【Introduction】
Butyric acid, a type of short-chain fatty acid produced in the guts by butyric acid-producing bacteria, has been pointed out to have the potential to suppress excessive inflammatory reactions and prevent various chronic diseases. BARLEYmax, a variety of barley, contains about twice as much dietary fiber and about four times as much resistant starch as ordinary barley, and human studies have shown that it increases the concentration of short-chain fatty acids in feces*1. However, no studies have focused on butyric acid and analyzed its effects on human gut microbiota. The present study has evaluated whether BARLEYmax increased butyric acid-producing bacteria in the guts of healthy subjects and increased the concentration of butyric acids in their feces.
【Materials and Methods】
The subjects were 20 healthy men and women (18-65 years old). They consumed 40 g of granola included BARLEYmax, once a day for more than four times per week, for four weeks. The participants were not instructed on the schedule and the method of intake granola. Stool samples were collected before (pre), one month after (post), and one month after the end of the intake period (1M) for 16S rRNA analysis of gut bacteria and measurement of the concentration of short-chain fatty acids.
A 40 g serving of granola contains 20.4 g of BARLEYmax, brown sugar syrup, oats, puffed brown rice, and dried fruits. It has 169 kcal of energy, 5.72 g of dietary fiber (including 1.2 g of beta-glucan and 2.32 g of fructan), and 0.48 g of resistant starch.
【Results】
Eighteen subjects, excluding two who took antibiotics during the study period, were included in the analysis. The median age was 35.9 years; the median BMI was 22.0; 14 of the subjects had allergic rhinitis, and 1 had atopic dermatitis; 12 were consuming probiotic foods (yogurt, cheese, miso, pickles, drinks, and supplements) at least four times per week; and the median age and median BMI of the subjects were 35.9 and 22.0, respectively. The percentage of days during which the subjects consumed granola was 78% (64-93%).
16S rRNA analysis showed no significant differences in the number of bacterial species, Shannon index, which is a measure of species diversity, and Simpson index among pre, post, and 1M. There was also no significant difference in the Bray-Curtis distance, an index of the degree of difference in the gut microbiota.
Clostridiales and Bacteroidales were predominant at the order level, with Clostridiales decreasing from 33% in pre to 30.4% in post, and Bacteroidales increasing from 16.1% to 19.5%. However, the difference was not significant. The proportion of butyric acid-producing bacteria increased significantly from 5.9% in pre to 8.2% in the post but decreased to 5.4% in 1M, the same level as in pre.
The butyric acid concentration in feces increased significantly from 0.99 mg/g in pre to 1.43 mg/g in the post, which decreased significantly to 0.87 mg/g in 1M. Similarly, propionic acid concentration increased significantly from 1.16 mg/g to 1.82 mg/g, which decreased significantly to 1.00 mg/g. Acetic acid concentration also increased significantly from 2.79 mg/g to 4.63 mg/g, which decreased significantly to 2.85 mg/g.
【Discussion】
BARLEYmax increased the proportion of butyric acid-producing bacteria without substantially altering the gut microbiota and increased the concentration of short-chain fatty acids, including butyric acid, in the feces. The presence of beta-glucan, fructan, and resistant starch in the experimental diet may have functioned as prebiotics and increased the number of butyric acid-producing bacteria.
Consumption of BARLEYmax tended to increase Bacteroidales and decrease Clostridiales, but this was not significant. However, a previous study using rats showed that the intake of BARLEYmax increased the Bacteroidetes (including Bacteriodales) and decreased the Firmicutes (including Clostridiales) in the distal colon digestive tract*2. This result is consistent with the results of this study.
There were many subjects with allergic diseases in the present study (15 subjects, 83%), it may not be easy to extrapolate this result to the healthy adult population in a generalized manner. However, recent reports indicate that the prevalence of allergic diseases in the Japanese population is about 50%, primarily allergic rhinitis*3. It is speculated that the subjects in this study may represent the characteristics of the Japanese population with respect to allergic diseases.
The increase of butyric acid-producing bacteria and butyric acid in the intestines due to the intake of BARLEYmax is expected to be helpful in the prevention and treatment of various chronic diseases.
*1 Br J Nutr 99, 1032–1040, 2008
*2 PLoS One 14, 6, e0218118, 2019
*3 Nippon Jibiinkoka Gakkai Kaiho 123, 485–490, 2020
Fiber-Rich Barley Increases Butyric Acid-Producing Bacteria in the Human Gut Microbiota
Metabolites 11, 8, 559, 2021
