The evidence for intermittent fasting to improve health and extend life is pretty good, but nowhere near as solid as that for calorie restriction. There appear to be separate mechanisms at work: researchers have demonstrated at least some benefits to result in laboratory animals for intermittent fasting without reduction in dietary calorie levels, for example. Additionally, the gene expression profiles of intermittent fasting and calorie restricted mice are noticeably different. Still, it seems likely that in most cases some of the benefits of intermittent fasting are derived from a reduction in overall calorie intake.
Thousands of people deliberately practice calorie restriction on the basis of the scientific evidence to date, and over the past decade or two the Calorie Restriction Society has spurred research programs that have produced pretty compelling human data. Given that intermittent fasting is in many ways an easier sell to the public in this age of obesity and low-cost food, I imagine that we'll see a similar growth in research and awareness in the years ahead. That says nothing about the relative merits or level of scientific support for outcomes at this point, of course - and on that count calorie restriction is far ahead.
Periods of deliberate fasting with restriction of solid food intake are practiced worldwide, mostly based on traditional, cultural or religious reasons. There is large empirical and observational evidence that medically supervised modified fasting (fasting cure, 200-500 kcal nutritional intake per day) with periods of 7-21 days is efficacious in the treatment of rheumatic diseases, chronic pain syndromes, hypertension, and metabolic syndrome. The beneficial effects of fasting followed by vegetarian diet in rheumatoid arthritis are confirmed by randomized controlled trials.
Further beneficial effects of fasting are supported by observational data and abundant evidence from experimental research which found caloric restriction and intermittent fasting being associated with deceleration or prevention of most chronic degenerative and chronic inflammatory diseases. Intermittent fasting may also be useful as an accompanying treatment during chemotherapy of cancer.
A further beneficial effect of fasting relates to improvements in sustainable lifestyle modification and adoption of a healthy diet, possibly mediated by fasting-induced mood enhancement. Various identified mechanisms of fasting point to its potential health-promoting effects, e.g., fasting-induced neuroendocrine activation and hormetic stress response, increased production of neurotrophic factors, reduced mitochondrial oxidative stress, general decrease of signals associated with aging, and promotion of autophagy. Fasting therapy might contribute to the prevention and treatment of chronic diseases and should be further evaluated in controlled clinical trials and observational studies.