Predictors of Vitamin C Status in National Health and Nutrition Examination Survey 2017–2018: The Effect of Body Weight and Body Mass Index in a Multiple Fractional Polynomial Analysis

Background: This study aims to set vitamin C requirements for all age, sex, and life cycle groups. Requirements calculated for healthy young males are extrapolated based on body weight. Increasing obesity prevalence in recent decades has led to higher body weights in many countries; hence, this may increase vitamin C requirements. Objectives: This study aims to assess the relationship between body weight/obesity, and other covariates, associated with serum vitamin C status. Methods: Data from adult participants of the cross-sectional survey National Health and Nutrition Examination Survey 2017–2018 (N = 4556) were used. The multiple fractional polynomial method was used to quantify linear or nonlinear associations between vitamin C status and obesity, including relevant covariates identified from the literature. The covariates included were gender, smoking, C-reactive protein (CRP), serum albumin, recent illness, physical activity, and vitamin C intake. Results: There was a positive, linear relationship found between serum vitamin C, and serum albumin and physical activity. There was a negative, linear relationship between serum vitamin C, and bodyweight or body mass index, and a weak U-shaped relationship with age. Serum vitamin C and vitamin C intake showed a positive, nonlinear relationship, whereas there was a negative, nonlinear relationship between vitamin C status and CRP. Vitamin C status was lower in males and in people with higher smoking exposure. Recent illness was not associated with vitamin C status. Conclusions: These results in a large, representative dataset indicate that greater body weight or obesity may increase vitamin C requirements. In addition, nonlinear associations such as between vitamin C intake and status should be taken into consideration when determining requirements.