August 1, 2016

expert reaction to study investigating the association between plant and animal protein consumption and risk of death

In assessing different sources of protein a group of scientists publishing in JAMA Internal Medicine have reported an association between consumption of animal protein and risk of death, and an inverse association between consumption of plant protein and risk of death.

All our previous output on this subject can be seen here.

Prof. Tim Key, Professor of Epidemiology & Deputy Director of the Cancer Epidemiology Unit, University of Oxford, said;

“This is a high quality analysis of two long-term observational studies. In this type of study it is important to examine whether the results seen could actually be due to “confounding” by other dietary and non-dietary factors – for example, in this study the people eating the most plant protein were slimmer and less likely to smoke than those eating the least, which would be expected to reduce their risk for mortality. The authors made extensive adjustments for such potential confounding factors, and although some statistically significant associations remained, the adjustments did tend to weaken the associations, which suggests that the results may still be influenced by some residual confounding.

“Few previous studies have specifically addressed the relationships of mortality with animal vs plant protein, but previous long-term studies on major animal and plant foods are broadly consistent with these findings, and there are several mechanisms which could explain the findings. Overall, the study adds to the view that healthy diets should emphasize plant foods, including plant sources of protein, and that intakes of animal source foods – especially processed meat – should be low.”

Dr Gunter Kuhnle, Associate Professor in Nutrition and Health, University of Reading, said:

“This has been a well conducted study where over 130,000 people were observed for up to 32 years. Diet and other lifestyle data were collected every two years, giving detailed data.

“The results presented in the actual paper are supported by the data, but the conclusions seem somewhat exaggerated and are likely to be confounded by other lifestyle factors. When looking at those participants with a ‘healthy’ lifestyle separately, there was no association between animal protein intake and mortality. There was a slight benefit of consuming more plant-based protein in those with an ‘unhealthy’ lifestyle – but not those with a ‘healthy’ lifestyle. Interestingly, the authors include being overweight in their definition of ‘unhealthy lifestyle’. As animal-based foods are often more energy dense then plant based foods, it is possible that this might contribute to the associations observed.

“The authors conducted further analyses and found that in particular the consumption of processed meat is associated with an increased risk for an early death. Interestingly, processed meat consumption was not associated with increased cancer mortality, even though a link between processed meat intake and colorectal cancer risk is well established.

“It should be noted that this is an observational study and provides information about associations, which contributes to evidence towards causality but doesn’t firmly establish it. This study contributes to the evidence available to develop dietary guidelines. The results themselves do not suggest that a change in current dietary guidelines is necessary. The overall results of the study show that an overall healthy lifestyle, including – but not limited to – a healthy diet is beneficial.”

Dr Ian Johnson, Nutrition researcher and Emeritus Fellow, Institute of Food Research (IFR), said:

“This interesting and robust work seems to support the growing consensus that diets based largely on plant foods are better for long term health than diets containing large quantities of meat and dairy products but it tells us little about mechanism. It is far from clear whether plant proteins are protective or animal proteins are detrimental to health, or whether these protein levels are simply markers for something else.”

‘Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality’ by Song et al. published in JAMA Internal Medicine on Monday 1st August. 

Declared interests

Prof. Tim Key: Member of the Vegan Society & PI of MRC grant “Health of Vegetarians”

Dr Gunter Kuhnle: “Employment: Associate Professor at the University of Reading.  Grant funding: Investigation of links between polyphenol intake and health – EU, Mars, Horizon.  Appointments: EFSA Working group – risk assessment of soy isoflavones.  Memberships: British Mass Spectrometry Society, British Nutrition Society, Registered Nutritionist (Reg. Nr. 8236); 2011 to 2012 member of ‘Biomarker group’ at ILSI Europe.  Other financial interests: Vineyard owned by family.”

Dr Ian Johnson: None declared