, but a few older trials were conducted on the potential of oats in treating addictions with conflicting results.50, 51, 52 A study in rats receiving increasing alcohol doses up to 8 g/kg/day demonstrated a protective effect of oats on gut leakiness associated with endotoxemia and liver injury.53
The FDA recognizes that beta-glucan may play a role in reducing the risk of coronary heart disease.8
In vitro experiments using human aortic endothelial cells showed reduced monocyte adherence and decreased production of cytokines involved in inflammatory response with oat phenolic avenanthramide pretreatment. A role in reducing the risk of atherosclerosis has been suggested.5
In a trial of overweight dyslipidemic adults (N = 30), oats had no effect on endothelial dysfunction over a 6-week period compared with placebo. Subgroup analysis (not powered for) suggested a beneficial effect.9 A pilot study showed a reduction in systolic and diastolic blood pressure in 18 patients with mild or borderline hypertension who received oat cereal. Beta-glucan content of the cereal was standardized to 5.52 g/day.10 However, these results were not confirmed by another small trial that showed no change in blood pressure despite a similar daily intake of beta-glucan.11 In a larger trial of 97 patients with hypertension, beta-glucan administered for 12 weeks did not affect blood pressure except in subgroup analysis, which was not powered for in the study design.12 A decrease in systolic pressure (but not diastolic) was reported in a 12-week trial of 206 healthy middle-aged volunteers who consumed three 30 to 40 g portions of whole grain foods daily. No changes in markers of inflammation were noted.13
The role of oats in the diet of patients with celiac disease is controversial and possibly compounded by results of older, uncontrolled trial data or the use of contaminated oats.14 A smaller proportion of immunogenic storage proteins is found in oats than in wheat, barley, and rye, and oat-derived proteins are more readily digested by the protease enzymes in the gut. In addition, the proline found in oat avenin is more readily digested by protease enzymes, which results in rapid degradation of potentially harmful peptides and may help to prevent the initiation of an immune response against oats in the small intestine.4, 15 Some people with celiac disease possess avenin-reactive mucosal T cells that can cause mucosal abnormality.16, 17
Advantages of incorporating oats into gluten-free diets have been described and include the provision of nutrients including vitamin B1, magnesium, and zinc, as well as the health benefits of increased dietary fiber.18, 19, 20
A long-term study of oat ingestion in adults with celiac disease showed no effect on duodenal villous architecture, inflammatory cell infiltration of the duodenal mucosa, or antibody titers after 5 years.15, 21 Similar results were obtained in children with newly diagnosed celiac disease.20, 22 Two systematic reviews, as well as the World Gastroenterology Organisation, and the Finnish, United Kingdom, and Canadian celiac associations consider that uncontaminated oats can be consumed by most patients with celiac disease. Because of the possibility for oat intolerance in some people, the reviewers suggest eliminating oats initially and only adding moderate amounts in well-established, gluten-free diets. The American Celiac Sprue Association regards the use of oats as not risk-free.16, 19, 23 Studies suggesting that oats are safe in patients with dermatitis herpetiformis have also been published.4, 15, 24, 25, 26
Despite their promotion for management of dry, itchy skin conditions, claims about the benefits of colloidal oat-containing preparations are largely based on older trials3, 56 but the popularity of colloidal oatmeal in the management of pruritus is increasing.56, 57, 58 Although listed in the United States Pharmacopeia, the FDA has added a caution to the skin protectant monograph regarding over-drying that may occur from prolonged soaking in colloidal oatmeal.59 Additionally, the use of oatmeal-containing products may cause sensitization in children with atopic dermatitis.60
Meals high in soluble fiber have been shown to reduce the rise in postprandial blood glucose and insulin concentration, attributed in part to an increase in the viscosity of the contents of the stomach and small intestine, with a subsequent reduction in the rate of absorption of digested nutrients.27, 28 The results of studies of oats in diabetic patients are conflicting. As a component of medical nutrition therapy for patients with type 1 or type 2 diabetes, the American Diabetes Association Standards of Care (2014) recommend an increase in foods containing n-3 fatty acids (EPA and DHA; from fatty fish), viscous fiber (eg, oats, legumes, citrus), and plant-based stanols or sterols to help treat dyslipidemias in most diabetic patients (ie, those who do not have severe hypertriglyceridemia) (high-quality evidence).76
Fasting plasma glucose, insulin, and glycosylated hemoglobin (HbA1c) were unaffected by a dietary regimen containing oat bran (beta-glucan 3 g) concentrate in a number of trials. Dosages of beta-glucan in these trials ranged from 2.25 to 6 g/day over 6 to 12 weeks.12, 29, 30, 31, 32, 33
A slight effect on the glucose response curve was shown in a trial in healthy adults.34 In a small trial of patients with mild type 2 diabetes, oat bran flour produced a lower glycemic response than glucose.28 Similar responses were obtained in a few additional trials.35, 36, 37, 38
The use of a beta-glucan–enriched bedtime snack to reduce the prevalence of silent nocturnal hypoglycemia in children with diabetes mellitus has been investigated.39 Children receiving the enriched snack experienced flattening of the blood glucose curve before midnight, but the incidence of hypoglycemia after 2 AM remained unchanged.
