ENC Summary and Review of Low Protein Intake Study

Published: 7 March 2014

This study investigated whether high protein intakes were associated with increased mortality from cancer, cardiovascular disease (CVD), diabetes and all causes by assessing dietary data in 6,381 US men and women aged 50 years and over who participated in the NAHNES III dietary survey. The authors hypothesized that higher protein intakes may increase mortality through an effect on increasing GHR-IGF-1 activity (growth hormone receptor-insulin like growth factor).

Dietary data was collected once at baseline using a food frequency questionnaire and mortality was assessed over the subsequent 18 year period.

Participants were classified as having a high protein intake (>20% energy from protein), moderate (10-19%) or low (<10%).

When all subjects were considered, high and moderate protein intake was not associated with all cause, CVD or cancer mortality but was associated with an increased risk of diabetes-related mortality. However, the authors note that in relation to diabetes, the numbers were small and larger studies are required to test this finding further, in particular, because people with diabetes may adopt a lower carbohydrate, higher protein intake when diagnosed.

When the group was split into those aged 50-65 years and those aged over 65 years, it was found that in the 50-65 year age group, higher protein intakes were associated with increased risk of all cause and cancer mortality (but not CVD mortality) whereas in those >65years, higher protein intakes were associated with the opposite outcomes for overall and cancer mortality but a similar outcome was found for diabetes mortality. When animal and plant protein sources were considered, it was found that higher intakes of animal protein promoted mortality in the 50-65 year age group, while plant based proteins had a protective effect. For those over 65 years, the source of protein did not have any impact on the findings.

Additional analysis carried out by the authors found IGF-1 did not account for the association between protein consumption and mortality but was a moderator of the association. Further experiments in mice (implanted with cancer cells) showed those on high protein diets had a greater incidence of certain types of cancers and the mean tumour size was greater compared to mice on low protein diets and that this effect was associated with different levels of serum IGF-1.

Key points:

  • This study is observational only and does not prove a cause and effect relationship between protein intake and mortality
  • No conclusions can be drawn about individual protein rich foods and their association with mortality as a result of this study as individual foods were not assessed
  • Dietary data were collected at one time point only (18 years prior to completion of the study) and people’s dietary habits can change over time
  • The statistical analysis did not adjust for saturated fat intake or fruit and vegetable intake, two important factors associated with development and protection from chronic diseases
  • Data in mice need to be tested further with research conducted in humans

Take home message:

It is recommended that people follow the advice as outlined in the Dietary Guidelines for Australians and select a variety of lean, protein rich foods in an amount suitable for their age, height, gender and level of activity. These foods include eggs, lean meat, skinless poultry, nuts and seeds, legumes, tofu and fish.

 

Study reference

Levine ME, Suarez JA, Brandhorst S, et al. 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. Cell Metabolism 19:407-417, March 2014.