Oddbean

"Some mice were placed on calorie-limited diets, another group followed intermittent fasting regimens, and others were allowed to eat freely." Sounds like only some of the mice were fasted intermittently, others had calories restricted. I'm certainly open to the benefits of intermittent fasting and have experimented with it myself. Having practiced (natural) bodybuilding for years in the past I was never able to maintain muscle mass whilst restricting calories. This may just speak to my lack of talent as a bodybuilder. Pros are typically still using some sort of exogenous testosterone derivative while dieting for competition so I don't think that's a good example. As a competitive weightlifter in multiple weight classes for over 15 years I feel I have a good understanding of how my body reacts to various dietary interventions and in my experience anything but brief caloric restriction leads to loss of both fat and muscle (not to mention strength). BTW the Cambridge Dictionary's definition of frail seems to cover both our usages. https://m.primal.net/LRbC.jpg

The 1 day per week intermittent fasting group appears the most interesting to me as overall caloric intake was maintained but body weight slightly reduced (possible improvement in body composition?) and some measured health benefits. To me the loss of lean mass (and probable hunger) experienced by the 40% and 20% caloric restriction groups (if applicable to humans) would considerably reduce quality of life while increasing the risk of hospitalisation and early death from falling injuries in later life. Each to their own I suppose. My preference would be to live an averagely long life with a high capacity over a long life with compromised capacity.