They found that:
- Participants aged 50 and over who said they ate a high-protein diet were four times more likely to die from cancer or diabetes, and twice as likely to die from any cause, in the following 18 years.
- Those who consumed moderate amounts of protein had a three-fold higher chance of dying of cancer.
- These effects either reduced or disappeared altogether among participants whose high-protein diet was mainly plant-based.
- However, in those aged 65 and over, the effect was nearly the opposite - high protein intake was linked to a 60% reduced risk of dying from cancer and a 28% reduced risk of dying from any cause, with similar effects for moderate protein intake.
Growth hormone, amino acid damage, ability to process protein may be key factorsThe team suggests, because of evidence from other studies, that growth hormone and the growth factor IGF-1 may be responsible for these effects, as Prof. Longo explains:
"Notably, the activity of these factors, but also body weight, declines naturally with aging, which may explain why older people not only did not benefit but appeared to do worse if they ate a low-protein diet."
Cell experiments have suggested the amino acids that proteins are made of can reduce cellular protection and increase damage to DNA, both of which might explain why high-protein intake is linked to cancer.
Also, experiments in mice have shown that the body's ability to process protein declines with age.