Research into calorie restriction has, as I'm sure you know, been picking up in pace and breadth over the past decade or so. The drug industry is interested in trying to replicate the end results without the dietary changes, which should ensure continuing funds and interest for the foreseeable future. Lowering calorie intake while still obtaining optimal levels of micronutrients is a practice that, much like regular exercise, has long term beneficial effects on health and longevity in healthy animals and people. These benefits are very reliable and exceed those that can be provided by any presently available medical technology - they also exceed the results of many of the interesting ways to extend healthy life in laboratory animals. This state of affairs won't last, given the pace of progress in biotechnology, but based on the present landscape for medical development we should expect calorie restriction and exercise to remain the best available tools for most of us for a good twenty years yet.
I noticed a good open access review paper today (the full thing is in PDF format only) that covers all of what has been learned of calorie restriction (CR) in the past decade or two. That includes the results of a fair number of human studies, what is now known of the underlying biochemistry and mechanisms, differences between intermittent fasting versus constant dietary restriction (DR), and more:
CR has been demonstrated to extend the maximal lifespan of a diverse group of species. This extension of life is maximized when: 1) the magnitude of CR is elevated to the highest possible value before inducing malnutrition and 2) the duration of CR is maximized. Animals on CR regimens exhibit a variety of improvements in overall health in general and cardiovascular health in particular. Unfortunately, the likelihood of discovering whether or not CR extends human life is rather remote due to the ethical and logistical limitations of research design. The optimal magnitude and duration of CR for humans will also likely never be known for the same reason. Nonetheless, many human CR studies have noted favorable changes in biomarkers related to cardiovascular and glucoregulatory function, which likely relate to quality of life and may relate to longevity.
Regarding DR, neither carbohydrate restriction nor lipid restriction extend life. However, protein restriction appears to extend maximum lifespan by 20%. Recent findings suggest that methionine restriction may be the single cause of life extension observed in protein restriction studies. Future studies should examine the effects of different magnitudes of methionine restriction on life extension.
It's pessimistic to think that optimal calorie restriction practices will not be uncovered for humans, however. Scientists can measure changing levels in biomarkers over years of calorie restriction, so identifying optimal practices really rests on the larger area of work that relates to linking biomarkers with aging and longevity. If measurable aspects of our biology can be firmly linked to future life expectancy - and there is no reason to think that the development of such biomarkers is implausible - then a whole range of medical and health practices become open to far greater optimization than is presently possible.