In the past decades, the human colon has come to light as a hub for microbial activities that impact many aspects of human health. The gut microbiota is composed of hundreds to thousands of species with biological activities including nutrient metabolism, vitamin synthesis, immune modulation, pro- and anti-microbial activities, and modulation of gut homeostasis [1
]. Dysbiosis, or microbial imbalance, has been implicated in a number of chronic progressive conditions, one of which is obesity. More than 70% of adults in the United States are classified as overweight or obese; not only do these conditions threaten quality of life, but they also increase the risk of chronic diseases such as type 2 diabetes, heart disease, and stroke [2
]. Evidence from animal and human studies indicates a role of the gut microbiota in body weight maintenance and development of obesity, although exact mechanisms remain unclear [4
]. Marked differences in bacterial population have been seen in the microbiota of overweight/obese individuals when compared to normal weight individuals [5
]. These findings implicate the gut microbiota as a potential target of nutritional therapies to prevent/reduce weight gain and obesity.
While most food consumed is digested and absorbed in the stomach and small intestine, complex carbohydrates such as resistant starch and fiber reach the colon where they are utilized by the gut microbiota [9
]. These compounds are fermented by certain species to produce short-chain fatty acids (SCFA) which have numerous health implications including body weight maintenance [8
]. Prebiotics were previously defined as “selectively fermented ingredient(s) that result in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health” and the compounds that fit these criteria were limited to mainly inulin, galactooligosaccharides (GOS), and fructooligosaccharides (FOS) [13
]. In 2017 however, prebiotics were redefined as “a substrate that is selectively utilized by host microorganisms conferring a health benefit” [14
]. This new definition may allow for inclusion of non-carbohydrate foods that do not undergo fermentation but are utilized by beneficial species in the gut, thus enhancing their expansion and positive health benefits. Such compounds include polyphenols.
Polyphenols are biologically active compounds produced by metabolic pathways in plants, with numerous roles including pathogen protection, antimicrobial, and antioxidant activities [15
]. Most polyphenols consumed by humans are polymerized and/or glycosylate. These high molecular weight compounds reach the colon where they are broken down by the gut microbiota to smaller, absorbable compounds [16
]. Grain sorghum (Sorghum bicolor
L. Moench) commonly called sorghum, is the world’s fifth highly produced cereal crop, far behind the top four (rice, corn, wheat, and barley) but commonly grown in specific areas. Sorghum contains various classes of polyphenols, which are in the bran fraction [17
]. The polyphenol composition of sorghum bran varies with its color, with sumac varieties rich in proanthocyanidins and black varieties rich in 3-deoxyanthocyanins, which include luteolinidin and apigeninidin [18
]. Though previous studies have identified positive modulatory effects of polyphenols from polyphenol-rich foods on the human gut microbiota [19
], to our knowledge no research has been conducted with sorghum polyphenol extracts. The objectives of this research were to characterize the major polyphenol components of two sorghum brans, and to evaluate the change of gut microbiota composition and the effect on SCFA production in response to sorghum bran polyphenols in fecal samples from normal weight (NW) and overweight/obese (OO) subjects.
As metabolic conditions such as obesity continue to burden large proportions of the U.S. population, the topic of gut health, and more specifically the gut microbiota, has gained increasing attention in the realms of health and nutrition. For this reason, a recent focus of nutrition research has been identifying whole foods as well as bioactive components with the potential to positively shift dysregulated bacterial populations towards more desirable profiles [34
]. Of particular allure are plant polyphenols, as they are widely spread in nature and have been credited with the ability to positively modulate the gut microbiota. Not only is grain sorghum bran is a source of polyphenols, but it is also cost effectively and efficiently produced in the United States, making it an attractive candidate for nutraceutical applications.
Our current study found both black and sumac sorghum brans to contain various classes of polyphenols. In sorghum bran analysis, our findings of 27.5 ± 1.5 mg/g for black and 43.0 ± 2.0 mg/g for sumac are consistent with previous reports of total phenolics ranging from 7.6–35.6 mg/g and 22.5–88.5 mg/g for black and sumac sorghum bran, respectively [17
]. The two varieties had strikingly distinct phenolic profiles, with higher concentrations of 3-Deoxyanthocyanins (3-DXA) in black bran, and higher concentrations of proanthocyanidins in sumac bran. These results are in agreement with multiple previous analyses of black and sumac sorghum, in which black sorghum is established as enriched in 3-DXA, and sumac in proanthocyanidins [17
It has been well established that grain sorghum bran has significant antioxidant capabilities [17
], and our current analyses sought to compare the radical scavenging properties of the two different varieties of sorghum bran extract. Despite lower levels of total polyphenols in black sorghum bran extract (BSE), both BSE and sumac sorghum bran extract (SSE) showed similar radical scavenging capabilities.
