I guess Iâm a bit confused by the questions, too. Iâll try to explain better. I donât think the point Iâm trying to make is very deep, since I donât have any expertise in this area.
In summary, if caloric restriction is both negative affectively and reduces aging in modern humans not living in poverty (and some captive animals), this is because evolution hasnât had the time (or the evolutionary pressures are too weak) to get us to just want to eat less; caloric restriction (compared to how much they currently eat in the wild) probably just doesnât reduce aging overall in the wild. Since this research is for animals living in the wild, the proposed examples of caloric restriction (in humans, and Iâd add in some captive animals) and smoking not covered by the theory might not be very relevant. I suppose this might only be the case in the very long-term if evolutionary pressures still apply or in the short-term if we donât make the lives of wild animals substantially easier. If we do make the lives of wild animals substantially easier, then biological aging might correlate less with welfare in the short-term than it had before, so it would be relevant.
To expand more:
I think we should doubt that animals eat (and our ancestors ate) more than is best for their fitness (or welfare) in the wild, since how much they eat is subject to natural selection. If eating less reduced biological aging, then this would mean that biological aging wouldnât track some harms (to fitness and welfare) that result from eating less. But I think itâs also plausible that eating less just doesnât reduce biological aging *in the wild*, since animals probably need (and our ancestors probably needed) more energy in the wild for survival and reproduction than in captivity (than modern humans not living in poverty, resp.), where the experiments were performed. It could also be that animals eat more in captivity than in the wild because of ease of access or boredom or other environmental factors, and the amount they eat in the wild is roughly optimal for fitness, welfare and biological aging (regardless of predation, trouble getting food, disease recovery, etc.) The same could be true of modern humans not living in poverty compared to our ancestors; our ancestors were undergoing caloric restriction compared to what we eat today, but this might have been what was best for them because they had more important things to do than look for even more food.
Maybe itâs the case that metabolism inevitably causes some harm, e.g. oxidation, and this harm canât be âevolved outâ, but the extra energy is worth some such extra harm in the wild, so that animals have a better chance to escape predators, or find food, or recover from disease, which are not really issues in captivity (and for modern humans not living in poverty). Spending less energy on the functions they have might be more harmful to fitness than eating less.
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 thereâs actually a disagreement here.
I certainly agree that âwe should doubt that animals eat (and our ancestors ate) more than is best for their fitnessâ. If they were eating more than was good for their fitness, youâd 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 âmaking the best of a bad situationâ 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 doesnât 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/âlongevity, but further energy is diverted away from health/âlongevity 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?
I think dietary restriction could be dangerous in expectation, because animals might really need that extra energy on rare occasions.
Okay, so from this I think you mean the metabolic response to dietary restriction, not the actual restriction of diet.
If thatâs dangerous in expectation, why would it have evolved?
I guess Iâm a bit confused by the questions, too. Iâll try to explain better. I donât think the point Iâm trying to make is very deep, since I donât have any expertise in this area.
In summary, if caloric restriction is both negative affectively and reduces aging in modern humans not living in poverty (and some captive animals), this is because evolution hasnât had the time (or the evolutionary pressures are too weak) to get us to just want to eat less; caloric restriction (compared to how much they currently eat in the wild) probably just doesnât reduce aging overall in the wild. Since this research is for animals living in the wild, the proposed examples of caloric restriction (in humans, and Iâd add in some captive animals) and smoking not covered by the theory might not be very relevant. I suppose this might only be the case in the very long-term if evolutionary pressures still apply or in the short-term if we donât make the lives of wild animals substantially easier. If we do make the lives of wild animals substantially easier, then biological aging might correlate less with welfare in the short-term than it had before, so it would be relevant.
To expand more:
I think we should doubt that animals eat (and our ancestors ate) more than is best for their fitness (or welfare) in the wild, since how much they eat is subject to natural selection. If eating less reduced biological aging, then this would mean that biological aging wouldnât track some harms (to fitness and welfare) that result from eating less. But I think itâs also plausible that eating less just doesnât reduce biological aging *in the wild*, since animals probably need (and our ancestors probably needed) more energy in the wild for survival and reproduction than in captivity (than modern humans not living in poverty, resp.), where the experiments were performed. It could also be that animals eat more in captivity than in the wild because of ease of access or boredom or other environmental factors, and the amount they eat in the wild is roughly optimal for fitness, welfare and biological aging (regardless of predation, trouble getting food, disease recovery, etc.) The same could be true of modern humans not living in poverty compared to our ancestors; our ancestors were undergoing caloric restriction compared to what we eat today, but this might have been what was best for them because they had more important things to do than look for even more food.
Maybe itâs the case that metabolism inevitably causes some harm, e.g. oxidation, and this harm canât be âevolved outâ, but the extra energy is worth some such extra harm in the wild, so that animals have a better chance to escape predators, or find food, or recover from disease, which are not really issues in captivity (and for modern humans not living in poverty). Spending less energy on the functions they have might be more harmful to fitness than eating less.
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 thereâs actually a disagreement here.
I certainly agree that âwe should doubt that animals eat (and our ancestors ate) more than is best for their fitnessâ. If they were eating more than was good for their fitness, youâd 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 âmaking the best of a bad situationâ 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 doesnât 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/âlongevity, but further energy is diverted away from health/âlongevity 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?