Background: Health claims regarding the cholesterol-lowering effect of soluble fiber from oat products, approved by food standards agencies worldwide, are based on a diet containing ≥3 g/d of oat β-glucan (OBG). Given the number of recently published randomized controlled trials (RCTs), it is important to update the findings of previous meta-analyses.
Objective: The objective was to quantify the effect of ≥3 g OBG/d on serum cholesterol concentrations in humans and investigate potential effect modifiers.
Design: A meta-analysis was performed on 28 RCTs comparing ≥3 g OBG/d with an appropriate control. Systematic searches were undertaken in PubMed, AGRICOLA, and Scopus between 1 January 1966 and 6 June 2013, plus in-house study reports at CreaNutrition AG. Studies were assessed with regard to inclusion/exclusion criteria, and data were extracted from included studies by reviewers working independently in pairs, reconciling differences by consensus. Estimates of the mean reduction in serum cholesterol from baseline between the OBG and control diets were analyzed by using random-effects meta-analysis models and meta-regression.
Results: OBG in doses of ≥3 g/d reduced low-density lipoprotein (LDL) and total cholesterol relative to control by 0.25 mmol/L (95% CI: 0.20, 0.30; P < 0.0001) and 0.30 mmol/L (95% CI: 0.24, 0.35; P < 0.0001), respectively, with some indication of heterogeneity (P = 0.13 and P = 0.067). There was no significant effect of OBG on high-density lipoprotein (HDL) cholesterol or triglycerides and no evidence that dose (range across trials: 3.0–12.4 g/d) or duration of treatment (range: 2–12 wk) influenced the results. LDL cholesterol lowering was significantly greater with higher baseline LDL cholesterol. There was a significantly greater effect for both LDL and total cholesterol in subjects with diabetes compared with those without (although based on few studies).
Conclusions: Adding ≥3 g OBG/d to the diet reduces LDL and total cholesterol by 0.25 mmol/L and 0.30 mmol/L, respectively, without changing HDL cholesterol or triglycerides.
Alex’s Notes: Oats and oatmeal are great. Even if a person shuns grains, they would be foolish to demonize whole-oat products simply because of their “grain” status. It is akin to saying all fats are whatever, when there are clear differences between the fatty acids types and lengths. Or to say all carbohydrates are bad; what about nondigestible polysaccharides (fiber)? Indeed, this is not the first time I have brought the wonder of oats to our attention. Oatmeal has been shown to induce satiety and lower appetite, and its beta-glucan fiber specifically has demonstrated the ability to reduce food intake, weight gain, and adiposity during times of overfeeding. They even have a powerful antioxidant unique to them.
So to expand our list of reasons to eat oatmeal daily, I present to you a meta-analysis of the literature looking at the impact of high molecular weight (MW) oat beta-glucan (OBG) on blood lipids. The researchers searched for all RCTs until June of 2013 that investigated the effect of at least three grams of OBG per day for a minimum of two weeks. It could be consumed in any form provided it met the minimum molecular weight (100 kDa), which basically excluded any heavily processed oat products and can be estimated from the type of product used in the study based on the researchers understanding of how processing methods impact it. The study populations included were generally healthy free living normocholesterolemic or hypercholesterolemic adult men and women from the general population, and subjects could be lean, overweight, or obese, and could have type 2 diabetes.
Twenty-eight studies were included in the meta-analysis, with OBG ranging from 3-12.4 g/day, treatment duration ranging from 2-12 weeks, average age ranging from 25-62 years, and number of subjects ranging from 8-235 persons. Twelve studies were in healthy subjects, 13 in those with hypercholesteremia, and 3 in type-2 diabetics.
What was found?
In a nutshell, diets containing at least 3 grams of OBG per day reduced serum total and LDL cholesterol relative to the control group by 11.6 ng/dL and 9.6 ng/dL, respectively, with no effects seen on HDL cholesterol or triglycerides. The effects of OBG were significantly more pronounced in diabetics, suggesting that OBG reduces serum cholesterol at least in part through a mechanism that is related to dysglycemia, insulin resistance, and/or insulin secretion. Although, since only three diabetic studies were included we must be cautious making these conclusions. Additionally, the effects on LDL cholesterol tended to be more pronounced with increasing baseline LDL values (p=0.074), age (p=0.08), percentage of subjects who were male (p=0.054), and for hypercholesterolemic subjects relative to healthy controls (p=0.079).
Surprisingly, treatment duration had no effect, suggesting that the benefits can be obtained in as little as two weeks and are durable for as long as one consumed OBG. More surprisingly, there was no dose-response relationship, suggesting that 3g/day is just as good as 12g/day. Both I and the study authors have no real explanation for why this would be. Granted, a previous meta-analysis also found no dose-response for the benefits of oatmeal on glycemic control and insulin sensitivity, but as the authors of the current study point out, “a significant effect of dose was found in both previous meta-analyses” that assessed OBG effects on serum lipids. The authors propose several ideas for why.
- Previous meta-analyses used doses of OBG below 3g
- The cholesterol lowering effects depend on the viscosity of the chime that enters the duodenum because it binds bile salts and prevents their reabsorption. The viscosity is dependent on the MW. The MW could have been lowered via processing and heating before being utilized into the ingredients of the studies analyzed (at least in the ones that used baked goods).
- OBG solubility is dependent on water content (i.e. hydrated oatmeal makes it goopy), and many of the products were dry when consumed, thus lowering the intestinal viscosity.
Regardless of the reason, I am skeptical that there is no dose-response relationship. That said, the overall results still hold and with the analyzed studies being conducted in Europe, North America, Asia, and Australia utilizing a range of common foods such as rolled oats, whole oat flour, oat bran, breads, muffins, muesli, breakfast cereals, and biscuits, it appears that the results are applicable to the general population.
Also, for what it is worth, most of the control groups consumed wheat bran, wheat fiber, or wheat noodles, suggesting that oats definitely beat out wheat for these benefits.
Eat oatmeal dammit! Taken in conjunction with previous studies on insulin and glycemic control, as well as satiety and appetite, one standard serving of oatmeal is all it takes. This is about 3-5g of oat beta-glucan.