The research, published in the journal Cell Metabolism, followed the diets and health of more than 6,000 people aged over 50 for 18 years, who took part in the US nationally representative dietary survey, NHANES III.
It found that cancer death risk among those who consumed the most protein was four times higher than those who ate the least – a risk level comparable with smoking, according to the University of Southern California (USC), which was behind the research. Death risk from any cause among the highest protein consumers was 74% higher during the period than those who ate the least protein.
"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 USC's Valter Longo, the study's senior author.
Cancer prevention?
However, these associations were almost completely eliminated when the proteins were plant-derived, the researchers found.
"The research shows that a low-protein diet in middle age is useful for preventing cancer and overall mortality, through a process that involves regulating IGF-1 and possibly insulin levels," said co-author Eileen Crimmins, the AARP Chair in Gerontology at USC. "However, we also propose that at older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty."
Growth hormone IGF-1 is linked to cancer susceptibility, but is important for normal growth.
The study’s authors defined protein consumption as ‘high’ when it accounted for more than 20% of calories, ‘moderate’ at 10-19% of calories, and ‘low’ at less than 10%.
Meat and cheese in moderation
However, experts warned against comparing cancer risk associated with animal protein consumption with cancer risk from smoking.
Heather Ohly, of the University of Exeter Medical School, and registered nutritionist, said: “Smoking has been proven to be entirely bad for us, whereas meat and cheese can be consumed in moderation as part of a healthy diet, contributing to recommended intakes of many important nutrients. However, excessive consumption of these foods will take people over the recommended intakes of saturated fats and protein, which may be harmful as indicated by these studies.
“In the UK, it is recommended that protein makes up 10-15% of our average daily energy intake.”
Gunter Kuhnle, a food nutrition scientist at the University of Reading, said: “While this study raises some interesting perspectives on links between protein intake and mortality, it’s certainly not true that this is the first study to make such a link. It is also wrong, and potentially even dangerous, to compare the effects of smoking with the effect of meat and cheese in such a way.”
Benefits for the elderly
The study comes at a time when added protein is appearing in more and more products, touted as a way to quell hunger pangs, or to deal with muscle wastage in the elderly in particular.
However, while the research found that even moderate protein consumption increased risk of death from cancer among the middle aged, it did suggest that moderate to high protein consumption among older adults – aged 65 and over –may indeed be beneficial and could help optimise health and longevity.
The World Health Organisation recommends that adult women consume about 48 g of protein a day, and adult men about 56 g. In Europe, average protein consumption ranges from about 99-115 g a day, and the WHO has said that protein deficiency was eliminated from the European Union after the Second World War.
Source: Cell Metabolism
Vol. 19, Iss. 3, pp. 407-417, doi:10.1016/j.cmet.2014.02.006
“Low Protein Intake Is Associated with a Major Reduction in IGF-1, Cancer, and Overall Mortality in the 65 and Younger but Not Older Population”
Authors: Morgan E. Levine, Eileen M. Crimmins, Valter D. Longo, et al.