This open access study has something for everyone to argue about, no matter your previous position on diet and health. The basic idea that lower dietary protein levels are beneficial and increase life expectancy is straightforward and supported by research on calorie restriction and methionine restriction. But the results showing that low protein intake becomes disadvantageous and increases mortality in old age run contrary to past studies that demonstrated calorie restriction to be beneficial in old age.
This is not even to start in on talking about competing theories as to why animal protein may be worse for health over the long term than plant protein: because vegetarians tend to be more health-conscious in other ways; because of greater levels of dietary AGEs in cooked meat; because of greater methionine intake associated with meat eating; and so forth.
So this all suggests, as usual, that greater complexity is buried here. This is all interesting, but of course somewhat irrelevant to the future of longevity, which will arrive via new medical technology to repair the damage of aging, not via dietary changes. You'll want to click through and look at the infographic at the head of the paper for a better summary of the findings than the description in the abstract:
Mice and humans with growth hormone receptor/IGF-1 deficiencies display major reductions in age-related diseases. Because protein restriction reduces GHR-IGF-1 activity, we examined links between protein intake and mortality.
Respondents aged 50-65 reporting high protein intake had a 75% increase in overall mortality and a 4-fold increase in cancer death risk during the following 18 years. These associations were either abolished or attenuated if the proteins were plant derived. Conversely, high protein intake was associated with reduced cancer and overall mortality in respondents over 65, but a 5-fold increase in diabetes mortality across all ages.
Mouse studies confirmed the effect of high protein intake and GHR-IGF-1 signaling on the incidence and progression of breast and melanoma tumors, but also the detrimental effects of a low protein diet in the very old. These results suggest that low protein intake during middle age followed by moderate to high protein consumption in old adults may optimize healthspan and longevity.