Is Bioavailable Vitamin D Better Than Total Vitamin D to Evaluate Vitamin D Status in Obese Children?

Abstract

Objective: Free hormones are biologically more active in target tissues. Thus, measurement of vitamin D taking into account bioavailability and free vitamin D may be preferable, especially when evidence is contradictory, as in obese children. In order to assess bioavailablity and free vitamin D, using a previously reported formula, vitamin D-binding protein (VDBP) level was measured and VDBP polymorphisms were also evaluated because of variations in binding affinity. Methods: Eighty-four obese and 78 healthy children were included. Anthropometry, calcium, phosphorus, alkaline-phosphatase, parathyroid hormone (PTH), 25 hydroxyvitamin D [25(OH)D], bioavailable-free vitamin D, and VDBP concentration and polymorphism were evaluated in the whole group. Results: Obese girls had significantly higher PTH than normal weight girls (p=0.001). Regardless of gender, obese children had significantly higher concentrations of VDBP (p=0.008) and PTH (p=0.002). When samples taken in winter were analyzed, PTH and VDBP were found to be higher and bioavailable and free vitamin D lower in the obese group. There was no difference in terms of total vitamin D between groups during the winter season. Conclusion: While total, free, and bioavailable vitamin D in the obese group was similar to the control group in autumn, free and bioavailable vitamin D in the winter was lower in the obese than the control group. In addition, PTH was higher in the obese group in both autumn and winter. Therefore, more research is needed to evaluate the variability of free and bioavailable vitamin D according to body habitus, season and the effect any differences may have.

Association of vitamin D levels with anthropometric and adiposity indicators across all age groups: a systematic review of epidemiologic studies

“…Bioavailable and free vitamin D were lower in the obese group. However, the total vitamin D level between the two groups did not differ, according to Küçükali et al ( 19 ). In another study by Turer et al , vitamin D deficiency is highly prevalent in overweight and obese children.…”

“…After careful screening of 2803 full texts, we also excluded 2731 more studies because they evaluated the relationship between serum levels of 25(OH)D with an outcome other than anthropometric/adiposity indices, were clinical trials, animal or in vitro studies in design, editorial, and reviews, or the participants of the studies were unhealthy (with chronic diseases, such as diabetes, cancer, or chronic kidney disease). Ultimately, 72 studies ( 12 , 13 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , …”

“…The current systematic review identified 27 cross-sectional studies ( 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ) and one longitudinal study ( 43 ) that assessed the potential relationship between serum 25(OH)D levels and anthropometric/adiposity indices in children. The studies about the relation between 25(OH)D serum level and anthropometric indices in pediatric were carried out in the following countries: four studies from Turkey, three from Spain, two from Denmark, five from the USA, two from Brazil, four from Italy and eight others from Iran, Korea, China, Thailand, England, Saudi Arabia, Sri Lanka, and Germany.…”

Genetic influence on urinary vitamin D binding protein excretion and serum levels: a focus on rs4588 C>A polymorphism in the GC gene

“…Given the lack of similar studies and available data, we conducted a sample size calculation to ensure an adequate number of participants with the rs4588-A allele. For this, we used the G*Power software, taking into account the allele frequencies reported in previous studies in Türkiye (14,15). This calculation factored in important parameters including a significance level (a) of 0.05, a statistical power (1-b) of 0.95, and the anticipated proportions of rs4588-C and rs4588-A allele carriers based on prior studies conducted in Turkey.…”

“…The resulting calculated sample size for each allele group was 42, leading to a total sample size of 84 participants. Recognizing the significance of obtaining robust and meaningful outcomes, we aimed to include between 110-115 participants, a number aligned with Kücükali et al's study (15), to establish a strong and well-founded statistical basis for our findings.…”