Discretizing continuous variables in nutrition and obesity research: a practice that needs to be cut short

Osvaldo F Morera; Mosi I Dane'el; Brandt A Smith; Alisha H Redelfs; Sarah L Ruiz; Kristopher J Preacher; Leah D Whigham

doi:10.1038/s41387-023-00248-0

Discretizing continuous variables in nutrition and obesity research: a practice that needs to be cut short

Nutr Diabetes. 2023 Nov 8;13(1):20. doi: 10.1038/s41387-023-00248-0.

Authors

Osvaldo F Morera¹, Mosi I Dane'el², Brandt A Smith³, Alisha H Redelfs⁴, Sarah L Ruiz⁵, Kristopher J Preacher⁶, Leah D Whigham⁷

Affiliations

¹ Department of Psychology, University of Texas at El Paso, 500 W. University Ave., El Paso, TX, 79968, USA. omorera@utep.edu.
² State of New Mexico, New Mexico Children, Youth and Families Department, 4775 Indian School Rd, Albuquerque, NM, 87110, USA.
³ Department of Psychology, Columbus State University, Arnold Hall 272, 4225 University Avenue, Columbus, GA, 31907, USA.
⁴ Department of Public Health, Brigham Young University, 2136 LSB, Provo, UT, 84602, USA.
⁵ Interdisciplinary Health Sciences Program, University of Texas at El Paso, 500 W. University Ave., El Paso, TX, 79968, USA.
⁶ Department of Psychology and Human Development, Vanderbilt University, Peabody #552, 230 Appleton Place, Nashville, TN, 37203, USA.
⁷ Center for Community Health Impact, Department of Health Promotion and Behavioral Sciences, The University of Texas Health Science Center at Houston School of Public Health, 5130 Gateway East Blvd., MCA 114, El Paso, TX, 79905, USA.

Abstract

Background/objectives: Nutrition and obesity researchers often dichotomize or discretize continuous independent variables to conduct an analysis of variance to examine group differences. We describe consequences associated with dichotomizing and discretizing continuous variables using two cross-sectional studies related to nutrition.

Subjects/methods: Study 1 investigated the effects of health literacy and nutrition knowledge on nutrition label accuracy (n = 612). Study 2 investigated the effects of cognitive restraint and BMI on fruit and vegetable (F/V) intake (n = 586). We compare analytic approaches where continuous independent variables were either discretized/dichotomized or analyzed as continuous variables.

Results: In Study 1, dichotomization of health literacy and nutrition knowledge for 2 × 2 ANOVA revealed health literacy had an effect on nutrition label accuracy. Nutrition knowledge has an effect on nutrition label accuracy, but the health literacy by nutrition knowledge interaction was not significant. When analyzed using regression, the nutrition knowledge effect was significant. The simple effect of health literacy was also significant when health literacy equals zero. Finally, the quadratic effect of health literacy was negative and significant. In Study 2, dichotomization and discretization of cognitive restraint and BMI were used for three ANOVAs, which discretized BMI in three ways. For all ANOVAs, the BMI main effect for predicting fruit and vegetable intake was significant, the interaction between BMI and cognitive restraint was non-significant, and cognitive restraint was only significant when both variables were dichotomized. When analyzed using regression, the continuous mean-centered variables, and their interaction each significantly predicted F/V intake.

Conclusions: Dichotomizing continuous independent variables resulted in distortions of effect sizes across studies, an inability to assess the quadratic effect of health literacy, and an inability to detect the moderating effect of BMI. We discourage researchers from dichotomizing and discretizing continuous independent variables and instead use multiple regression to examine relationships between continuous independent and dependent variables.

MeSH terms

Cross-Sectional Studies
Eating*
Humans
Nutritional Status*
Obesity