In the present study we aimed to evaluate the impact of increasing WG or FV intake on inflammatory makers and gut microbiota composition in individuals affected by overweight or obesity with normally low intakes of these foods. Subjects increased their intake of these foods from < 1 serving/d to 3 servings/d. Subjects did not all consume the same foods, but rather consumed their choice of foods from a particular food category. In this respect, our intervention was very modest, focusing on a potentially sustainable change in participants’ diet pattern. Anecdotally, many subjects in each group commented on how different they felt their diet was. Many commented that they “[hadn’t] eaten this healthy in years.” During the weekly visits to the study facility, several subjects in the FV group often indicated that it was very difficult to adhere to the treatment regimen. No such comments were made by individuals in the other two groups. Many participants indicated informally that they planned to continue trying to consume more FV or WG after the study period.

One limitation of this experimental design is that subjects used diet diaries to record their food intake. While diet diaries have the advantages that they can provide detailed food intake data without the constant presence of study personnel, they do have disadvantages including accidental or intentional under or over-reporting of foods consumed or portion sizes [48]. For example, our results suggested that participants in the FV group may have under-reported their intake of refined grain foods. However, the weekly visits to the clinical facility coupled with the positive biological effects observed in the treatment groups suggests that subjects generally conformed to the treatment protocols.

The WG and FV diets had significant positive impacts on inflammatory markers. The FV treatment decreased LBP and IL-6, while the WG foods decreased LBP and TNF-α. Previous studies with FV and WG have also reported reductions in inflammatory markers [6,7,8,9,10, 12, 24, 25], although some other studies have not shown significant differences [23, 30, 31, 49]. The differences in experimental design and treatment foods make direct comparisons among studies difficult. Lipopolysaccharide is a component of the cell walls of gram-negative bacteria; increased levels of LBP in the blood is suggestive of endotoxemia [50]. The change in LBP in both groups suggested a positive impact on gut barrier function. The decreases in either IL-6 or TNF-α suggest reductions in subclinical inflammation, which is associated with a lower risk of metabolic syndrome.

One limitation of this study is that with the variety of food options provided to subjects it was not possible to pinpoint which component of the foods were responsible for the biological effects observed. Both treatment groups were good sources of dietary fiber, although the composition of the dietary fibers within each group were very different. The major dietary fibers in WG are β-glucan, arabinoxlyan, and cellulose, while FV primarily contains pectins, xyloglucan and cellulose [51, 52]. FV also contain more soluble fiber, and WG contain more insoluble fiber [52,53,54]. Additionally, FV have more free polyphenols, while WG have a predominance of bound phenolics [55]. WG foods can also contribute vitamin E, and phytoserols that have been linked to lowering markers of metabolic syndrome [56]. In contrast, FV include folate, flavonoids, vitamin C, and β-carotene, which have been inversely correlated with hs-CRP and IL-6 [57].

We did not find any effects of WG or FV on hs-CRP, even though previous studies have reported significant effects [6, 9, 13, 27]. Post-hoc power analysis revealed that we had only 12% power to detect differences in hs-CRP due to our modest sample size. Therefore, even if WG or FV had significant effects on hs-CRP we would likely not be able to detect it. Notably, our study was powered to detect differences in IL-6, where we did find differences. We also found differences in LBP and TNF-α, indicating sufficient power for those inflammatory markers.

One unique aspect of the present study is the comparison between WG and FV. There are several previous studies that test WG in comparison with refined grain or high and low FV diets [6, 7, 9,10,11,12,13, 19, 24,25,26,27,28, 30,31,32, 49]. However, no previous studies have included both food groups in the same study. Interestingly, while both treatment groups decreased inflammatory markers, each decreased a different biomarker. These unique changes suggest that the beneficial effects of WG and FV on inflammation may be mediated via different mechanisms. Furthermore, perhaps consuming both FV and WG together could have a synergistic effect to help lower inflammation. There are some diets that combine, among other dietary recommendations, WG and FV to improve health (e.g., DASH). For instance, a clinical trial of the DASH diet on type 2 diabetic patients showed a significant reduction in CRP [58, 59], although other similar feeding trials have not shown reductions in CRP or markers of inflammation [60]. In a study where participants followed a Nordic diet, a reduction in expression of genes related to inflammation in subcutaneous adipose tissue was reported [61].

The significant decrease in LBP during the interventions suggested a link between the changes in inflammatory state and the gut microbiota. However, save for an increase in α-diversity in the FV group, we found no significant changes in gut microbiota composition by treatment group during the intervention. Rather, found that gut microbiota composition at baseline was more related to changes in LBP than changes in the gut microbiota during the intervention. Others have also found that baseline microbiota composition is associated with changes in outcomes during an intervention trial. Korpela et al. [18] reported that baseline microbiota composition had the greatest ability to predict changes in host cholesterol levels using data from three independent intervention studies. Kovatcheva-Datchary et al. [28] found that baseline abundances of Prevotella were associated with host improvements in glucose tolerance. In our study, it appeared that certain members of the Clostridiales order were perhaps involved in the reduction in LBP following either a WG or FV intervention. The Clostridiales order contains a number of important gut microbiota families, such as Lachnospiraceae and Clostridiaceae, which contain bacteria that are important in the degradation of complex carbohydrates [62, 63].

The increase in α-diversity during the FV diet may have been due to the introduction of the wide variety of new dietary fibers to subjects’ diets. In contrast, the WG diet was mainly composed of wheat-based products which contain similar dietary fibers to the refined grain products that subjects consumed as part of their habitual diet prior to the study.

There have been several studies that report changes in the gut microbiota following a WG intervention. Most studies report only modest to no changes in gut microbiota composition in accordance with our study [10, 11, 13, 26, 32, 49]. Others have reported more substantial changes, such as increases in beneficial bacteria and even phylum-level changes [14, 64]. The differences in gut microbiota changes may be due to differences in the types, forms, or quantities of WG consumed in each of the studies. Notably, among those microbial taxa that changed the most during the study, our power to detect significant shifts only ranged from 25 to 50%. Therefore, the treatments may have induced shifts in the microbiota that we were unable to detect due to insufficient sample size. Notably, our study was powered to detect differences in inflammatory markers (IL-6) and not microbiota changes.

Very few studies have reported on the changes in gut microbiota composition following a FV intervention. Li et al. [19] reported shifts in the gut microbiota structure following a cruciferous vegetable intervention, but the shifts were dependent on individuals. Despite the individualized responses, four taxa were associated with the cruciferous vegetable diet: Eubacterium hallii, Phascolarctobacterium faecium, Alistipes putredinis, and Eggerthella spp. In another study also feeding cruciferous vegetables, the authors reported significant reductions in sulfate-reducing bacteria [65]. This is desirable, since sulfate is reduced to hydrogen sulfide by sulfate-reducing bacteria, which has been associated with several GI disorders [66, 67].

This present study did not show significant changes in fecal SCFA. This has also been reported by others: a three-week treatment of WG cereal showed no significant difference between whole grain treatment and the control in SCFA production [64]. SCFA are rapidly absorbed from the gut, which could be why many studies, including our study, show no significant changes in SCFA production [68]. However, when using a more dramatic treatment of a 12 week diet rich in WG foods, the authors reported increased plasma propionate concentrations [11].