Study finds high-sodium diet may modulate gut microbiome

Reducing salt intake levels appears to be good for both the gut microbiome and blood pressure, according to a new study published in the journal Hypertension.

For the study, researchers looked at blood samples from 145 adults with untreated hypertension. They found, particularly for the females, six weeks of a daily sodium intake close to the recommended 2,300 milligrams resulted in increased levels of short-chain fatty acids, an indicator of a healthy microbiome, circulating in the blood. The hypertensive adults also experienced decreased blood pressure and more compliant blood vessels.

The gut microbiota are all the bacteria, viruses, protozoa, and fungi populating the gastrointestinal tract, which have a wide range of functions from helping digest your food to your immune response to influencing a propensity to gain weight. Problems with the microbiome are associated with a wide range of diseases from cancer to gastrointestinal problems to allergies.

Short-chain fatty acids are known to play a role in blood pressure regulation. These small metabolites originating from the gut, get absorbed into the entire circulation, binding to receptors on the lining of blood vessels and in the kidneys, regulating things like the release of renin, an enzyme that works to keep the kidneys well perfused and a major player in blood pressure control. Blood levels of short-chain fatty acids can be considered an indicator of the health of the gut microbiome.

The researchers said their hypothesis was even a modest reduction in salt intake would alter concentrations of circulating short-chain fatty acids and lower blood pressure.

The study looked at both males and females of different races, ages 30 to 75 years old, with untreated high blood pressure who were enrolled in a previous study at the Queen Mary University of London. Because stool samples were not taken on the study participants, they could not look more directly at the gut microbiota, so instead measured circulating short-chain fatty acids, the main metabolite produced by gut microbiota.

All the individuals were given two weeks of detailed instruction by nurses on how to lower their sodium intake to about 2,000 milligrams daily, information that was reinforced over the course of the study. Then in what is called a randomized, placebo-controlled study, half the participants got either a sodium tablet or placebo tablet nine times daily for six weeks, then switched groups.

The researchers found sodium reduction increased all eight of the short-chain fatty acids, the end-product of the fermentation of fibers we consume by our microbiota. Humans don't naturally contain enzymes to digest many of these fibers, the researchers said. The increased short-chain fatty acids levels they found were consistently associated with lower blood pressure and increased blood vessel flexibility.

While periods of higher salt intake drove up blood pressures in both males and females and improvements were noted in both sexes with a move to lower salt, the shifts were most dramatic in females, according to researchers. While all humans have slightly distinctive microbiota, influenced by things like diet and environment, there tend to be consistent differences between males and females generally.

The researchers said in the future they want to do a larger study that also examines fecal samples to assess microbiome content and health more directly.

“Sodium is a factor in both sexes but the impact in relationship to the gut microbiome seems more in females,” said Haidong Zhu, MD, PhD, lead author of the study and molecular geneticist at the Georgia Prevention Institute at the Medical College of Georgia at Augusta University, in a statement. “It may be that high-salt affects blood pressure through different pathways in males and females. We need to study it further to see if that is true and why it's true if it holds.”