Oats
Scientific Name(s): Avena sativa L. Family: Gramineae
Common Name(s): Oats , Hafer (German), ma-karasu-mugi (Japanese), avena (Spanish)
Clinical Overview
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Uses of Oats
Oats and oatmeal are used primarily as a food source. Use in celiac disease is debated. Benefits in dermatology, hypercholesterolemia, cardiovascular conditions, and diabetes mellitus remain controversial.
Oats Dosing
The recommended intake of beta-glucan for reduction of cholesterol is 3 g/day, an amount found in approximately 90 g of oats.
Contraindications
None well documented.
Pregnancy/Lactation
Generally recognized as safe when used as food. Avoid dosages higher than those found in food because safety and efficacy are unproven.
Oats Interactions
Oat bran may decrease absorption of medications. There are reports of decreased absorption of HMG-CoA reductase inhibitors (statins) and iron with oat bran ingestion.
Oats Adverse Reactions
Oat bran increases the bulk of stools and frequency of defecation, resulting in distention, flatulence, and possible perineal irritation. Oat sensitization and allergy have been described.
Toxicology
Data are lacking.
Botany
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 about 61 to 91 cm in height with straight, hollow, blade-like leaves. The flowers, which contain 2 or 3 florets, 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
History
Derived from wild grasses, the oat evolved into today's cultivated plant. The oldest known oat grains were found in Egyptian remains from about 2000 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, and atonia of the bladder, and, externally, as a skin cleanser and emollient. 2 , 3
Chemistry
The dietary value of oats is very high compared with 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 grains, 3 with about 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
Oats Uses and Pharmacology
The widespread use of oats and oatmeal preparations makes the findings of animal experiments largely redundant.
CardiovascularThe Food and Drug Administration (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
Clinical dataIn a trial among 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 both 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
Celiac diseaseThe role of oats in the diet of patients with celiac disease is controversial; possibly compounded by results of older, uncontrolled trial data or the use of contaminated oats. 13 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 oats avenins 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 Some people with celiac disease possess avenin-reactive mucosal T-cells that can cause mucosal abnormality. 14 , 15
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. 16 Similar results were obtained in children with newly diagnosed celiac disease. 17 Two systematic reviews, as well as the World Gastroenterology Organisation, the Finnish Coeliac [Disease] Society, and Coeliac UK, have found 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 when well established on a gluten-free diet. 14 , 18 Studies suggesting that oats are safe in patients with dermatitis herpetiformis have also been published. 4 , 19 , 20 , 21
DermatologyDespite their promotion for management of dry, itchy skin conditions, claims about the benefits of colloidal oat-containing preparations are largely based on older trials. 3 A more recent clinical trial reported reductions in itching as measured by daily requests for antihistamine in patients with healing burn injuries using 5% colloidal oatmeal in liquid paraffin versus liquid paraffin alone. 22
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. 23 Additionally, the use of oatmeal-containing products is suggested to have the potential for causing sensitization in children with atopic dermatitis. 24
Diabetes mellitusMeals high in soluble fiber have been shown to reduce the rise in postprandial blood glucose and insulin concentration. This has been 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 the digested nutrients. 25 , 26 The results of studies of oats in diabetic patients are conflicting.
Clinical dataFasting plasma glucose, insulin, and glycosylated hemoglobin (HbA1c) were unaffected by a dietary regimen containing oat bran (beta-glucan 3 g) concentrate in a study of 13 diabetic men. 27 A slight effect on the glucose response curve was shown in a trial in healthy adults. 28 In 4 larger trials (n = 74 to 110) among hyperlipidemic or metabolic syndrome patients no changes in either glucose or insulin response curves were shown. 12 , 29 , 30 , 31 Dosages of beta-glucan in these trials ranged from 2.25 g to 6 g per day over 6 to 12 weeks.
In a small trial of patients with mild type 2 diabetes, oat bran flour produced a lower glycemic response than glucose. 26 Similar responses were obtained in 2 additional trials of overweight women, 32 , 33 and a larger trial (N = 100) of hypercholesterolemic adults. 34
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. 35 Children receiving the enriched snack experienced flattening of the blood glucose curve before midnight, but the incidence of hypoglycemia after 2 AM remained unchanged.
HyperlipidemiaOat 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. 30 , 36 , 37 , 38
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. 37 , 39
Clinical dataA large number of studies have been conducted to evaluate the effect of oat bran supplementation on blood lipid levels. The results of trials have been included in several meta-analyses. 36 , 37 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. 29 , 30 , 31 , 34 , 36 , 37 , 39 , 40 , 41 In addition, nonlinearity was observed at higher doses, suggesting a possible diminished adherence or biological maximum being reached at these doses. 34 , 36 , 37
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. 37
Other UsesAddiction
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, but with conflicting results. 42 , 43 , 44 A study in rats receiving increasing alcohol doses up to 8 g/kg/day demonstrated a protectant effect of oats on gut leakiness associated with both endotoxemia and liver injury. 45
Dosage
The recommended intake of beta-glucan for reduction of cholesterol is 3 g/day, an amount found in approximately 90 g of oats. 36 , 37
Pregnancy/Lactation
Generally recognized as safe when used as food. Avoid dosages above those found in food because safety and efficacy are unproven.
Interactions
In 2 patients with hypercholesterolemia, concomitant ingestion of oat bran 50 to 100 g and lovastatin 80 mg resulted in an increase in LDL, compared with taking lovastatin alone. 46 A similar interaction can be expected with other HMG-CoA reductase inhibitors (statins) 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. 47
Adverse Reactions
Oat bran increases stool bulk, which may cause discomfort, and more frequent defecation may result in perineal irritation. 48 The increase in stool bulk has been used to advantage in stoma reversal. 49 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. 50 A study linking life-threatening, recurrent, exercise-induced anaphylaxis with gliadin-containing grains, including oats, has been published. 51
A higher than expected incidence of oat sensitization in children with atopic dermatitis has been reported. 24
Toxicology
Data are lacking.
Bibliography
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48. Valle-Jones JC . An open study of oat bran meal biscuits (‘Lejfibre') in the treatment of constipation in the elderly . Curr Med Res Opin . 1985 ;9(10):716-720.
49. Saunders RN , Thomas WM . Antegrade porridge enema to assess anorectal function after severe perineal sepsis . Ann R Coll Surg Engl . 2006 ;88(1):74-75.
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51. Varjonen E , Vainio E , Kalimo K . Life-threatening, recurrent anaphylaxis caused by allergy to gliadin and exercise . Clin Exp Allergy . 1997 ;27(2):162-166.
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