Fermentation with the wine/grape and black tea polyphenols resulted in significantly increased acetate concentrations compared to blank in a study by Gross et al. [40
]. However, in the present study, the levels of acetate and butyrate did not change during the fermentations with BSE and SSE compared to NC. Propionate production was lower in SSE compared to the NC, which is not in agreement with the report of Gross et al. [40
] who found no change of propionate production with the wine/grape and tea polyphenols. FOS-containing treatments in this study increased butyrate production over 24 h. Lactobacillus
are both known to utilize FOS to produce lactic acid [41
], which can be further metabolized to butyrate by genera such at Anaerostipes
]. We found increases in Bifidobacterium
, and Anaerostipes
in response to FOS-containing treatments, so it is plausible that the increases in butyrate were due to these cross-feeding pathways. This is one of the first studies to examine the impact of sorghum bran polyphenols on SCFA production and further work is needed to corroborate these findings.
Although NMDS did not reveal a significant impact of sorghum polyphenols on the overall microbial communities, the sorghum extracts modulated the gut microbiota at both phyla and genus levels, and combined sorghum polyphenols and FOS worked to enhance specific beneficial genera. Our data showed relative shifts in the two major phyla, Firmicutes increasing in abundance and Bacteroidetes decreasing. This is not surprising, as many carbohydrate utilizers, including the probiotic Lactobacillus, are within Firmicutes, and the conditions of the experiment promote utilization of these substrates, including those already present in the fecal samples.
Proteobacteria contains several potentially pathogenic bacteria [43
] avoiding the overgrowth of this phylum may be a positive outcome of nutrition strategies. Lower abundance of Proteobacteria likely reflect the increases in groups that utilize FOS. Though not statistically significant, BSE and SSE caused a marked decrease in Proteobacteria compared to the NC, suggesting they may act antagonistically against some pathogenic species. These results are similar to those of Pham et al. [44
], who found that during in vitro fecal fermentation Proteobacteria was decreased by FOS, feruloylated arabinoxylans (FAXO), proanthocyanidin-rich rice bran polyphenols, and FAXO and rice bran polyphenols combined.
is often associated with long-term high carbohydrate diets [45
was stimulated but there were no significant differences between sorghum polyphenols with and without FOS. A study with 62 obese subjects found that individuals with a high Prevotella:Bacteroides
ratio lost significantly more weight in response to a high-fiber diet than individuals with lower levels of Prevotella
]. Although further research is required to reach a deeper understanding of this interaction, enhancing the ratio of Prevotella:Bacteroides
may assist in weight-loss strategies that are based on increased fiber intake. In the present study, OO samples resulted in increased Prevotella:Bacteroides
with higher levels of Prevotella
after 24 h.
Though health-promoting bacterial genera in the colon are not limited to Lactobacillus
, they are the traditional targets of prebiotic supplementation [14
proliferation was enhanced by FOS-containing treatments. Compared to the NC, FOS + BSE and FOS + SSE tended to increase abundance of Bifidobacterium
in OO. This data suggests utilization of FOS by Bifidobacterium
is altered in OO microbiota, and that sorghum polyphenols may enhance fermentation of FOS by this species in OO subjects. As decreased proportions of Bifidobacterium
have been seen previously in overweight/obese subjects [6
], nutritional therapies to increase this genus may be extremely beneficial to individuals combatting excessive body weight gain. Past in vitro fermentation studies have observed stimulation of Bifidobacterium
by polyphenols such as the anthocyanin malvidin-3-glucoside [47
], tart cherries [31
], and grape and red wine polyphenols [23
]. In our experiment, however, this genus was not apparently impacted by sorghum polyphenols alone.
In the present study, FOS and sorghum polyphenols worked synergistically to enhance Lactobacillus
. As Lactobacillus
was present at low abundance, further studies are needed to corroborate these effects. Success of prebiotic research in stimulating growth of Lactobacillus
has been markedly lower than with Bifidobacterium
], and the ability of polyphenols to enhance oligosaccharide utilization by Lactobacillus
would provide a new avenue of prebiotic supplementation. As human clinical trials have attributed several strains of Lactobacillus
with anti-obesogenic actions [49
], this mode of supplementation may be especially beneficial in body weight maintenance. In addition, previous studies have observed stimulation of Lactobacillus
by red wine polyphenols, grape seed extract monomers, and anthocyanin malvidin-3-glucoside [19
Additional targets of prebiotic supplementation include butyrate-producing bacteria. Butyrate is not only the main energy source of colonocytes, but research has purported numerous roles in colon-cancer antagonism, inflammation suppression, and colonic barrier function [50
]. In our study, FOS-containing treatments increased butyrate production and this trend was paralleled by increased abundance of Anaerostipes
produces butyrate by metabolizing lactate produced by other species or utilizing acetate through the butyryl CoA: acetate CoA transferase pathway [51
], and our results suggest that this genus was responsible for much of the butyrate production during this in vitro fermentation.
is a genus with the ability to produce butyrate through the butyryl CoA: acetate CoA transferase pathway [52
]. Though little research has been done with this genus, reduced abundance of Roseburia
has been a marker of dysbiosis in both ulcerative colitis [53
] and colorectal cancer [54
]. Previous studies have identified stimulation of Roseburia
by various carbohydrate sources [55
], but this was not significantly impacted by FOS-containing treatments in the present study. SSE, on the other hand, caused significant increases compared to both NC and FOS, indicating utilization of sumac sorghum polyphenols by Roseburia
. This is not the first report of Roseburia
stimulation by polyphenols, as this genus also increased in response to red rice bran polyphenols [44