Body adiposity predictors of vitamin D status in nondialyzed patients with chronic kidney disease: A cross-sectional analysis in a tropical climate city

Highlights

• Vitamin D was sufficient (25[OH]D ≥ 30 ng/dL) in 43% of nondialyzed patients with chronic kidney disease (CKD).

• Overweight/obesity was observed in most (∼60%) of the nondialyzed patients with CKD.

• Total body adiposity was the independent predictor of vitamin D deficiency in CKD.

• Vitamin D deficiency was related with hyperparathyroidism and higher serum phosphorus.

• Insulin resistance was not independently associated with 25(OH)D concentrations.

Abstract

Objectives

The association of vitamin D status with high body adiposity is poorly investigated in the chronic kidney disease (CKD) population. The aim of the present study was to describe vitamin D status and to identify body adiposity predictors of vitamin D deficiency, in a nondialyzed CKD population inhabiting a tropical city.

Methods

This cross-sectional study included patients with CKD, defined as estimated glomerular filtration rate (eGFR) <60 mL/min, regularly treated by an interdisciplinary team in an outpatient university clinic, set in a Brazilian city (latitude: 22°54′S; 43°12′W). Adiposity parameters analyzed were body mass index (BMI), total body adiposity (dual-energy x-ray absorptiometry [DXA] and body adiposity index [BAI]), and central body adiposity (DXA-trunk fat and waist-to-height ratio [WHtR]). Laboratory parameters included serum concentrations of 25-hydroxyvitamin D, phosphate, parathyroid hormone, and insulin (insulin resistance [IR evaluation: homeostasis model assessment; HOMA]).

Results

We studied 244 patients (54.9% men; n = 134) with median eGFR = 29.1 mL/min and BMI 26.1 kg/m², comprising 58.2% (n = 142) with overweight/obesity. The vitamin D status was sufficient (≥30 ng/dL) in 43%, insufficient (20–30 ng/dL) in 37%, and deficient (<20 ng/dL) in 20%. Total body adiposity was the independent predictor of vitamin D deficiency (DXA: odds ratio [OR], 2.3; 95% confidence interval [CI], 1.1–5; P = 0.03; BAI: OR, 1.9; 95% CI, 1–3.8; P = 0.02), whereas BMI, DXA-trunk fat, and WHtR showed no correlation. Higher serum phosphorus and hyperparathyroidism were related (P < 0.05) to vitamin D deficiency. IR was not independently associated with 25-hydroxyvitamin D concentration.

Conclusions

Just under half of the CKD population presented sufficient concentration of 25-hydroxyvitamin D. Total body adiposity, independent of age and eGFR, regardless if evaluated by DXA or BAI, was the predictor of vitamin D deficiency, which in turn was associated with higher serum phosphorus and hyperparathyroidism, but not with IR.

Introduction

Overweight/obesity is commonly observed in the chronic kidney disease (CKD) population and follows the growing increase in worldwide prevalence. Overall, the proportion of the world's adult population with overweight/obesity is about 36.9% to 38% [1]. In Brazil, according to the 2008–2009 census, the scenario is not different. In adults >20 y, the prevalence of overweight was 50.1% in men and 48% in women, whereas the prevalence of obesity was 12.4% in men and 16.9% in women [2]. Obesity/overweight raises the risk for insulin resistance (IR), type 2 diabetes, hypertension, dyslipidemia [3], and vitamin D deficiency [4].

The definition of insufficiency and deficiency of vitamin D remains controversial, limited by the lack of a consensus regarding the cutoff point for serum 25-hydroxyvitamin D (25 [OH]D). However, there is a common understanding that low serum 25(OH)D concentration causes calcium malabsorption accompanied by negative calcium balance, secondary hyperparathyroidism, and bone disorders among other alterations [4], [5], [6], [7]. CKD is accompanied by alteration in bone mineral metabolism early in the evolution of the disease, and association with overweight/obesity might worsen this condition. Patients with CKD have a high burden of cardiovascular disease (CVD), and traditional risk factors only partially account for this risk. Multiple epidemiologic studies have demonstrated a survival benefit associated with higher serum concentration of 25(OH)D [4], [5], [6], [7].

There are few investigations concerning vitamin D status and its association with adiposity in nondialyzed CKD population, especially in those inhabiting a tropical area, where the ultraviolet index is high all year around [8], [9]. Additionally, data associating vitamin D with adiposity in patients with CKD are controversial and the majority of studies use body mass index (BMI) as body fat parameter [8], [10], [11], [12], [13]. However, BMI presents limitations in the accurate assessment of body fat [14], [15]. Furthermore, although the relation of body fat with IR is consistent, the association of vitamin D with IR is less explored in CKD population [16].

The hypotheses of this study are that the BMI is not a good body adiposity predictor of vitamin D deficiency, considering its known limitations to assess body adiposity; the total body adiposity, rather than central adiposity, is a predictor of vitamin D deficiency; the vitamin D deficiency is associated with IR, impaired homeostasis of parathyroid hormone (PTH), phosphorus, and lipids.

Thus, the aim of the present study was to describe the vitamin D status and to identify body adiposity predictors of vitamin D deficiency, in a nondialyzed CKD population under regular treatment, dwelling in a tropical city. A secondary objective was to examine the association of vitamin D status with clinical abnormalities commonly observed in CKD.