Request for Study Design Modification in Examining Nutritional Intake and Muscle Strength in Individuals with Airflow Limitation
Article information
We read with interest the cross-sectional, observational study by Heo et al. [1], which utilized data of the Korean National Health and Nutrition Examination Survey from 2014 to 2016. Their study aimed to evaluate associations of nutritional intake with physical activity (PA) and handgrip strength (HGS) in adults aged ≥40 years with airflow limitation defined as forced expiratory volume in 1 second (FEV1)-forced vital capacity (FVC) ratios <0.7. Nutritional intake was assessed using a 24-hour dietary recall questionnaire administered through a computer-assisted personal interview system and PA was evaluated using the Korean version of the International Physical Activity Questionnaire, while HGS was measured with a digital grip strength dynamometer.
Their study enrolled 622 individuals (mean age of 66.8 years, 26.7% female) categorized into two groups based on the presence of active aerobic PA (with vs. without) and HGS (normal vs. low). Their findings revealed that participants with active aerobic PA (n=299) and normal HGS (n=509) had significantly higher daily intakes of all nutritional variables except carbohydrates than their counterparts without active aerobic PA (n=323) and with low HGS (n=113). Specifically, these variables included total food, water, calories, protein, and lipids. Their study found weak correlations between HGS and these nutritional intake variables, with Pearson’s correlation coefficients ranging from 0.147 to 0.346 (all p=0.001). Although some nutritional intake variables were identified as factors associated with active aerobic PA and normal HGS in univariate logistic regression analysis, these associations were insignificant in the multivariate model [1].
Issues of nutritional intake and clinical outcomes for individuals in varied clinical settings, including individuals with airflow limitation, are of high importance and clinical relevance. An increased body of literature has addressed nutritional management of adult patients with airway limitation [2]. However, there is a knowledge gap for nutritional interventions in this population [3]. As a nationwide study covering a large number of Asian participants, Heo et al.’s study [1] provides essential knowledge on this topic. However, results of their study appear to diverge from the existing body of knowledge without fully explaining the observed diversity. Therefore, we would like to raise some concerns regarding their study design and results and offer corresponding suggestions to enhance Heo et al.’s study [1].
Firstly, the authors calculated nutritional intake using grams or kilocalories per day. However, using kilocalories or grams per kilogram of body weight (BW) per day (kcal/kg BW/day or g/kg BW/day) is considered more accurate than using kilocalories or grams per day (kcal/day or g/day) for discussing human nutrition because it accounts for individual differences in body size and composition [4]. Thus, the authors need to revise units of these nutritional intake variables by considering BW.
Secondly, their study investigated associations of nutritional intake with PA and HGS, respectively. However, a meta-analysis of 82 randomized controlled trials found that protein supplementation-related muscle strength improvement could only be observed when protein supplementation was combined with resistance exercise. Such improvement was absent in those without combined resistance exercise [5]. With concurrent resistance exercise, the increase in muscle strength demonstrated a dose-dependent positive association with increased total protein intake up to a maximum of 1.5 g/kg BW/day [5]. Applying this concept to the study of Heo et al. [1], the authors need to perform subgroup analyses using individuals with and without active aerobic PA as separate populations to evaluate associations between HGS and nutritional intake variables in these two populations. This design can determine whether the presence of PA affects the association between nutritional intake and HGS.
Thirdly, the authors applied HGS cut-off values (28.6 kg for men and 16.4 kg for women) to categorize patients into two groups with significantly unequal numbers of individuals (113 vs. 509). Clinically, applied cut-off values are determined from a healthy elderly population for defining sarcopenia [6], not for individuals with airway limitations. It is already known that HGS has a significant positive association with pulmonary function, including FEV1 and FVC, in healthy older adults [7]. It is also known that peripheral muscle strength is significantly lower in patients with airway limitations than in healthy individuals [8]. Statistically, having equal numbers in each group when comparing two groups can help improve the power of a statistical test and make it easier to detect a significant difference if one exists [9]. Therefore, the authors need to lower cut-off values in their study probably by dividing all patients into two or three groups. This change in grouping method might clinically improve the differential power for individuals with airway limitations and show a statistically significant difference. Hopefully, these comments could help Heo et al. [1] demonstrate significant findings and make essential contributions to this topic for the medical community.
Notes
Authors’ Contributions
Conceptualization: Shiao CC. Writing - original draft preparation: all authors. Writing - review and editing: Ma HK, Shiao CC. Approval of final manuscript: all authors.
Conflicts of Interest
No potential conflict of interest relevant to this article was reported.
Funding
No funding to declare.