Epidemiological research has shown that whole grains are protective against diseases including cardiovascular disease and many forms of cancer. Previous clinical research has also exhibited a positive association between intake of whole foods, such as fruits, vegetables and red wine, and increased plasma antioxidant capacity [5–7]. However, we found that administration of a diet high in commercially available whole grain foods did not significantly alter the antioxidant activity of healthy adults when compared to a diet high in refined grain foods.
A slight but insignificant increase of approximately 4% was found in the mean antioxidant activity in whole plasma along with a 1% decrease in the deproteinized plasma. These results demonstrate that components in the plasma did not cause a significant delay in the oxidation of the target protein, β-phycoerythrin. In addition, lipid peroxidation increased slightly (by approximately 2%) in evaluation of thiobarbituric acid reactive substances. Also, there was an increase in oxidation of arachidonic acid to form 8-epi-prostaglandin F2α with the whole grain diet, though again these differences were not significant. These results revealed that components in the diet did not result in a decrease in lipid peroxidation in vivo.
The antioxidant micronutrients, calculated using the nutrition data systems software, were significantly greater in the whole grain diet treatment than the refined grain diet treatment. It is possible that the addition of six or eight servings per day of whole grains may be insufficient to warrant a significant increase in plasma antioxidant activity. However, we believe that this quantity would be sufficient to produce a change in antioxidant activity when compared to the quantities fed in other dietary intervention antioxidant studies [5, 21].
An additional factor that may have affected overall antioxidant potential of the diets, in addition to micronutrients calculated with the diet software, is the presence of other antioxidant components such as phenolic compounds (quercetin, caffeic acid or ferulic acid) and synthetic antioxidants (BHT or TBHQ). The presence of phenolic compounds in both diet treatments would have affected overall antioxidant activity, though we expect that these compounds would not be found in significant amounts in refined grain foods because of the nutrient losses that occur in the milling process.
Synthetic antioxidants, however, may more likely have affected antioxidant potential of the diet treatment foods. Several of the products provided to subjects from both diet treatments were known to contain synthetic antioxidants, including BHT and THBQ used for preservation. Research has not yet been conducted to evaluate the antioxidant potential of these synthetic antioxidants in comparison to the "natural" antioxidants found in whole grains. Because we do not know their potential, it is feasible that these antioxidants may have greater antioxidant potential than "natural" antioxidants and may have masked any effect that might have been seen without their presence.
We do believe that the antioxidant nutrients in the whole grain diet were greater than the refined grain diet because of nutrient losses in grain refining. However, it is possible that these nutrients are not being absorbed. The absorption of antioxidant nutrients would be affected by subject characteristics such as age, overall health, or other dietary components, and the bioavailability of these nutrients in the food products provided to subjects. A lack of absorption would explain the fact that no significant change occurred in plasma antioxidants or their protection against lipid peroxidation. Antioxidant micronutrient research has demonstrated an increase in plasma antioxidants with their consumption [22–24]. In addition, whole food research has shown an increase in plasma antioxidants with the consumption of certain fruits and vegetables [5–7, 21, 25, 26]. However, studies have not evaluated the effects of whole grain consumption and antioxidant potential. Therefore, it is not known at this time whether these nutrients are being absorbed in the colon. One way to measure nutrient absorption would be to analyze total antioxidant activity in the food consumed and fecal excretion of antioxidants. Subtracting excretion from consumption would give us a better understanding of nutrient absorption.
The characteristics of the subjects in this study should also be considered in analyzing our results. Physiological stage, aging, and exercise have been shown to be associated with increased oxidation [27, 28]. In addition, other factors such as hormonal changes and physical or emotional stress are also believed to be associated with oxidative stress . Because our subjects were healthy, young adults who may not be in a state of excess oxidative stress or antioxidant deficiency, it is possible that a dietary intervention such as this may not have warranted a change in antioxidant protection.
One of the main objectives of this study was to evaluate the effects of a whole grain diet in a free-living situation in the average individual. Controlling subjects' consumption and lifestyle may have provided more easily measured dietary antioxidant potential, though would not have provided practical information that could be applied to the average individual. Also choosing a population in a state of known oxidative stress might have provided a situation in which a change was more visible. However, this would not answer the question of whether or not a dietary intervention that is practical for the average individual would be effective in increasing antioxidant potential in vivo.
Other whole grain feeding studies have found few changes in biomarkers. While whole-grain wheat breakfast cereal had a prebiotic effect in human subjects, there were no differences blood glucose, insulin, or blood lipids with whole grain consumption compared to refined grain consumption . Also, whole-grain foods did not affect insulin sensitivity or markers of lipids peroxidation and inflammation in health, moderately overweight subjects .
It is possible that the diet composition or subject characteristics were not ideal for significant alteration of antioxidant activity in our study. However, we believe that it is more likely that the mechanism by which the protection of whole grains occurs is not due solely to its antioxidant potential but to some of the other components of the whole grain, or a combination of many of its components.