What sort of evidence would it take to challenge my assessment of the data to date that methods of raising NAD+ levels with age, such as nicotinamide riboside supplementation, are not worth pursuing as a major area of focus in research and development? Given the history of work in this area of metabolism, mostly that relating directly to sirtuins and their manipulation, one has to be a little skeptical. Initially promising (and overhyped) results in mice went essentially nowhere, or turned out to make the condition of obesity a little less harmful, while showing little evidence of utility for healthy individuals.

To answer the question, human data showing meaningful benefits that could not be achieved via exercise or calorie restriction would be very interesting. Human data showing some reliable level of reproduction of the benefits of exercise or calorie restriction without side-effects would be good news for the present majority who don't put in the effort to stay in shape. Good news for supplement sellers as well - there is no shortage of people who would pay rather than exercise or eat less, even if the results were mixed or marginal.

In either case, the cost-benefit analysis runs along the lines of (a) as an individual, how much it is worth spending on a supplement that can capture a fraction of the benefits of exercise or calorie restriction, but also (b) is it worth making this a major focus of the research community, versus the rejuvenation biotechnology that can achieve far greater gains? I think (b) is always going to be answered in the negative, for me at least. No calorie restriction mimetic or exercise mimetic can possibly be as good as functional SENS repair biotechnologies. They cannot achieve the results produced by senolytics, or any of the other ways to remove the root causes of aging. If one looks at NAD+ research as the final stage of sirtuin-related calorie restriction research as a whole, it has taken as much funding to get here as it would to completely implement the SENS rejuvenation therapy package in mice. Yet we know that exercise and calorie restriction cannot add decades to healthy life, as is possible in principle for repair therapies.

The data here on human nicotinamide riboside supplementation seems promising in comparison to the results of past sirtuin research, but I'd like to see a larger study group. If that larger group shows similar results, then maybe this is worth it for individuals. Either way, it is appreciated that the authors avoided running a study in overweight individuals - in this part of the field, that just muddies the waters, given the very different effects of sirtuin manipulation on thin versus fat animals. Nonetheless, it still appears to be the case that this is essentially a way to gain some of the beneficial long-term effects of fitness without putting in the physical effort. I expect future NAD+ studies and exercise studies in older individuals to converge in some ways, showing overlapping effects on cellular biochemistry. It is arguable as to whether taking up exercise, eating less, or artificially increasing NAD+ levels should be termed rejuvenation. There is a certainly a sizable grey area at the intersection of repair, compensation, and overriding regulatory signals that respond to aging.