- Researchers found that when healthy adults ate the equivalent of three servings of grapes daily for two weeks, the diversity of bacteria in the gut was not affected, but some types of gut bacteria increased while others decreased.
- In some people, changes in the gut microbiome as well as associated changes in enzyme levels and biological pathways persisted even up to 30 days following grape consumption.
- More studies are needed to determine if these changes observed in this study are the reason behind the various previously-established health benefits of grapes.
The human body contains an astonishing hundred trillion bacterial cells. While these bacteria are present on all surfaces of the body, the majority are found in the gastrointestinal tract.
Gut bacteria – also known as the gut microbiome, microbiota, or microflora – play an important role in many areas of human health, including food digestion, immunity, and the regulation of brain function (known as the “gut-brain axis”).
One area of interest is how our diet can affect the microbiome and our overall health. Researchers have looked into how different nutrients in food, like protein, carbohydrates, and fats, can change the composition of the microbiome and the substances it produces.
As part of ongoing research to better understand how diet can influence the microbiome and ultimately impact our health, some researchers are focusing on the effects of specific foods, such as grapes.
Previous epidemiological research has shown that grape consumption supports heart health, and some show improvements in memory. Animal studies have shown that it protects healthy colon tissue from cancer, and protects the retina in the eye from damage caused by a toxin. A small-scale study on humans taking grape powder showed a reduction in markers of inflammation.
“Since we know that diet can modulate the gut microbiome, and we know that dietary grapes can have some effects on health, it is reasonable to ask: can grapes modulate the gut microbiome? This may be related to the overall mechanism of action,” Dr. John M. Pezzuto, dean and professor of pharmaceutics of the Western New England University College of Pharmacy and Health Sciences, explained to Medical News Today.
A previous in vitro study of human intestinal microbiota found that grape seed polyphenols altered the populations of certain microbes and the short-chain fatty acids they produced. Another study in healthy adults found that grape powder consumption significantly modified the gut microbiome and cholesterol/bile acid metabolism.
Now, a new study led by Dr. Pezzuto and partly funded by the California Table Grape Commission looked at the influence of grape consumption on the human microbiome and urine and plasma (blood) metabolites in healthy adults.
The findings appear in the Nature journal Scientific Reports.
3 servings of grapes per day
The trial was conducted over a period of two months and involved 41 healthy volunteers, of whom 29 completed the study. Twenty-two study participants (53.7%) were female, and 19 (46.3) were male. The ages of the participants ranged from 20.9 to 55.7 years, and the mean age was 39.8 years.
For the first two weeks of the study, the participants followed a restricted diet which excluded or limited the intake of specific foods.
In the next two weeks, the study participants continued to follow a restricted diet but supplemented with the equivalent of three servings of grapes per day in freeze-dried powder form. A standard grape powder was used instead of grapes to ensure consistency among study participants.
The study participants then stopped consuming grapes for a month to allow for a “washout” period.
The researchers collected plasma, urine and stool samples from each study participant on days 15, 30, and 60.
How grapes affect the gut microbiome
When the researchers analyzed the whole study population (N = 29), they found that, except for a subgroup of females ages 29 to 39, grape consumption did not significantly alter the overall diversity of the microbiome in the study population.
However, the researchers did observe changes in the abundance of certain gut bacteria.
Following two weeks of grape consumption, the levels of some gut bacteria, like Holdemania spp., decreased, while those of others, like Streptococcus thermophiles, increased. They also saw shifts in various enzyme levels and biological pathways.
Even 30 days after stopping grape consumption, some individuals still exhibited changes in their microbiome, enzymes, and pathways, suggesting that the effects of eating grapes can be delayed.
Analyses of the chemicals in urine and plasma showed that certain substances like 2′-deoxyribonic acid, glutaconic acid, and 3-hydroxyphenylacetic acid increased when grapes were consumed and then returned to normal levels after the washout period.
As expected, the researchers noticed differences in the microbiome between individuals, with each person having their distinct patterns of microorganism distribution throughout the study.
How grapes benefit overall health
MNT asked Dr. Pezzuto to shed some light on how grapes might alter the amounts of certain gut bacteria, enzyme levels, and biological pathways.
“It is logical to think some of the microorganisms found the grapes desirable and flourished, whereas others did not. The matter is very complex, but also, if one member of the microbial community begins to flourish, that may in itself affect the abundance of others,” Dr. Pezzuto explained.
“Since each of the microbial members within the community have their own enzymes that participate in various metabolic pathways, shifting abundance will shift enzyme and pathway levels, either up or down,” he added.
When asked how the changes induced by a grape-enriched diet may translate into health benefits, Dr. Pezzuto said that the altered enzyme and pathway levels “may affect the generation of specific chemical metabolites potentially capable of reaching the organs of the body. It remains to be shown which, if any, of these metabolites contribute to the health effects of grapes, but it is reasonable to expect there would be some effect.”
Dr. Hana Kahleova, Ph.D., director of clinical research for the Physicians Committee for Responsible Medicine, told MNT that the beneficial effects of grapes on human health could be explained through the fiber in grapes, which “feeds the beneficial gut bacteria that produce short-chain fatty acids that have multiple health benefits.”
Another possible mechanism, according to Dr. Kahleova, is the activity of polyphenols, naturally occurring molecules that are abundant in grapes.
These “have been shown to increase Bifidobacterium and Lactobacillus abundance, which provide anti-pathogenic and anti-inflammatory effects and cardiovascular protection,” Dr. Kahleova explained.
Dr. Franck Carbonero, assistant professor at Washington University’s Elson S. Floyd College of Medicine, agreed that “polyphenols are the most likely drivers of changes.”
“3-hydroxyphenylacetic acid is well known as a major microbial metabolite of polyphenols, and its increase after grape consumption appears logical,” he noted.
Dr. Carbonero told MNT that the “microbial conversion of polyphenols is now considered important in mediating their health properties, either by making the molecules small enough to be absorbed by human cells, or by providing different metabolic effects (or both).”
Study limitations
Dr. Carbonero believes that the study authors did “not tak[e] advantage of the strengths of their datasets” when conducting statistical analyses.
He also cautioned, “the metagenomics technique used [in the study] cannot be trusted for quantitative taxonomic measures, and thereby the changes reported are questionable.”
Dr. Kahleova said “the relatively short duration [of the study], and inclusion of only healthy men and women,” were some of the study’s limitations.
“Further longer-term studies that will include also people who have diabetes, cardiovascular disease, and other health issues are warranted,” she added.
In comments to MNT, Dr. Pezzuto noted that “there are many more metabolites produced and changed by grape consumption than we reported in this study. These analyses are ongoing, and we hope to be able to more clearly establish cause and effect relationships in the near future.”
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