Associations of underweight and stunting with impaired vitamin D status in Ecuadorian children provides insights into the vitamin’s biology

Thirty years ago, Michael Holick’s group showed that the low angle of penetration of sunlight into Boston, Massachusetts, USA (latitude 42°N) attenuated UVB radiation so substantially that it failed to convert 7-hydroxycholesterol, the skin precursor of vitamin D, into the vitamin itself during the winter months. The Republic of Ecuador (named from the Spanish word for ‘equator’), however, possesses the essence of tropical sunshine, with a near perpendicular solar angle all year round. Moreover, Ecuador, an Andean country, has an average altitude as a nation of 1117m (3665 ft), with the capital of Quito situated at 2850m (9350 ft). The lower gaseous density at altitude attenuates the atmospheric filtering of UVB radiation. So, from first principles, one would identify Ecuador as a propitious setting for the solar generation of vitamin D and the maintenance of adequate vitamin D status. A 2017 editorial in Public Health Nutrition identified four ‘critical gaps’ in vitamin D research at a population and community level. A paper in this issue of the journal addresses one of these gaps – vitamin D deficiency and its association with health risk – by providing the first evidence of vitamin D deficiency in Ecuador and assessing its associations with growth status. Using stored samples, the authors quantified serum 25-hydroxyvitamin D (25(OH)D), a widely accepted biomarker of vitamin D status, and compared its concentration in 6–36-month-old children according to weightand heightfor-age Z-scores (WAZ and HAZ, respectively). Mean 25(OH)D did not differ between underweight (WAZ<− 1·0) and non-underweight children, although the prevalence of vitamin D<42·5nmol/l (a population-customized cut-off point) was about twice as high in children with v. without underweight. Mean 25(OH)D was slightly lower in children with stunting (HAZ<−2·0) than in non-stunted children; nevertheless, the prevalence of stunting was 1·4 times higher in children with 25(OH)D< 42·5 nmol/l than in those with higher vitamin D concentrations. Blood levels of parathyroid hormone (PTH) were not related to anthropometric status and there was only a weak inverse association between PTH and 25(OH)D. The paper confirms a finding from both of our groups in different settings, namely of endemic poor vitamin D status in highland populations of Latin American countries near the equator. This has been demonstrated in premenarcheal girls from Bogota, Colombia and elderly Mayan Guatemalans. Although Smith et al. provided evidence for widespread vitamin D deficiency in sunny tropical countries, specifically of the South-East Asian rim, this is not necessarily the case in other areas that are sunbathed year-round, as recently shown by Robinson et al. in Mesoamerica and the Dominican Republic. While the Ecuadorian findings on the prevalence of vitamin D deficiency are not inordinately noteworthy per se, a more revealing feature of the paper is the effort to draw out associations of vitamin D status with growth parameters of young children. Previous investigations of vitamin D and child growth in tropical countries were conducted in school-aged children. In the Bogota School Children Cohort study, plasma 25(OH)D<50 nmol/l at the time of recruitment was associated with rapid BMI gain and, among girls, with slower growth in height over a 2·5-year follow-up period. In the cross-sectional NiMeCoMeS study of Central American nations and the Dominican Republic, 25(OH)D was not associated with heightor BMI-for-age Z-score in children 7–12 years of age. The children surveyed around Quito have the typical pattern of negligible wasting, modest underweight and abundant stunting, which characterizes lowand moderate-income countries in general; as such, the sampling strategy employed to over-represent underweight children provided adequate numbers of low-WAZ and low-HAZ individuals for the desired disaggregation and modelling analyses. This opportunity first led to an exercise to find a population-specific cut-off point of 25(OH)D to define low status, based on growth measures. The analysis determined that a vitamin D concentration of < 42·5 nmol/l (rather than the conventional <50 nmol/l based on bone metabolism) was the cut-off level for inadequate vitamin D status in this population. In fact, although intriguing, an approach to customize a population-specific cut-off point based on an alternative association frame is hampered for two reasons. First, it was necessary – and will always be necessary – to use the Public Health Nutrition: 21(11), 1971–1973 doi:10.1017/S1368980018000927