Association Between Protein Intake, Occlusal Force, and Walking Speed in Older Adults

Abstract

In a recent study published in the Journal of the American Geriatrics Society, Okada and colleagues reported that lower protein intake mediates the association between walking speed and occlusal force in older Japanese adults.1 This is an important finding because people with poor chewing ability due to loss of teeth have low nutrient and protein intake, causing skeletal muscle loss, but we have several concerns about this study. First, the amount of protein intake studied in this analysis should have been adjusted for the total energy intake using the residual method,2 but it was calculated using the density method, which does not measure protein intake independent of total energy intake. The residual method has no such limitation and has been used in important epidemiological studies (e.g., Health, Aging, and Body Composition (Health ABC)3 and a nationwide Japanese study4) focused on protein intake assessed using validated questionnaire tools. It would have been possible to perform the analysis using the residual method because protein intake was evaluated based on results of a brief self-administered diet history questionnaire. Otherwise, the authors should have described total energy intake when they used the density method to analyze the relationship between occlusal force, protein intake, and walking speed. The density method alone, which calculates energy from protein per total energy intake (% energy), cannot calculate the adequacy of protein intake independent of total energy intake. This was shown in the results of Health ABC.3 In addition, the unit for protein intake in Table 1 in reference1 (g/kcal) might be unnecessary, and the unit for protein intake percentile in Table 2 in reference1 (g/kcal) might be incorrect. Second, protein intake seemed to be sufficient in this study. In the Dietary Reference Intakes for Japanese 2015,5 the tentative dietary goals for preventing lifestyle-related disease (DG) include proteins as a desirable percentage of energy (% energy). For individuals aged 70 and older, the DG for protein was determined to be 13% to 20% (median 16.5%). Table 1 in reference1 shows that median protein intake (percentage of energy) was 15.6% for a walking speed of 0.8 m/s of less and 16.2% for a walking speed of greater than 0.8 m/s. These results were not lower than the DG mentioned in the Dietary Reference Intakes for Japanese 2015. To compare the differences in lower protein intake, energy-adjusted protein intake (g/d) should be reported. Third, the prevalence of chronic kidney disease (CKD) was not reported in the study. Older adults are more likely to develop CKD. Is it usually recommended that individuals with severe CKD not undergoing dialysis have low protein intake. Therefore, CKD may affect the relationship between occlusal force, walking speed, and protein intake. Fourth, sarcopenia and malnutrition, which are associated with occlusal force, walking speed, and protein intake, were not included in the structural equation modeling analysis. Chewing ability and sarcopenia, and sarcopenia and age have equal odds ratios.6 Walking speed is included as a diagnostic criterion for sarcopenia in the consensus reports of the European Working Group on Sarcopenia in Older People7 and the Asian Working Group for Sarcopenia.8 Dietary protein intake is associated with sarcopenia,3 and sarcopenia and malnutrition are associated with poor rehabilitation outcomes and physical functions.9 Therefore, body mass index (BMI) can be associated with occlusal force, walking speed, and protein intake, but only walking speed was adjusted for BMI. Occlusal force and protein intake should have also been adjusted for BMI. Structural equation modeling analysis should also include sarcopenia and malnutrition as latent variables. A clarification from the authors about these concerns would be welcome. Conflict of Interest: The editor in chief has reviewed the conflict of interest checklist provided by the authors and has determined that the authors have no financial or any other kind of personal conflicts with this paper. Author Contributions: Saino, Wakabayashi, Maeda: drafting and critical revision of manuscript for important intellectual content, approved final version of manuscript. Sponsor's Role: None.