Introduction

Vitamin D and its metabolites are a number of cholesterol-like compounds related to mineral metabolism. The best-known effect of vitamin D is to promote intestinal absorption of calcium, although it also stimulates absorption of phosphate and magnesium. These minerals also play important roles in the muscle; therefore vitamin D is essential for the maintenance and function of the musculoskeletal system, bone health, and child growth.¹

Humans obtain vitamin D3 from exposure to sunlight, which provides more than 90% of the body´s vitamin D requirements.² Ultraviolet rays trigger the synthesis of vitamin D through the skin, converting 7-dehydrocholesterol into vitamin D3, which then enters into the blood circulation.³ Vitamin D is sequentially metabolized in the liver and kidneys into 25-hydroxyvitamin-D (25-OH-D), and 1,25-dihydroxyvitamin D, respectively, the latter being its biologically active steroid hormone form, also known as calcitriol.³

The widespread distribution of the vitamin D receptor (VDR) and vitamin D metabolizing enzymes CYP24A1 and CYP27B1, as well as the impressive number of genes (almost 3% of human genome) subject to 1,25-dihydroxyvitamin D regulation, explain the pleiotropic actions of vitamin D in many organs and systems in the body. These actions range from bone formation, calcium and phosphorus homeostasis, regulation of cell proliferation, and insulin production, to the modulation of the immune system, among others.⁴

Globally, studies have demonstrated that vitamin D deficiency is a public health issue across all age groups, even in countries near the equator, where, in theory, enough sunlight is consistently available to prevent deficiency.⁵ An estimated one billion children and adults around the world are affected by vitamin D deficiency.⁶ In Mexico alone, this deficiency has been documented in 25.9% of pre-school-age children and 36.6% of school-age children.⁷

Furthermore, some studies have shown that vitamin D deficiency is associated with excess body weight and obesity,⁸ and that it is more frequent in children who spend greater amounts of time indoors, whether at home, school or a childcare center, as well as in cases in which dietary vitamin D intake is insufficient for their metabolic needs.³ In Mexico, the average vitamin D dietary consumption is 3.38 (SD 0.09) µg/d (135.2 [SD 3.6] IU/d) in pre-school-age children, and 2.85 (SD 0.06) µg/d (114.0 [SD 2.] IU/d) in school-age children; the daily intake recommended for the latter is 400 IU/d according to the Institute of Medicine (IOM).⁹ Milk contributes 64.4% of dietary vitamin D in pre-school-age children, and 54.7% in school-age children.⁷

The prevalence of vitamin D deficiency is a nutritional problem among Mexican children; therefore, the objective of the present study is to evaluate vitamin D status and deficiency, as well as related factors, in these children, with updated data obtained from a representative national survey.

Materials and methods

Results

Information from 4 691 children (1 209 of pre-school age and 3 482 of school age), representing 22.3 million Mexican children, was analyzed.

Table I describes the characteristics of the studied sample. The distribution by sex was divided evenly between male and female (approximately 50% each). The mean age was 2.8 years for the pre-school-age group, and 8.2 years for the school-age group. According to nutritional status, based on anthropometric measures, 14.7% of pre-school-age children were stunted, and 5.4% were overweight. Among school-age children, 37.5% were overweight or obese. Over 70% of the children resided in an urban area, and more than 46%, within the Central-Mexico City region. Over 40% of the children had a low HWI, and around 60% of their mothers lacked formal education or had completed only up to elementary education; 71-78% of these mothers were overweight or obese.

The average concentration of 25-OH-D was 60.93 nmol/L (CI95% 58.85-63.0) in pre-school-age children, and 65.36 nmol/L (CI95% 64.26-66.46) in school-age children (data not shown).

Regarding the vitamin D status (table II), deficiency (25-OH-D<50 nmol/L) was observed in 27.3% of preschool-age children and 17.2% of school-age children. Sufficient levels (25-OH-D≥75 nmol/L or ≥30 ng/mL) were recorded in 26.8% of the pre-school-age children and in 25.0% of the school-age children.

Vitamin D deficiency affected female children to a greater extent (36.8%), and reached 29.2% among the children aged 3-4 years. A positive trend was observed between vitamin D deficiency and high BMI in school-age children: 14.9, 18.2 and 24.7% in children with normal body weight, excess body weight and obesity, respectively.

