In the drying puddles of Mozambique lives a fish whose entire life fits into a single season. The African killifish Nothobranchius furzeri hatches, matures and grows old in four to six months. For scientists this is not a curiosity but time-lapse photography: everything that happens to human muscles over decades shows up here within weeks.
The secret turned out to lie not in the quantity of muscle tissue but in its quality. In females, fasting brought back fast muscle fibers and restored the connections between nerves and muscles. In males, other mechanisms kicked in: cells began using fats and sugar more efficiently. Think of it as home renovation: the females were fixing the wiring, while the males were overhauling the heating system.
This difference in repair work explains why one-size-fits-all diets against age-related muscle weakness perform unevenly. After menopause, female muscles lose the fast fibers needed for sudden movements. Male muscles start absorbing energy poorly earlier in life. The same diet cannot fix both sexes in the same way.
Now it is clear why doctors should ask about a patient's sex before recommending fasting. The little fish that survived its native puddle drying up is showing us how people can hold on to their strength well into old age.
The killifish Nothobranchius furzeri lives in ephemeral pools in Mozambique and Zimbabwe that dry up within a few months. Its eggs can survive complete desiccation in diapause for a year or more. In the laboratory the fish live 4-6 months, making them the shortest-lived vertebrates bred in captivity.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.