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 said that it is 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.
However, while 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 US National Health and Nutrition Examination Survey, 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 percent and reduced cancer deaths by 60 percent, 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.
“People need to switch to a diet where only around 9 or 10 percent of their calories come from protein, and the ideal sources are plant-based,” Longo said. “We are not saying go and do some crazy diet we came up with. If we are wrong, there is no harm done, but if we are right, you are looking at an incredible effect that in general is about as bad as smoking.”
“Spend a couple of months looking at the labels on your food. There is a little bit of protein everywhere. If you eat breakfast, you might get 4g protein, but a piece of chicken for lunch may have 50g protein,” said Longo, who skips lunch to control his calorie and protein intake.
People who took part in the study consumed an average of 1,823 calories a day, with 51 percent coming from carbohydrates, 33 percent from fat and 16 percent from protein, of which two-thirds was animal protein. Longo divided them into three groups. The high-protein group got 20 percent or more of their calories from protein, the moderate group got between 10 percent and 19 percent of their calories from protein, and the low group got less than 10 percent of calories from protein.
Teasing out the health effects of individual nutrients is notoriously difficult.
“I would urge general caution over observational studies, and particularly when looking at diet, given the difficulties of disentangling one nutrient or dietary component from another. You can get an association that might have some causal linkage or might not,” said Peter Emery, head of nutrition and dietetics at King’s College London.