March 4, 2014

expert reaction to two new studies looking at dietary protein, health and longevity

Two groups of researchers publishing in Cell Metabolism each suggest that low protein intake may be a key factor in leading a long and healthy life; one found a correlation between consuming moderate to high levels of animal protein and an increase in cancer risk and mortality in adults, while the other found that a high-protein, low-carbohydrate diet led to a shorter lifespan in mice. 

Prof Tom Sanders, Head of the Nutritional Sciences Research Division, King’s College London, said:

“The headline of the press release from the University of Southern California is running ahead of the evidence, and the comparison with smoking is really unwarranted in terms of the relative risks and the certainty of the adverse effects of smoking. The study shows a relationship with growth factor IGF-1 and cancer risk which is already known. However, the relationship between IGF-1 levels and protein intake is far more tenuous in humans. Cross-sectional data i.e. omnivores vs vegans  suggest animal protein to be associated with increased IGF-1 levels but there is a lack of evidence from controlled feed studies to show that IGF-1 levels fall when animal protein intake is restricted.  Much of the supporting work is based on studies in mice not humans. Dietary guidelines should not be based on animal experiments.

“Although the follow-up on the NHANES survey* shows that those with the highest reported protein intake were at greater risk of all-cause mortality, it fails to adjust for other confounding factors such as socioeconomic status, smoking, and obesity. The sample size is also modest at 6381, compared with over 448,568 in the European Prospective Investigation into Cancer which only found a weak association (14% increase in risk of mortality with red meat consumption, which was more consistent for processed meat (11% increase in risk)). The European data suggest a much smaller effect than the 74% increased risk claimed in this paper.

“Also, the study does not control for the overall balance of the diet. People who eat large amounts of animal proteins often have other aspects of their diet which are imbalanced such as low intakes of fruit and vegetables.  I think the next step would be to show that changing protein intake in the range of normal human intakes influences IGF-1 levels.  IGF-1 levels may well be programmed in early development and dietary protein intake in adult life may well be less important in later life. This would be consistent with the observation that accelerated growth in childhood is associated with increased height and a high risk of cancer in later life.”

Professor Tim Key, a Cancer Research UK epidemiologist based at the University of Oxford, said:

“This study is too small to provide any robust conclusions. This area of research is important, but further research is needed to establish whether there is any link between eating a high protein diet and an increased risk of middle aged people dying from cancer.

“There is, however, strong evidence that the following things can help reduce your risk of cancer: giving up smoking, keeping a healthy weight, drinking less alcohol and staying physically active.”

Dr 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.

“Sending out statements such as this can damage the effectiveness of important public health messages. They can help to prevent sound health advice from getting through to the general public. The smoker thinks: ‘why bother quitting smoking if my cheese and ham sandwich is just as bad for me?’

“Other research, from long-running studies in Iowa and Sweden, already suggests that diets very high in animal protein increase mortality risk – particularly from those with diets very high in red or processed meat. Sabine Rohrmann has shown recently in a much larger European study that the increased risk of mortality from processed meat is about 20% (relative risk) for every 50g per day increase. This compares to studies into smoking, which suggests a doubling of mortality risk for smokers compared to non-smokers. The increased risk of cancer for smokers is higher still.”

Heather Ohly, Associate Research Fellow at the University of Exeter Medical School’s European Centre for Environment and Human Health, and Registered Nutritionist, said:

“While these new studies*^ǂ highlight important concerns about excessive dietary habits, it would be a shame if extreme headlines caused yet more confusion among the public about food and nutrition. 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. It is a good idea to consume a range of protein sources to obtain the full range of amino acids – the building blocks of protein – which all have different roles in the body. Plant sources of protein include nuts, seeds, beans and lentils. Nutritionists recommend including these foods in your diet as well as animal sources of protein such as meat, fish, cheese, milk and eggs. A key message from the study is “don’t get extreme in cutting out protein”. Just be sensible about where it comes from, choose high quality whole foods, and don’t over-consume any one food group.”

‘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’ by Morgan E. Levine et al. published in Cell Metabolism on Tuesday 4 March 2014.

‘The ratio of macronutrients, not caloric intake, dictates cardiometabolic health, aging, and longevity in ad libitum-fed mice’ by Samantha M. Solon-Biet et al. published in Cell Metabolism on Tuesday 4 March 2014.