·      NMN can regulate the structure of the gut microbiota, increase the abundance of probiotics, and reduce the number of harmful bacteria.
·      These changes in gut microbiome colonization improve the intestinal lining of mice.
·      By exerting these protective effects on the intestinal lining, NMN may positively impact gut health.

The human intestinal mucosa covers an area of up to 200 to 300 square meters — about 2 to 3 football fields worth of guttural lawn rolled up inside of you — and contains 10 trillion different symbiotic microorganisms, known as the “microbiota.” The gut microbiome plays a vital role in the body’s digestive function and affects the operation of other bodily systems and tissues. Due to its importance and potential value, the gut microbiome has become a hot-button topic. Now, slews of scientists are trying to understand the role of the gut microbiome in human health and disease as well as how to manipulate it to improve medical outcomes and longevity.

Pan Huang and colleagues from Jiangsu University in Zhenjiang, China, show the positive effects of long-term Nicotinamide mononucleotide (NMN) treatment on modulating gut microbiota diversity and composition and intestinal function. Published in Frontiers in Nutrition, the study shows that mice drinking different concentrations of NMN for 15 weeks displayed an increased abundance of “good” gut microbes while the levels of several harmful bacteria decreased.  NMN also exerted a protective effect on the inner lining of the intestinal tract (intestinal mucosa), which positively affected gut health. This study indicates a new direction for the use of NMN in promoting gastrointestinal health.

The Gut Microbiome Influences Health and Disease

The basic functions of the gut microbiota include facilitating the decomposition of food, making it easier to absorb and digest, synthesizing essential vitamins, removing toxic compounds, resisting pathogens, maintaining the integrity of the cell layer lining the intestine, and regulating immune function. In addition, damage to the intestinal lining and imbalance in the gut microbiota can cause invasion of microbial communities into intestinal lining cells (mucosal cells) and affect lipid absorption and storage. These changes can lead to metabolic diseases like obesity, type 2 diabetes, and non-alcoholic fatty liver disease.

Although we don’t completely understand these bacteria’s specific effects and mechanisms on lipid and glucose metabolism, we know that they help maintain energy balance in the body. However, due to individual differences, the impact of the gut microbiota on the human body is not clearly or specifically understood.

Long-Term NMN Treatment Affects Species Abundance in the Mouse Gut Microbiota

Pan Huang and colleagues separated mice into groups, supplementing them with different concentrations of NMN (0.1 mg/mL, 0.2 mg/mL, 0.4 mg/mL, and 0.6 mg/mL) in their drinking water for 15 consecutive weeks. They then profiled the microbiome diversity by sequencing samples of fecal matter (the technical term for excrement). The research team found that with increasing amounts of NMN, the diversity of the gut microbiome decreased, and there were changes in specific bacterial populations.

The sequencing results revealed that NMN increases the abundance of butyrate-producing bacteria, such as Ruminococcaceae and Akkermansia (A. muciniphila). Ruminococcaceae are crucial to gastrointestinal health, processing various polysaccharides (carbohydrate-like starch, cellulose, or glycogen) and fibers to produce short-chain fatty acids — the primary energy source of cells in the colon. Akkermansia is an intestinal symbiotic bacterium that colonizes the mucous layer and is considered a promising probiotic candidate. Some studies have reported a decrease in the abundance of A. muciniphila in various diseases, including digestive system diseases, such as irritable bowel disorder (IBD).

Besides increasing the abundance of certain beneficial bacteria, NMN also reduced certain harmful bacteria, such as Bilophila, Oscillibacter, and Desulfovibrionaceae. Bilophila may be related to the occurrence and development of several infection-related diseases, such as sepsis and cholecystitis. These diseases are linked to the presence of the bacterium Oscillibacter, which has been associated with trimethylamino oxide (TMAO) — a risk factor for cardiovascular and cerebrovascular disease. Desulfovibrionaceae are Gram-negative bacteria that produce endotoxin, primarily including lipopolysaccharides (LPS). LPS is very likely to induce inflammation.

