Ketone bodies are the small molecules that are produced by the liver when the body is in a state of ketosis. These can be readily used by the body and, most notably, the nervous system, and one of their functions is to spare lean tissue during ketosis, since with the burning of ketones, the body does not have to break down muscle in order to make blood glucose.
The state of ketosis is readily entered when severely restricting carbohydrates in the diet for just a short while; for instance, if someone goes on the Atkins diet, or generally keeps carbs below 50 grams a day. (If one exercises a lot or is otherwise physically active, one can eat more carbs, say up to 100 grams, and remain in ketosis.)
It turns out that in the roundworm C. elegans, one of the ketone bodies, beta hydroxybutyrate, extends lifespan: D-beta-hydroxybutyrate extends lifespan in C. elegans.
βHB supplementation extended mean lifespan by approximately 20%. … βHB did not extend lifespan in a genetic model of dietary restriction indicating that βHB is likely functioning through a similar mechanism. βHB addition also upregulated ΒHB dehydrogenase activity and increased oxygen consumption in the worms.
So, the ketone functioned similarly to dietary restriction, increased lifespan by 20%, and caused increased metabolism.
It looks like being in ketosis much of the time could be, gasp, good for you.
The probable future Nobel Laureate Cynthia Kenyon discovered that a mutation in insulin signalling in C. elegans caused radically increased lifespan. When she made that discovery, she herself went on a low-carbohydrate diet.
So, add all this to the evidence for the healthiness of a low-carbohydrate diet.