In response to a recent study by scientists from the University of Southern California (Cell Metabolism​, Vol. 19, pp. 407-417), experts led by Donald Layman, PhD, from the University of Illinois, criticized that “inappropriate” study design and analyses, the “neglect” of key contradictory data, and the “unjustified” conclusion.

“As scientists with decades of experience studying the impact of protein on health, we are concerned that translation of these flawed data and exaggerated conclusions to the public could have serious negative health consequences for adults seeking to maintain muscle health and avoid sarcopenia,” ​they wrote.

To read the full statement, please click here​.

The experts also criticized the editors of Cell​ Metabolism​ for declining to publish the group’s comments as a letter to the editor. Instead, the group was told to publish their statement as a comment on the journal website.

“We view the decision to confine our views on this paper to online discussion forums as a severe limitation to academic discussion and debate that completely minimizes alternative evaluations,” ​they wrote. “We view the position taken by the Editors as inconsistent with goals for maintaining scientific integrity.”​

Controversial study ​

As reported recently by our European edition​, a diet high in animal protein during middle age may lead to a four-fold increase in risk of dying from cancer (Cell Metabolism​, Vol. 19, pp. 407-417).

Using NHANES III data for 6,381 people aged over the age of 50, scientists from the University of Southern California reported that cancer death risk among those who consumed the most protein was four times higher than those who ate the least. Death risk from any cause among the highest protein consumers was 74% higher during the period than those who ate the least protein.

The study’s conclusions received widespread media coverage at a time when added protein is appearing in more and more products

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.

Response​

Prof Layman and his co-responders, including Prof Arne Astrup (University of Copenhagen), Prof Peter Clifton (CSIRO, Australia), Dr Heather Leidy (University of Missouri), Dr Douglas Paddon-Jones (The University of Texas Medical Branch), and Prof Stuart Phillips (McMaster University) wrote: “Our overall assessment of this paper is that the conclusions and analyses are biased, and flawed. ​

“While there is growing consensus that a moderate protein intake between 1.0 and 1.5 g/kg/d may confer health benefits beyond those afforded by the current RDA for protein, we also recognize there are gaps in the current knowledge base and encourage discussion of important contradictory evidence/data. ​

“Future research must be well designed, rigorously reviewed, and credibility communicated. Unfortunately, the article by Levine et al. presents conclusions not supported by their own analyses or the greater literature.”​

Prof Layman told FoodNavigator-USA that he has not had direct contact from either the journal of the authors.

“I don't expect any response because our comments are not ones they can defend,” ​he said.

To read the full statement, please click here​.

‘Dietary guidelines should not be based on animal experiments’​

Commenting separately in a briefing for the UK-based Science Media Centre​, Prof Tom Sanders, Head of the Nutritional Sciences Research Division, King’s College London, said that USC’s press release headline (Meat and cheese may be as bad for you as smoking​​) was running ahead of the evidence, and the comparison between protein and smoking was “really unwarranted”.

“The study shows a relationship with growth factor IGF-1 and cancer risk which is already known,”​ said Prof Sanders. “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.”​