The ability of a variety of oat and barley food products to lower postprandial blood glucose was evaluated in a 2013 meta-analysis of 34 human studies; patients with type 2 diabetes mellitus (including NIDDM) were excluded as were trials that employed low viscosity extracts or deliberately depolymerized beta-glucan. A dose of at least 3 g beta-glucan per meal of intact oats or barley (cooked or fermented), or at least 4 g soluble beta-glucan (with a molecular weight greater than 250,000 g/mol) in processed oat and barley food products, which provided 30 to 80 g of available carbohydrate, was sufficient to produce a physiologically relevant reduction (at least −27 mmol min/L) in postprandial blood glucose. Glycemic response was greater for intact grains than for processed foods. The efficacy rate and average reduction in area under the curve (AUC), respectively, were as follows: intact kernel foods (96%, −99 mmol min/L); raw flours, flakes, and bran in beverages or puddings (75%, −75 mmol min/L); muffin batter (92%, −60 mmol min/L); dry cereal products (82%, −32 mmol min/L); and breads (64%, −29 mmol min/L). Oat and barley products were not significantly different with respect to average reduction in AUC or glycemic index.73
Data from studies assessing oat and barley beta-glucan effects on blood glucose levels, as well as cholesterol levels, were analyzed in a 2011 meta-analysis that included subjects with or without health conditions. Of the 126 eligible studies, 82 studied oat beta-glucan; daily beta-glucan doses ranged from 2 to 14 g/day. Analysis revealed a significant reduction in blood glucose (−2.58 mmol/L); heterogeneity was high. A beta-glucan dose of 1 g/day resulted in a change in blood glucose of −0.084 mmol/L, but showed uncertain changes in blood glucose levels with changes in beta-glucan dosage.74
A meta-analysis of randomized controlled trials has assessed the effect of beta-glucan extract on glycemic control and insulin sensitivity in patients with or without diabetes. A total of 18 trials met inclusion criteria; 7 trials (N=423) utilized beta-glucan extract. All 7 studies used beta-glucan extracted from cereals (oats, oat bran, barley) and the dose of the extract ranged from 3 to 10 g/day for up to 8 weeks. Significant heterogeneity was observed among the studies and subgroup analysis found the variability in study results to be impacted by study location (significant benefit associated with studies conducted in Asian but not Western countries), type of study (benefit in parallel but not crossover designs), participant health status (benefit in type 2 diabetics and hyperlipidemics), and duration of intervention (less than 8 weeks vs 8 weeks). Overall, beta-glucan extract was not as effective as whole oats in reducing HbA1c, fasting blood glucose, or insulin sensitivity.79
Oat fiber produces modest reductions in cholesterol levels and may exert a small positive effect on the risk of coronary artery disease, but the mechanism is unclear. Although evidence suggests that some soluble fibers bind with bile acids or cholesterol, resulting in an increased clearance of low-density lipoprotein (LDL) cholesterol, this action may be insufficient to account for the observed cholesterol reductions. Other proposed mechanisms include inhibition of hepatic fatty acid synthesis, changes in intestinal motility, and reduction in absorption of macronutrients, resulting in increased insulin sensitivity and satiety, with a consequent overall reduction in total energy intake.32, 40, 41, 42
Other factors to be considered when interpreting trial data include the solubility and molecular weight of beta-glucan, unfavorable changes during commercial preparation, storage conditions, and cooking processes.41, 43
