Maybe it would be helpful if I try to lay out my rough model of why fasting responses to limited food availability exist in the wild, and we can see if there’s actually a disagreement here.
I certainly agree that “we should doubt that animals eat (and our ancestors ate) more than is best for their fitness”. If they were eating more than was good for their fitness, you’d expect them to evolve to eat less. However, wild animals exist in a state of severe food insecurity, in which food may be abundantly available one day and scarce for weeks thereafter. It probably is quite difficult to have offspring while food is scarce, and probably not very valuable anyway since those offspring will be food-deprived during crucial developmental periods. So it makes sense to use what energy you have to maintain a healthy body, and wait for better times.
The response to DR would therefore be a “making the best of a bad situation” sort of thing: from a fitness perspective it would be better to eat lots of food, have lots of offspring, and die young, but since that option is unavailable due to food scarcity it is better to activate an energy-conserving fasting response that will keep you in better shape until the good times return.
Importantly, the claim is not that DR improves fitness. It is that it increases lifespan. Natural selection doesn’t care about increased lifespan, or even increased healthspan, except insofar as it increases the number of descendents you have. And food deprivation is certainly very costly: DR mice show dramatically reduced fertility relative to AL (=eat-as-much-as-you-want) mice. However, they also show less age-related decline in fertility, so if you later put them back on an AL diet they are more fertile than mice of the same age that have been on AL the whole time. I think that summarises the evolutionary point of a fasting response pretty well.
Ok, this makes sense.
Flipping it (comparing non-fasting to fasting), not only is the extra energy when not fasting used for functions other than health/longevity, but further energy is diverted away from health/longevity towards reproduction, because the best time to reproduce is when not fasting.
If the comparison were to starvation instead of fasting, some of the extra energy would be used for basic survival. But fasting is milder.
Is my understanding correct?
Maybe it would be helpful if I try to lay out my rough model of why fasting responses to limited food availability exist in the wild, and we can see if there’s actually a disagreement here.
I certainly agree that “we should doubt that animals eat (and our ancestors ate) more than is best for their fitness”. If they were eating more than was good for their fitness, you’d expect them to evolve to eat less. However, wild animals exist in a state of severe food insecurity, in which food may be abundantly available one day and scarce for weeks thereafter. It probably is quite difficult to have offspring while food is scarce, and probably not very valuable anyway since those offspring will be food-deprived during crucial developmental periods. So it makes sense to use what energy you have to maintain a healthy body, and wait for better times.
The response to DR would therefore be a “making the best of a bad situation” sort of thing: from a fitness perspective it would be better to eat lots of food, have lots of offspring, and die young, but since that option is unavailable due to food scarcity it is better to activate an energy-conserving fasting response that will keep you in better shape until the good times return.
Importantly, the claim is not that DR improves fitness. It is that it increases lifespan. Natural selection doesn’t care about increased lifespan, or even increased healthspan, except insofar as it increases the number of descendents you have. And food deprivation is certainly very costly: DR mice show dramatically reduced fertility relative to AL (=eat-as-much-as-you-want) mice. However, they also show less age-related decline in fertility, so if you later put them back on an AL diet they are more fertile than mice of the same age that have been on AL the whole time. I think that summarises the evolutionary point of a fasting response pretty well.
Ok, this makes sense. Flipping it (comparing non-fasting to fasting), not only is the extra energy when not fasting used for functions other than health/longevity, but further energy is diverted away from health/longevity towards reproduction, because the best time to reproduce is when not fasting. If the comparison were to starvation instead of fasting, some of the extra energy would be used for basic survival. But fasting is milder. Is my understanding correct?