Researchers know that whole grains are good for health, but the mechanisms at play remain unclear. However, a recent study in humans and mouse models now shows just how certain whole grains help regulate gut health.

Studies from the past few years have shown, variously, that eating whole grains and foods containing them can help maintain a healthy gut. It can even help prevent type 2 diabetes and reduce the risk of colorectal cancer. Although we know that consuming whole grains brings us benefits, it remains unclear what biological mechanisms are at play.

Recent research has pointed to the impact on metabolites, molecules formed and used during metabolic processes, when it comes to the positive effects of whole grains on gut health. A new study has looked further into how eating whole-grain rye and wheat impacts gut metabolism. Its findings may hold an answer to why whole grains can help prevent gut problems and conditions such as colorectal cancer.

The research—by scientists from the University of Eastern Finland in Kuopio and the International Agency for Research on Cancer in Lyon, France—suggests that consuming whole-grain rye or wheat has an impact on plasma (blood) serotonin levels, with implications for health. The team reports these findings in The American Journal of Clinical Nutrition.

Lower blood serotonin levels

In their new study, the scientists analyzed how whole grains affected the concentration of different metabolites in the blood—first in humans, and then in mouse models.

For the first part of the research, they recruited 15 adult participants. For 4 weeks, the participants ate between 6 and 10 slices of low-fiber wheat bread per day.

Then, for another 4 weeks, they each ate 6 to 10 slices per day of either whole-grain rye bread or wheat bread supplemented with rye fiber. Aside from this, none of the participants made any changes to their normal diets.

The researchers collected blood samples from each participant once at the end of the first 4-week period and again after the next 4 weeks. This was so they could compare the samples for any changes driven by the consumption of whole grains.

Analysis of these blood samples indicated that people who had added whole-grain rye into their diets had significantly lower plasma serotonin levels, compared with when they had eaten low-fiber white bread.

In the second part of their research, the investigators worked with mice in an attempt to find out whether introducing cereal fiber to one's diet can affect the levels of serotonin the intestines produce.

People may be familiar with the concept of the hormone and neurotransmitter serotonin, which is linked to the regulation of emotions, being present in the brain. However, the gut also produces this hormone independently. Gut serotonin serves other functions, such as the regulation of gut motility and the gastrointestinal tract's muscles' ability to relax and contract, which allows food to pass through.

Some long-sought-after explanations?

During their research in mice, the scientists fed them additional rye bran, wheat bran, or cellulose flour for a period of 9 weeks. The rodents whose diets the scientists enriched with rye or wheat bran had much lower serotonin levels in the colon, compared with rodents who had been on the cellulose flour diet.

These findings could explain why consuming whole grains could help prevent diabetes, since high plasma serotonin levels are also associated with high blood sugar. "Whole grain, on the other hand, is known to reduce the risk of diabetes, and on the basis of these new results, the effect could at least partly be due to a decrease in serotonin levels," explains study co-author Kati Hanhineva, PhD.

The researchers also note that their results could shed further light on whole grains' effect on the risk of colorectal cancer. "Some recent studies," notes study co-author Pekka Keski-Rahkonen, PhD, "have found [people with cancer] to have higher plasma serotonin levels than healthy controls."

Possible clinical implications

Other study findings indicate that incorporating whole-grain rye bread into one's diet is also linked to lower blood levels of taurine, a compound present in many tissues and some biological fluids, such as bile. A diet high in these whole grains was further linked to lower plasma levels of other metabolites, such as glycerophosphocholine, and two types of glycerophospholipid. Up to 15 phytochemicals (plant-derived chemicals) from rye were present in higher concentrations in the blood of people who had eaten these whole grains regularly.

To conclude their study paper, the researchers explain that others should continue to study these mechanisms and associations, and that their recent findings may have implications for numerous health conditions. They say:

"Collectively, these results suggest that whole-grain cereal intake could have a role in the biosynthesis of peripheral serotonin by reducing colonic production of serotonin, which recent studies have linked to prevention of obesity, metabolic dysfunction, diabetes, and various gastrointestinal disorders including colorectal cancer."