Total cholesterol concentrations dropped 16.2 ± 6.3 mg/dL in the oat cereal group (P = .030), with a slight (nonsignificant) increase in the control group (P = .48). Additionally, a comparison of the changes in total cholesterol between the 2 groups revealed a significant mean difference of 21.1 ± 9.1 mg/dL (P = .035). LDL cholesterol was also reduced significantly after the oat cereal intervention by 15.8 ± 5.9 mg/dL (P = .025). The nonsignificant increase in LDL cholesterol in the control group (P = .231) combined with the significant reduction in the treatment group resulted in a significant difference between the groups after intervention (P < .015). Neither group experienced significant changes in HDL cholesterol or triglyceride concentrations.
An analysis of the side effect data showed no significant difference in the occurrence of side effects between groups. There was an overall decrease in the frequency of dietary fiber-related and hypertension-related side effects in both groups, with a more substantial reduction occurring in the oat cereal group (P = .11). Total body weight did not change significantly in either group. Additionally, both groups were very compliant (approximately 90%) in terms of cereal consumption Table 3.
Discussion
The results of this pilot study suggest that the inclusion of oats into the standard American diet of people with borderline or mild hypertension may reduce both SBP and DBP. In persons consuming 5.52 g/day of beta-glucan soluble fiber from oat cereal for 6 weeks, we found a statistically and clinically significant decrease in both SBP and DBP (7.5 mm Hg and 5.5 mm Hg, respectively) and a trend toward improved OGTT-determined insulin sensitivity. These findings warrant a large-scale clinical trial to explore further the relationship between whole-grain oat consumption and blood pressure, especially considering the limitations of this pilot study.
As with all small-scale trials, this one lacked sufficient power to detect true changes in both primary and secondary outcome variables. It is possible that regression to the mean explains at least part of the treatment effect, since participants in the oats group began the study with higher SBP, DBP, and LDL cholesterol levels than controls. In addition, it is possible that the reported blood pressure changes could have been caused by “other” undetected dietary change made by members of the oats group. Future trials will need to collect and analyze dietary data carefully; feeding trials should be considered. Such dietary analyses may indicate that certain micronutrients partially explain the hypotensive effects of whole-grain oat consumption. The DASH trial and others have consistently demonstrated that diets rich in certain micronutrients can reduce blood pressure.30,31
Soluble fiber-rich oat cereals may affect blood pressure by modulating changes in insulin metabolism. The mechanism of action is thought to involve the slowed absorption of macronutrients from the gut, resulting in a flattening of the postprandial glycemic curve.29 These lower postprandial blood glucose levels elicit a lower insulin response to accommodate its clearance from the plasma. This process may lead to improved insulin sensitivity if the lower circulating insulin levels lead eventually to upregulation of the insulin receptors in peripheral tissues. A recent animal trial demonstrated that soluble fiber feeding improved insulin sensitivity by increasing skeletal muscle plasma membrane GLUT-4 content.32 Findings in this pilot suggest that over time, oat ingestion may reduce the amount of insulin needed to clear a glucose load. However, the study was underpowered to detect significant differences in more sensitive measures of insulin resistance. The causal mechanistic relationship among whole grain oat consumption, blood pressure, and insulin resistance might be best studied using a long-term feeding study design.
Alternate mechanisms, such as attenuation in endothelial function, may have affected blood pressure responses in this study.33 Drugs specific to endothelial cell receptors mediating vasodilation are known to lower blood pressure.34 Moreover, plasma cholesterol reductions are associated with improvements in endothelium-mediated vasodilation.35,36 In addition, preliminary evidence in animals supports a direct relationship between changes in plasma cholesterol concentrations and blood pressure.37 In the present study, plasma cholesterol levels were significantly reduced in participants who ingested whole grain oat-based cereals compared to a more refined grain wheat, corn, and rice control. Thus, it is possible that the blood pressure reduction observed in the subjects consuming oats resulted in part from improved endothelial function due to a drop in plasma cholesterol. Additional research is needed to fully investigate this pathway.
From a practical standpoint, improvements in SBP and DBP such as those observed in this study would be a useful contribution to the clinical management of hypertension. The cereal feeding intervention was well tolerated. Participants were very compliant for the 6-week treatment period. Substantial improvements in blood lipids could serve as an added incentive for patients to maintain long-term compliance with feeding recommendations.18,19 Since treatment of hypertension is a lifelong process for most patients, future studies would need to assess the effectiveness of oat cereals to maintain blood pressure benefits over a longer time. Such studies may need to consider dietary options such as soluble fiber-rich fruits in addition to cereal consumption in efforts to deliver the desired quantity of soluble fiber. Future trials will have to investigate the antihypertensive effect of whole oats in other populations, such as people with diabetes, and to study not only surrogate endpoints such as blood pressure but also patient-oriented outcomes such as mortality and morbidity.