A large number of studies have been conducted to evaluate the effect of oat bran supplementation on blood lipid levels, with results included in several meta-analyses.40, 41, 74, 77 Substantial heterogeneity among individual studies suggests that the effects of fiber are not uniform, possibly as a result of inconsistent dosages. Many, but not all, trials show reductions in LDL-cholesterol.30, 31, 32, 33, 37, 38, 40, 41, 43, 44, 45, 46, 47, 48, 49, 74, 77 In addition, nonlinearity was observed at higher doses (eg, above 3 g/day), suggesting a possible diminished adherence or biological maximum being reached at these doses.37, 40, 41, 74 In a 2011 meta-analysis that included subjects with or without health conditions, significant reductions occurred in total cholesterol, LDL, and triglycerides/triacylglycerol after beta-glucan consumption. Of the 126 eligible studies, 82 studied oat beta-glucan; daily beta-glucan doses ranged from 1.2 to 10 g/day in total cholesterol studies. Analysis revealed a significant dose-response reduction in total cholesterol with 1 g/day yielding a −0.079 mmol/L change, but no significant dose-response relation was noted for LDL, high-density lipoprotein, or triglycerides/triacylglycerol. 74 A 2014 meta-analysis of data from 28 randomized clinical trials on the consumption of at least 3 g/day of oat beta-glucan for at least 2 weeks in generally healthy normo- or hypercholesterolemic adults found significant reductions in LDL and total cholesterol (P = 0.0001 each) but not high-density lipoproteins or triglycerides.77
A small, uncontrolled study in AIDS patients on highly active antiretroviral therapy (n = 31) failed to produce any clinically or statistically significant improvement in cholesterol parameters or uricemia with daily supplementation of 20 g of oat bran contained in a formulation that also contained flaxseed and soy protein (10 g/day each).75
Dietary supplementation with a commercially available beta-glucan product (Betaglucare; 3 g/day for 8 weeks) was observed to reduce LDL cholesterol by 0.3 mmol/L (P=0.0002) compared to baseline in patients with myasthenia gravis in a prospective, uncontrolled study. Patients with an initial baseline LDL of at least 3 mmol/L experienced a slightly greater reduction. LDL was reduced equally regardless of diabetes diagnosis or baseline BMI. Mean HbA1c was not significantly affected.78
The FDA has endorsed the relationship between inclusion of beta-glucan soluble fiber in the diet and a decrease in serum cholesterol, and advises a dosage of beta-glucan 3 g/day soluble fiber. A causal relationship with decreased cardiovascular disease has not been demonstrated.41
Nutrition in the elderly population
Clinical trials evaluating the effect of adding oats to energy deficient diets have largely found no additive effect.46, 49, 63 Satiety is increased with the addition of fiber in some, but not all, studies.64, 65, 66
The recommended intake of beta-glucan for reduction of cholesterol is 3 g/day, an amount found in approximately 90 g of oats.40, 41, 46 An estimate of decreases in LDL by 5 mg/dL (0.13 mmol/L) is produced by inclusion of 3 g/day beta-glucan in the average American adult.46
Moderate dietary amounts of oats, 20 to 25 g/day in children and 50 to 70 g/day in adults, are suggested in celiac disease.15
Pregnancy / Lactation
Generally recognized as safe when used as food. Avoid dosages above those found in food because safety and efficacy are unproven.
Oat bran may decrease absorption of medications. In 2 patients with hypercholesterolemia, concomitant ingestion of 50 to 100 g of oat bran and lovastatin 80 mg resulted in an increase in LDL compared with taking lovastatin alone.67 A similar interaction can be expected with other HMG-CoA reductase inhibitors and oat bran ingestion.
Absorption of iron is decreased with concomitant consumption of oats, attributed to the presence of phytic acid in the fiber fraction of cereals.68
Oat bran increases stool bulk, which may cause discomfort, and more frequent defecation may result in perineal irritation.69 The increase in stool bulk has been used to advantage in stoma reversal.70 Digestion of fiber by colonic bacteria may cause gaseous distention and flatulence. Adequate fluid intake is recommended to ensure hydration and dispersion of fiber in the GI tract. Contact dermatitis from oat flour has been reported.71 A study linking life-threatening, recurrent, exercise-induced anaphylaxis with gliadin-containing grains, including oats, has been published.72
A higher than expected incidence of oat sensitization in children with atopic dermatitis has been reported.60
Data are lacking.
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