A diet rich in meat, eggs, milk and cheese could be as harmful to health as smoking, according to a controversial study into the impact of protein consumption on longevity.
High levels of dietary animal protein in people under 65 years of age was linked to a fourfold increase in their risk of death from cancer or diabetes, and almost double the risk of dying from any cause over an 18-year period, researchers found. However, nutrition experts have cautioned that it's too early to draw firm conclusions from the research.
The overall harmful effects seen in the study were almost completely wiped out when the protein came from plant sources, such as beans and legumes, though cancer risk was still three times as high in middle-aged people who ate a protein-rich diet, compared with those on a low-protein diet.
But whereas middle-aged people who consumed a lot of animal protein tended to die younger from cancer, diabetes and other diseases, the same diet seemed to protect people's health in old age.
The findings emerged from a study of 6,381 people aged 50 and over who took part in the National Health and Nutrition Examination Survey (NHANES) which tracks a representative group of adults and children in the US.
The study throws doubt on the long-term health effects of the popular Atkins and Paleo diets that are rich in protein. Instead, it suggests people should eat a low-protein diet until old age when they start to lose weight and become frail, and then boost the body's protein intake to stay healthy. In the over-65s, a high-protein diet cut the risk of death from any cause by 28%, and reduced cancer deaths by 60%, according to details of the study published in the journal Cell Metabolism
Valter Longo, director of the Longevity Institute at the University of Southern California, said that on the basis of the study and previous work, people should restrict themselves to no more than 0.8g of protein a day for every kilogram of body weight, equivalent to 48g for a 60kg person, and 64g for an 80kg person.