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.

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.

If you take antibiotics that wipe out your gut bacteria, your body can't turn a supplement called nicotinamide riboside into a form that helps make an important molecule called NAD+, so it doesn't work anymore.

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.

When mice get NMN or NR through an IV, their bodies quickly break it down, send it to the gut, where gut bacteria turn part of it into another form that gets recycled back to the liver to make an important energy molecule—so the gut-liver loop is key, even with injections.