Too much protein in middle age 'as bad as smoking'

Two new studies conclude that low protein intake may hold the key to a long and healthy life, at least until old age. They also emphasize the need to examine not only calories when deciding what constitutes a healthy diet, but also where those calories come from - such as whether protein is animal or plant-based.

Another key finding is the suggestion that while a high-protein diet may in the short term help people lose weight and body fat, in the long term it may harm health and reduce lifespan.

Both studies are published in the journal Cell Metabolism.

The first study was led by Valter Longo, a professor at the University of Southern California, who counts longevity and cell biology among his areas of expertise.

He and his colleagues showed that high protein consumption is linked to increased risk of cancer, diabetes and death in middle-aged adults, although this was not the case for older adults who may benefit from moderate protein consumption. Also, the effect is much reduced when the protein comes from plant sources.

The second study was led by Stephen Simpson, a professor at the University of Sydney in Australia, whose group works at the interface of physiology, ecology, and behavior. From studying mice, he and his fellow authors concluded that diets low in protein and high in carbohydrates are linked to the longest lifespans.

Both studies suggest it is not just calories, but also diet composition - particularly in terms of amount and type of protein - that may determine the length and health of a lifespan.

Prof. Longo says:

"We studied simple organisms, mice and humans, and provide convincing evidence that a high- protein diet - particularly if the proteins are derived from animals - is nearly as bad as smoking for your health."

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.

Prof. Simpson says:

"We have shown explicitly why it is that calories aren't all the same - we need to look at where the calories come from and how they interact. This research has enormous implications for how much food we eat, our body fat, our heart and metabolic health, and ultimately the duration of our lives."

He and his colleagues suggest the ideal diet for a long and healthy life is one with moderate amounts of high-quality protein, low in fat, and high in complex carbohydrates.