The concentration of bile acid-related metabolites was significantly increased in the G1 group. Metabolites related to bile acid are closely related to the gut microbiota, which can have a strong inhibitory effect on the growth of bacteria. Thus, gut microbiota faced strong survival selection pressure from bile acids. This may explain why the diversity of gut microbiota decreased after long-term NMN treatment.

Long-Term NMN Treatment Improves the Intestinal Barrier

NMN didn’t only affect gut microbiome diversity, as treatment with the NAD+ precursor appeared to improve the functionality of the gut. Huang and colleagues revealed an increase in intestinal goblet cells and mucus thickness in response to NMN. Mucins secreted by goblet cells form a barrier that prevents external bacteria from directly contacting the epithelial layer. Its main ingredient, mucins, is a source of nutrients for gut bacteria because it comprises building blocks of proteins (amino acids) and carbs (oligosaccharides). Also, the abundance of tight junctions, which control the gut’s permeability (i.e., more tight junctions enable greater transport of nutrients through the intestinal barrier), increased with NMN treatment.

(Huang et al., 2021 | Frontiers in Nutrition) NMN increases the levels of proteins found in barriers that link cells in the intestinal wall in mice. The levels of tight junction proteins Claudin-1 (Cla-1) and ZO-1 in the colonic epithelium of mice. The darker the amount of grey area stained, the greater the levels of the proteins in question. The NMN-treated mice (G1, bottom row) had an intestinal lining that was darker grey than the untreated mice (CON), indicating higher levels of proteins that create barriers between cells. CON, control mice; G1, 0.1 mg/mL NMN treated mice.

Since a healthy gut should not leak its contents, Huang and colleagues analyzed how well the guts of these mice blocked the movement of a glowing dye. They found that the NMN-treated mice had far less of the compound leak out from the gut. These findings indicate that NMN reduces intestinal mucosal permeability and maintains mucosal barrier integrity.

(Huang et al., 2021 | Frontiers in Nutrition) Long-term NMN treatment improves the integrity of intestinal mucosal lining in mice. Leakage of a glowing compound (FITC-Dextran, FD4) from the gut was measured in mice that drank pure water or water supplemented with NMN. In the images on the left, you can see that the FD4 (red) was spread throughout the body in the untreated mice, indicating leakage from the gut likely due to increased permeability. However, in the NMN treated mice, the FD4 was mainly contained to the intestines, which indicates a healthy intestinal wall that does not leak its contents. Quantification of the amount of FD4 found in the liquid portion of blood (serum), which is inversely linked to intestinal absorption (i.e., the less FD4 in the blood, the greater the intestinal absorption), corroborated the image findings that NMN improves intestinal permeability. CON, control mice; G1, 0.1 mg/mL NMN treated mice.

NMN also promotes intestinal autophagy — the body’s recycling processes for dysfunctional proteins and defunct cells. Recent studies have shown that the integrity of the intestinal cell barrier (epithelial layer) is regulated by autophagy. By establishing a starvation model, Huang and colleagues found that autophagy could enhance these barriers between cells called tight junctions that control the flow of compounds through cell layers. Still, only the flux of small-sized molecules between these tight junctions decreased, as the flux of large-sized probes was not affected. Taken together, these results show that NMN maintains the integrity of the intestinal epithelium by strengthening tight connections and promoting mucus secretion.

Can NMN nurture a microbiome for a healthy human gut?

The findings from this study support that NMN can enhance the gut’s microbiome composition and improve the intestinal lining’s integrity. This study lays the foundation for exploring NMN’s utility in clinical research regarding gut health and microbiome-related health conditions, of which there are many. If these findings do translate to humans, NMN may provide a nutraceutical approach to improve digestive health and help fight off diseases linked to the microbiome, a growing list that is gaining more recognition with the rise in research on gut microbes.