High-protein diet had highest risk, except in older adults

In their study, Prof. Longo and colleagues analyzed data on over 6,800 American adults who took part in the National Health and Nutrition Examination Survey (NHANES) III, a US national survey that assesses health and diet.

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.

The researchers defined a high-protein diet as one where at least 20% of the calories consumed come from protein.

Growth hormone, amino acid damage, ability to process protein may be key factors

The 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.

Researchers trialed 25 different diets in hundreds of mice

In the second study, Prof. Simpson and his group trialed the effects of 25 different diets on hundreds of mice to see how different amounts and types of proteins, fats and carbohydrates affected energy intake, metabolic health, aging and lifespan.

They discovered that:

Mice on diets high in protein and low in carbohydrates had reduced food intake and lower levels of body fat, but they also died earlier and had poorer cardiometabolic health.
Mice on low-protein, high-fat diets had the poorest health and shortest lifespans.
The healthiest, longest living mice were those on diets high in carbohydrates and low in protein – this was in spite of increased food intake and having higher levels of body fat.
A calorie-restricted diet did not increase lifespan – which is contrary to evidence from previous studies on mice, other animals, yeast and worms that show calorie restriction lengthens life as long as supplemented with essential nutrients.