High-salt diet throws microbiome out of whack

It’s no longer just your doctor telling you to cut back on the salty foods. Your gut might be trying to send you a signal too. Gut bacteria in mice change significantly after the animals have eaten a high-salt diet, researchers have found (Nature 2017, DOI: 10.1038/nature24628). And feeding the animals microbes to replace those lost because of this diet can help ameliorate some negative health effects, a finding that could lead to future treatments for conditions caused by high salt intake, the scientists say. At Germany’s Experimental & Clinical Research Centre, jointly operated by the Max Delbrück Centre for Molecular Medicine and Charité—Medical University of Berlin, Nicola Wilck and co-workers gave one group of mice a high-salt diet and another group normally salted meals. Over the course of the three-week diet, the team sequenced ribosomal DNA in the animals’ faeces and coupled the results with a machine-learning algorithm to identify the microbes residing in their guts. After just 14 days of evaluation, the researchers observed that the mice eating more salt played host to lower levels of the mouse-specific bacterium Lactobacillus murinus. To test what role this microbe plays in mouse health, Wilck and his colleagues gave L. murinus to mice with high-salt-induced hypertension and to mice with a type of brain inflammation called actively induced experimental autoimmune encephalomyelitis (EAE). The real-world diseases mimicked by these lab-generated conditions have been linked to the malfunctioning of a certain type of immune cell, the TH17 cell. These cells also become more prevalent in animals and people who eat a lot of salt. When given L. murinus, the mice with hypertension experienced reduced blood pressure, and those with EAE displayed less inflammation. The researchers also found that the number of TH17 cells decreased when these mice were given the replacement bacteria. Encouraged by these results, the team ran a small-scale, limited-scope trial in humans. Twelve healthy men ingested 6 g of salt (NaCl) each day for 14 days. Similar to the mice, levels of various Lactobacillus microbes in their guts declined. “The mouse microbiome varies on a daily basis, but compared with human beings, it is a fairly simple system,” Wilck says. For example, not every adult human has Lactobacillus in his or her microbiome. So he urges caution when specifically considering administering this type of bacteria as a probiotic treatment in humans. David Relman at Stanford University agrees that these results won’t necessarily translate to humans. “The salt dose used in these mice was far beyond that which would be relevant to or tolerated by humans,” Relman says. But he notes that the results should prompt others to investigate with deliberate, well-controlled studies in humans. “If it can be shown that components of the human microbiota play a meaningful role in mediating the effect of dietary salt in promoting hypertension in some humans, this would be big,” Relman adds.

Chemical & Engineering News, 20 November 2017 ; http://pubs.acs.org/cen/news