BMI is a misleading indicator in this case. It doesn't differentiate by body composition at all (e.g. fat vs muscle).
I doubt very much that adding muscle to an underweight person would decrease that person's life expectancy. Probably it would improve it (the health benefits of exercise are well known). Either way, it would improve his quality of life.
I don’t think this is true. Pretty much the only proven thing you can do if you want to live longer is calorie restriction. Supposedly this is because the fewer calories you consume the less exposure your body has to free radicals that are important in aging. The corollary of this is that building muscle, which requires eating a lot, is inferior to being a skinny rake for longevity alone. Then, if you eat barely anything, you would be better with only a small amount of exercise.
And, just to rule out any suspicion of bias, know that I have a job where I’m constantly lifting weight, I have my own strength exercise regime outside work, I drink protein shakes, and I eat quite a lot for my frame.
I wonder if calorie restriction works because it slows down your metabolism, and thus slows down cell division, and effectively makes it so you are just living more slowly/aging more slowly than other people. However, I would think you would also have less energy, and get less done. So effectively you would live longer as the calendar turns but you would not live any longer in terms of life experienced.
(this is all just pure speculation, I have never looked into any of the science regarding calorie restriction)
"The corollary of this is that building muscle, which requires eating a lot, is inferior to being a skinny rake for longevity alone."
Not true. Stronger people are less likely to die. Strength reduces your risk of death from all causes - and cancer in particular.
"[The inverse relationship between] muscular strength and death from all causes and cancer persisted after further adjustment for cardiorespiratory fitness; however, the association between muscular strength and death from cardiovascular disease was attenuated after further adjustment for cardiorespiratory fitness."
I doubt that eating less and getting less exercise will really lead to longer life. It would be hard to measure such a thing, but my guess is that with less exercise and eating there would be a corresponding decline in happiness which alone could knock years off a person's life.
But I don't subscribe to the "live long an weak" mentality. I believe that your body is healthier and more capable when well nourished and physically strong. I'm 6'2" 190lbs and continually trying to gain muscle mass.
I stay fit by running and playing 5-a-side soccer but I am 5'9 weight 197lbs, I ran 10k in 50 minutes last weekend. BMI is a strange measure if I can be overweight without trying to put on muscle but be fitter than 95% of the population.
Calorie restriction is actually one of the fiew proven longevity tricks.
Nothing proven about it.
In mice anyway.
Not exactly. The calorie restricted mice do live longer unless they get an infection or some other unexpected source of stress, then they die at much grater rates. It is as if they have no reserves.
In humans calorie restriction is absolutely and utterly unproven.
Humans and mice are very different. They are tiny short lived animals, and we are large long lived animals. That is why a lot of the cancer cures you hear about in mice don't work in humans. Because often we already are born with them. There are known tumor suppressing genes, humans have more copies of them than mice, elephants have even more.
This is why it is not reasonable to assume that simple hacks like calorie restriction, which work in mice... sort of, will also work in humans.
A health diet and exercise on the other hand, have been proven to work in humans.
The one proven thing you can do to live longer is to not die.
Risk prevention / avoidance is a large part of this, but that includes a lot other than bodyweight. Seat belts, condoms, not smoking, little or no alcohol/drugs, pollution.
Several long term athlete studies show longevity, and quality of life benefits, to exercise (Stanford runners study, Scandinavian olympian study). And the specific mechanism of calorie restriction, telomere repair, is shown in both cardio and strength training.
I doubt very much that adding muscle to an underweight person would decrease that person's life expectancy. Probably it would improve it (the health benefits of exercise are well known). Either way, it would improve his quality of life.