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: