In the past few years two ongoing studies of long term calorie restriction (CR) in primates have started to publish their results on longevity. Both research programs have been underway for more than 20 years, one run by the National Institute on Aging and the other by the University of Wisconsin-Madison. Researchers have followed small groups of rhesus monkeys to see how the benefits to health and life expectancy resulting from a restricted calorie intake compare with those obtained in mice and other short-lived species. At this point the results are ambiguous, unfortunately: one study shows a modest gain in life expectancy that has been debated, while the other shows no gain in life expectancy, and that result has also been debated.
Calorie restriction does produce considerable benefits in short term measures of health in rhesus monkeys and humans, that much is definitive, but the present consensus in the research community is that it doesn't greatly extend life in longer-lived primates - perhaps a few years at most in humans. Differences and issues in the two primate studies mean that effects of this size on longevity may never be clear from the data generated. Other factors will wash it out, such as differences in the diet fed to the control groups, or the different age at which calorie restriction started. Certainly the results so far support the conjecture that calorie restriction is exceedingly good for health but doesn't have the same impressive effects on longevity as it does in short-lived animals. Why that is the case is a puzzle to be solved - but not one that has a great deal of relevance to the future of human longevity. One would hope that we'll be a long way down the road to rejuvenation therapies by the time another set of better constructed primate studies are nearing completion.
You'll find a long article over at the SENS Research Foundation that examines the NIA and Wisconsin primate studies, their differences, and their results in great detail - but I'm just going to skip ahead and quote some of the conclusions:
In this post, I have sketched out in detail two major possible interpretations of the disparate mortality outcomes in the NIA and WNPRC nonhuman primate CR studies. The "Diminishing Returns" hypothesis posits that the health and longevity benefits of "CR" reported in the WNPRC study were merely the unsurprising results of one group of animals being fed a high-sucrose, low-nutrient chow on a literally ad libitum basis, and another group being kept to portions of that diet low enough to avoid the deranged metabolisms flowing from obesity and (possibly) fructose toxicity. In this interpretation, the more severe restrictions of energy intake imposed at the NIA - particularly when the chow to which access was restricted may have been healthier to begin with - led to no further health benefit, because there are none to be gained: the dramatic age-retarding effects of CR observed in laboratory rodents and other species do not translate into longevous species such as primates, and the sole benefit of controlling energy intake is avoidance of overweight and obesity.
The "Dose-Response" hypothesis begins from the same interpretation of the WNPRC study, but posits that far from being excessive (or, at best, superfluous) to that required for good health, the additional energy restriction imposed at NIA were too little, and imposed during too narrow a window, to elicit a clear signal in health and lifespan benefits; this is supported by the evidence that the NIA primates were not especially hungry, and only weakly and inconsistently exhibited improvements in risk factors and endocrine signatures of CR that are seen both in life-extending CR in rodents, and in humans under rigorous CR.
Unfortunately, it seems very unlikely that this question will be resolved. Even the narrow question of whether the age-retarding effects of CR in laboratory rodents translate into nonhuman primates could only be established with confidence after yet another trial in nonhuman primates. [Such] a study is extremely unlikely in light of the enormous expense of the first two trials, disappointment (and possibly embarrassment) with the results, [and] the ill winds for nonhuman primate research. [Even] if such a well-designed and well-executed study were initiated: what then? Supposing that support were maintained for the duration of the experiment [it] would be a further three decades before the earliest point at which survival data could be reported.
The timescales involved in resolving these questions cannot be reconciled with the immediate imperatives that drive us to pose them. With the scale of the humanitarian, economic, and social crisis that looms in the coming decades due to global demographic aging and associated ill-health, the near-term need for effective interventions against the aging process could not be greater. Whether CR can retard aging in nonhuman primates or not; whether it can retard aging in humans or not; whether even effective CR mimetics can somehow be shepherded through clinical trials - even the most optimistic projection for retarding aging through such approaches is inadequate to the needs and suffering of aging world.
The point made in the article is the same one that should be made for all means of slowing the pace of aging by altering metabolism, whether by the use of drugs to replicate some of the changes caused by calorie restriction or via other mechanisms. These are very difficult and challenging projects, certainly very expensive in time and funds, and which will produce poor and uncertain end results even if successful. Ways to modestly slow aging do nothing for people who are already old, and we will grow old waiting for success in the development of drugs that can safely tinker our metabolisms into a state of slower aging.
The better approach is that outlined by the SENS Research Foundation: targeted therapies to repair the known forms of cellular and molecular damage that cause aging. This path is cheaper, more certain, and the resulting therapies will be capable of rejuvenation - of reversing degenerative aging, not just slowing it down a little. They will be greatly beneficial for the old, and extend the length of life lived in health and vigor. This is why I say that calorie restriction studies are irrelevant to the future of our health and longevity: the only thing that really matters is whether or not the SENS vision or similar repair therapies are prioritized, funded, and developed.