The Contrived Association of Dietary Protein with Mortality

On March 18, we submitted the following Letter to the Editor (LTE) concerning the article by Levine et al (Cell Metab 2014;19:407). This article contains, in our views, numerous errors in design and analyses; and the authors present biased conclusions both in the article and in University press releases on the article which equated a higher protein diet to smoking. In our opinion the peer-review system has failed to adequately evaluate this paper. When this happens, the scientific community has the responsibility to provide additional oversight with scholarly evaluation and debate. However, the Editors of Cell Metabolism declined to publish our LTE and recommended that we post our comments 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. We view the position taken by the Editors of Cell Metabolism as inconsistent with goals for maintaining scientific integrity. With this view in mind our unpublished LTE is provided below.

Dear Editor:

We applaud efforts to improve human health by asking insightful questions that explore existing nutrition paradigms. Unfortunately, the paper by Levine et al. is a flawed attempt to link health risks of a single nutrient, protein, to chronic disease states of cancer, CVD and diabetes. The study design and analyses are inappropriate; key contradictory data are neglected; and conclusions are not justified by the data. 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.

The optimum dietary intake of protein for adults remains a topic of scientific debate; however, research indicates that balanced diets with protein intakes moderately above the RDA value of 0.8 g/kg/d (1) are beneficial for weight management, sarcopenia, diabetes and physical activity (2,3). While data overwhelmingly demonstrate short-term benefits of moderate protein intake on metabolic status and body composition, the long-term impact of protein on disease risk or mortality is more difficult to assess and requires expert interpretation of large data sets such as the National Health and Nutrition Examination Survey (NHANES).

In their study, Levine et al. indicate (Figure 1, Table S1, and Discussion) that “…the level of protein is … not associated with differences in all-cause, cancer, or CVD mortality.” In fact the data demonstrate that cancer mortality was actually ~10% higher in the low protein group compared with the higher protein group (ie. 9.8% versus 9.0% deaths). We would argue that these obvious findings are the most important.

Subsequent subdivision and reanalysis of the data raise serious questions about the validity of the authors’ approach and conclusions. First, the NHANES Linked Mortality Files contain information for almost 12,000 adults, however, without justification, the investigators eliminated almost one-half of the data and only reported results for 6,381 over the age of 50 yr. Second the investigators created ill-defined protein groups of low (LP:

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