- The researchers found that when healthy adults ate the equivalent of three servings of grapes a day 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 after consuming grapes.
- More studies are needed to determine whether these changes seen 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. Although these bacteria are found on all surfaces of the body, most are found in the gastrointestinal tract.
Gut bacteria, also known as the gut microbiome, microbiota or microflora, play an important role
An area of interest is like ours
As part of the ongoing quest 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
Since we know that diet can modulate the gut microbiome and we know that eating grapes can have some health effects, it is reasonable to ask: Can grapes modulate the gut microbiome? This could be related to the general mechanism of action, explained Dr. John M. Pezzuto, dean and professor of pharmaceuticals in Western New England University’s College of Pharmacy and Health Sciences Medical News Today.
A previous in vitro study of human gut 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 consuming grape powder significantly changed the gut microbiome and cholesterol/bile acid metabolism.
Now, a new study led by Dr. Pezzuto and funded in part by the California Table Grape Commission has examined the influence of grape consumption on the human microbiome and urine and plasma (blood) metabolites in healthy adults.
The findings appear in the journal Nature
The study was conducted over a two-month period 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. Participants’ ages ranged from 20.9 to 55.7 years, and the mean age was 39.8 years.
For the first two weeks of the study, participants followed a restricted diet that excluded or limited their intake of specific foods.
Over the next two weeks, study participants continued to eat a restricted but supplemented diet with the equivalent of three servings of grapes per day in freeze-dried powder form. A standard grape powder was used in place of grapes to ensure consistency among study participants.
The study participants then stopped consuming grapes for a month to allow for a withdrawal period.
The researchers collected plasma, urine and stool samples from each study participant on days 15, 30 and 60.
When the researchers analyzed the entire study population (N = 29), they found that, with the exception of a subset of women aged 29 to 39, grape consumption did not significantly alter overall diversity of the microbiome in the study population.
However, researchers have observed changes in the abundance of some gut bacteria.
After two weeks of grape consumption, the levels of certain intestinal bacteria, type Holdmania spp., have decreased, while those of others, such as Thermophilic streptococcus, increased. They also saw changes in various enzyme levels and biological pathways.
Even 30 days after stopping grape consumption, some individuals still showed changes in their microbiome, enzymes and pathways, suggesting that the effects of grape consumption may be delayed.
Analyzes of chemicals in urine and plasma showed that some substances such as 2-deoxyribonic acid, glutaconic acid and 3-hydroxyphenylacetic acid increased when grapes were consumed and then returned to normal levels after the period of interruption.
As expected, the researchers noted differences in the microbiome between individuals, with each person having their own distinct patterns of microorganism distribution during the study.
MNT asked Dr. Pezzuto to shed some light on how grapes might alter the amount of certain gut bacteria, enzyme levels, and biological pathways.
It stands to reason that some of the microorganisms found the grapes desirable and flowery, while others did not. The issue is very complex, but also, if one member of the microbial community starts to thrive, that can in itself affect the abundance of others, explained Dr. Piebald.
Since each of the microbial members within the community have their own enzymes that participate in various metabolic pathways, the shift in abundance will shift the levels of enzymes and pathways, either up or down, he added.
When asked how the changes induced by a grape-enriched diet might translate into health benefits, Dr. Pezzuto said altered levels of enzymes and metabolic pathways can affect the generation of specific chemical metabolites that can potentially reach the body’s organs. It remains to be demonstrated which, if any, of these metabolites contribute to the health effects of the grape, but it is reasonable to expect that there will be some effect.
Dr. Hana Kahleova, Ph.D., director of clinical research for the Physicians’ Committee on Responsible Medicine, said MNT extension that the beneficial effects of grapes on human health could be explained through grape fiber, which feeds beneficial intestinal 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 antipathogenic and anti-inflammatory effects and cardiovascular protection, explained Dr. Kahleova.
Dr. Franck Carbonero, an assistant professor at Washington University’s Elson S. Floyd College of Medicine, agrees that polyphenols are the most likely drivers of the changes.
3-Hydroxyphenylacetic acid is well known as a major microbial metabolite of polyphenols, and its increase after grape consumption appears logical, he noted.
Doctor Carbonero said so MNT extension that microbial conversion of polyphenols is now considered important in mediating their health-promoting properties, either by making the molecules small enough to be taken up by human cells or by providing different metabolic effects (or both).
Dr. Carbonero believes the study authors did not take[e] leverage 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 therefore the reported changes are questionable.
Dr Kahleova said the duration is relatively short [of the study]and the inclusion of only healthy men and women were some of the limitations of the studies.
More long-term studies that will also include people who have diabetes, cardiovascular disease and other health problems are warranted, he added.
In the comments to MNT, Dr. Pezzuto noted that there are far more metabolites produced and modified by consuming grapes than reported in this study. These analyzes are ongoing and we hope to be able to establish cause and effect relationships more clearly in the near future.
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