In mice, the body absorbs vitamin B3 (nicotinic acid) early in the gut, so gut bacteria farther down don’t get much of it — scientists saw almost no labeled B3 ending up in those bacteria’s cells.

When mice take NMN or NR supplements by mouth, most of it gets changed into another form—nicotinic acid—by gut bacteria before the body uses it. The liver mostly uses this changed form to make an important molecule called NAD+, and only a little bit gets used directly.

In mice, a nutrient made by the body called nicotinamide travels to the gut and becomes the main building block that gut microbes use to make an important molecule called NAD—most of it in the small intestine. This shows the microbes rely heavily on the host's nutrients to survive.

In mice, a supplement called nicotinamide riboside only boosts energy-related molecules if gut bacteria first turn it into a different form—without those bacteria, the supplement doesn’t work.

Your gut bacteria can turn a form of vitamin B3 from your body into another version that gets reused to make important energy molecules, especially when your body’s usual recycling system isn’t working well.