Low protein intake may be a key factor in longevity, according to two groups of researchers who published studies in the journal Cell Metabolism on 4 March 2014.

The first study, from researchers at the University of Southern California, suggested that consuming moderate to high levels of animal protein prompted a major increase in cancer risk and mortality in middle-aged adults, while elderly individuals had the opposite result.

Meanwhile, the second team of researchers from the University of Sydney found that a high-protein, low-carbohydrate diet led to a shorter lifespan in mice. Both studies found that not all calories are equal — diet composition and animal protein intake were key players in overall health and longevity.

High protein diet “nearly as bad as smoking”

Researchers from the University of Southern California analysed information on 6, 831 middled-aged and older adults participating in NHANES III, a nationally representative dietary survey in the US.

They found that individuals aged 50 years who reported eating a high-protein diet (with more than 20 per cent of their calories coming from protein) were four times more likely to die of cancer or diabetes and nearly twice as likely to die from any cause in the following 18 years. A moderate-protein diet was associated with a three-fold increase in cancer mortality. These effects were either abolished or reduced in individuals eating a  high-protein diet that was mainly plant based.

For people older than 65 years, however, the effects on mortality were reversed: those who consumed high amounts of protein had a 28 per cent reduced risk of dying from any cause and a 60 per cent reduced risk of dying from cancer. Similar beneficial effects were observed for the moderate-protein intake group.

“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,” said Dr Valter Longo, senior author of the paper, from the University of Southern California.

The researchers found that the effects of protein on an individual’s risk of dying may be caused in part by the activation of growth hormone and the growth factor IGF-1.

“Notably, the activity of these factors, but also body weight, declines naturally with ageing, which may explain why older people did not only benefit but appeared to do worse if they ate a low-protein diet,” Dr Longo said.

Additional experiments in mice suggested that ageing reduced the body’s ability to absorb or process proteins. Experiments in cells also revealed that certain amino acids, which make up proteins, could reduce cellular protection and increase DNA damage, both factors that researchers said were likely to be at the centre of the cancer-promoting effects of proteins in humans.

High protein, low carbs leads to shorter lifespan

In the second study, researchers from the University of Sydney analysed the effects of protein, fat and carbohydrate on energy intake, metabolic health, ageing and longevity and found that a high-protein, low-carbohydrate diet resulted in reduced food intake and body fat, but also led to a shorter lifespan and poor cardiometabolic health.

The researchers examined the effects of 25 different diets in mice, finding that a low-protein, high-fat diet had the most detrimental effects, while a high-carbohydrate, low-protein diet was best, resulting in a longer lifespan and better cardiometabolic health, despite also increasing food intake and body fat.

The study also showed that, contrary to popular opinion, calorie restriction had no beneficial effect on lifespan. Caloric restriction without malnutrition has been consistently shown to increase longevity in a number of animal models, including yeast, worms and mice.

“The advice we are always given is to eat a healthy balanced diet, but what does that mean?” said Professor David Le Couteur co-author of the paper, from the University of Sydney’s Charles Perkins Centre. “This research represents an important step in finding out,” he said.

The researchers predicted that a diet with moderate amounts of high-quality protein that is also relatively low in fat and high in complex carbohydrates would yield the best metabolic health and the longest life.

“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,” said Professor Steve Simpson, senior author of the paper, also from the University of Sydney’s Charles Perkins Centre. “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 said.

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