Childhood obesity and suboptimal nourishment status: The case of vitamin D

Growing ChildVitamin D seems to play a role in several body functions, beyond bone health, in early childhood. Since most circulating vitamin D is synthesized endogenously through sunlight exposure, it was believed that latitude would mainly influence its levels. Nowadays, there is growing evidence that obesity coexists with suboptimal vitamin D status. Although there is no clear explanation regarding this association, low vitamin D intake, limited exposure to sunlight and sequestration of vitamin D in the adipose tissue could potentially explain this finding.

What is suboptimal vitamin D status?

The best indicator for assessing vitamin D status is considered to be serum 25-hydroxivitamin D (25OHD), mainly due to its increased half-life time. Regarding sufficiency of vitamin D status, most scientific organization1-3 recommend serum 25OHD concentration higher than 50nmol/L for infants and children as a safe threshold, while some scientists suggest that the recommended level should be even higher at 75nmol/L.4,5

Consequences of suboptimal vitamin D status in early childhood

The central role of vitamin D in bone health is already known since the beginning of the 20th century, with poor vitamin D status adversely affecting bone physiology. Apart from the importance of vitamin D for optimal bone health, recent research data suggest its additional functions in muscle development, prevention of infectious diseases, allergic diseases and type I diabetes,1  while an inverse relationship between vitamin D levels and cardiovascular risk factors has also been reported.6 However, evidence so far is not sufficient enough to support relevant recommendations from scientific organizations.1

The prevalence of suboptimal vitamin D status and the role of obesity

Since most circulating vitamin D is synthesized endogenously through skin exposure to ultraviolet B (UVB) radiation, it was believed until recently that suboptimal vitamin D status would be prevalent mainly in high-latitude regions. However, it seems that the prevalence of suboptimal vitamin D status is influenced not only by latitude (Figure 1), but also by other factors, such as skin’s pigmentation (i.e. light skin populations produce much easier vitamin D), clothing, sunscreen use, season, time spent indoors, urbanization and air pollution.2,7

Figure 1 World map that relates latitude by geographic regions to skin ability to synthesize vitamin D (developing countries in color and developed countries in white)8

Moreover, there is growing evidence during the last decades that obesity coexists with suboptimal vitamin D status. Data from the National Health and Nutrition Examination Survey (NHANES) indicated that the prevalence of suboptimal vitamin D status in overweight and obese school children and adolescents in USA was 29% and 34% respectively, while the prevalence of suboptimal vitamin D status in their normal weight counterparts was 21% (Figure 2).9 Furthermore, recent data from the Belgian IDEFICS (Identification and Prevention of Dietary induced health Effects In Children and Infants) study cohort also revealed a negative association between body mass index (BMI) as well as markers of subcutaneous and abdominal fat and vitamin D status in preschool and school children.10  Similarly, studies on infants and toddlers11,12  reported that increased weight status, based on BMI, tended to be associated with a higher likelihood of suboptimal vitamin D status, with each 1Kg/ m² increase of BMI to be associated with 57% higher odd of suboptimal vitamin D status.11  

Figure 2. The prevalence of suboptimal vitamin D status (25OHD<50nmol/L) according to weight status among school children and adolescents9

There is no clear explanation regarding the association between obesity and a suboptimal vitamin D status. Inadequate vitamin D intake could contribute to this association, as one study showed that obese children with suboptimal vitamin D status reported lower dietary vitamin D intake.13 However, poor vitamin D absorption cannot be assumed, since there is no evidence at the moment indicating that obesity itself results in reduced absorption of dietary vitamin D in the absence of other diseases decreasing its absorption.14 Furthermore, obese children may avoid outdoor physical activity or exposure to the sun during physical activity because of lack of body confidence, making sunlight exposure a possible mediator between obesity and suboptimal vitamin D status.15 Finally, it is suggested that the relation between obesity and a suboptiman vitamin D status may be the result of its sequestration in the adipose tissue. 16,17

References

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  2. Misra, M., et al., Vitamin D deficiency in children and its management: review of current knowledge and recommendations. Pediatrics, 2008. 122(2): p. 398-417.
  3. Ross, A.C., et al., The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. J Clin Endocrinol Metab, 2011. 96(1): p. 53-8.
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  10. Sioen, I., et al., Determinants of vitamin D status in young children: results from the Belgian arm of the IDEFICS (Identification and Prevention of Dietary- and Lifestyle-Induced Health Effects in Children and Infants) Study. Public Health Nutr, 2012. 15(6): p. 1093-9.
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