Nicotinamide riboside supplementation is one of the ways to increase levels of NAD+ in mitochondria, thus improving mitochondrial function. This probably does little for young people, particularly young and physically fit people, but in old age NAD+ levels decline along with mitochondrial function. Mitochondria are the power plants of the cell, and with aging they suffer a general malaise that is detrimental to tissue function, especially in energy-hungry tissues such as muscles and the brain. The causes are still poorly understood, though a faltering of the quality control mechanism of mitophagy due to loss of mitochondrial fission appears to be involved. Increased NAD+ appears to override this decline to some degree, albeit without addressing any of the underlying and still problematic root causes.
In early human trials, NAD+ upregulation has been shown to modestly improve vascular function in older individuals, most likely by reversing some of the dysfunction in smooth muscle cell behavior. In mice a broader range of benefits has been demonstrated, though it remains to be seen how many of those also appear in humans to a significant degree. The work here is more along the same lines, in which researchers show that nicotinamide riboside supplementation can restore intestinal stem cell function in older mice. This should improve tissue function, but again it is worth bearing in mind that this is only overriding a reaction to the underlying damage of aging - it doesn't fix that damage, which still carries on to produce all of its other downstream issues.
Researchers have long studied the link between aging and sirtuins, a class of proteins found in nearly all animals. Sirtuins, which have been shown to protect against the effects of aging, can also be stimulated by calorie restriction. In 2016 it was found that, in mice, low-calorie diets activate sirtuins in intestinal stem cells, helping the cells to proliferate. In a new study, researchers set out to investigate whether aging contributes to a decline in stem cell populations, and whether that decline could be reversed.
By comparing young (aged 3 to 5 months) and older (aged 2 years) mice, the researchers found that intestinal stem cell populations do decline with age. Furthermore, when these stem cells are removed from the mice and grown in a culture dish, they are less able to generate intestinal organoids, which mimic the structure of the intestinal lining, compared to stem cells from younger mice. The researchers also found reduced sirtuin levels in stem cells from the older mice.
Once the effects of aging were established, the researchers wanted to see if they could reverse the effects using a compound called nicotinamide riboside (NR). This compound is a precursor to NAD, a coenzyme that activates the sirtuin SIRT1. They found that after six weeks of drinking water spiked with NR, the older mice had normal levels of intestinal stem cells, and these cells were able to generate organoids as well as stem cells from younger mice could.
To determine if this stem cell boost actually has any health benefits, the researchers gave the older, NR-treated mice a compound that normally induces colitis. They found that NR protected the mice from the inflammation and tissue damage usually produced by this compound in older animals. "That has real implications for health. Just having more stem cells is all well and good, but it might not equate to anything in the real world. Knowing that the guts are actually more stress-resistant if they're NR-supplemented is pretty interesting."