Vitamin D deficiency was more prevalent in children residing in urban areas, whether of pre-school- or school-age. Prevalence was highest in pre-school age children in the Central-Mexico City region. Among school-age children, the highest prevalence was observed in the North.

In relation to the HWI, both age groups displayed greater levels of vitamin D deficiency at the medium level. Prevalence of this deficiency was highest in preschool-age children whose mothers had a low BMI, as well as among school-age children whose mothers were overweight or obese.

As for dietary sources of vitamin D (table III), these were mainly milk, eggs and other dairy products, which, combined, provided 78.3% of the vitamin D consumption in pre-school-age children, and 71.1% in school-age children.

The average intake of vitamin D was 168.5 IU/d in pre-school-age children and 123.8 IU/d in school-age children, both far below the international recommendation of 400 IU/d (table IV). Among school-age children, vitamin D consumption was higher in residents of urban areas, those with a higher HWI, and those residing in the Northern region. School-age children with mothers who had a higher HWI exhibited higher intakes of vitamin D.

The logistic regression model, adjusted for potential confounding factors (table V), showed that, in pre-school-age children, being female was associated with a greater risk of having vitamin D deficiency (OR=2.7, CI95% 1.6-4.4), while, among school-age children, the factors associated with the same risk for deficiency (25-OH-D <50nmol/L) were the age range of 9-11 years (OR=1.4, CI95% 1.0-1.9) and obesity (OR=1.7, CI95% 1.1-2.7).

The results were not modified by adding information to the model regarding the consumption of supplements, which was noted to be of 6.13% in pre-school-age children (CI95% 4.3-8.6) and 0.44% (CI95% 0.2-0.8) in school-age children (data not shown).

Discussion

In this analysis of recent (2018) data of a nationally representative sample of pre-school-age and school-age Mexican children, it was observed that vitamin D deficiency affects 27% of children aged 1-4 years and 17% of children aged 5-11 years, which evidences a critical nutritional problem in the country.

Vitamin D deficiency was positively associated with being female, the age group of 9 to 11 years, and with the presence of obesity, in school-age children. Furthermore, it was higher in children residing in urban areas than in those within rural areas, as well as in those with a medium HWI.

The main dietary sources of vitamin D were milk, eggs and other dairy products, which together contribute 70-79% of daily vitamin D intake for all child participants. Average vitamin D consumption was less than 170 IU/d in both age groups, with more than 90% of the children failing to meet the international dietary recommendation of 400 IU/d.⁹

Vitamin D deficiency has previously been documented in this population through data from Ensanut 2012,⁷ as has been the association of this deficiency with BMI and urban residency. Serum concentrations in pre-school-age children in the present study are very similar to those described in the 2012 study mentioned; nonetheless, the present study shows slightly higher values in school-age children. Vitamin D consumption reported in the present study is 8-20% higher than that described in 2012, which could partially explain the increase observed in serum concentrations and the lower prevalence of deficiency in children. Other related factors may include the consumption of vitamin D supplements, which in the 2012 study was less than 3% and in the present study was 6% in pre-school-age children, although in school-age children the percent-age of children whose mothers reported consumption of supplements was less than 0.5%.

The findings of the present study partially reflect those reported in other countries. A meta-analysis of 72 studies on vitamin D in Brazil¹⁹ reported serum levels of 25-OH-D of 67.65 nmol/L (CI95% 65.91-69.38) in the general population; the prevalence of vitamin D deficiency was 28.16% (CI95% 23.90-32.40); in children, the mean serum concentration of 25-OH-D was 66.68 nmol/L (CI95% 35.51-97.86), and the prevalence of vitamin D deficiency was 22.95% (CI95% 10.00-35.89). These data are similar to those evidenced in the resent study, which is notable, given important differences in latitude, feeding habits, ethnicity and more, between the two studied populations.

In the National Health and Nutrition Examination Survey (NHANES) 2007-2010 study, which reports national estimates of vitamin D status in the general population of the United States of America,²⁰ vitamin D deficiency in children aged 1-11 years was 9.7% (CI95% 7.8-12.0). Specifically in Hispanic children, this prevalence rose to 13% (CI95% 10.0-16.0). Furthermore, serum levels of 25-OH-D were on average 76.5 nmol/L in children aged 1-5 years, and 72.2 nmol/L in children aged 6-11 years, all of whom exhibited higher concentrations of serum 25-OH-D than those reported in the present study, as well as significantly fewer measures below 50 nmol/L. Nonetheless, among the NHANES population, nearly 30% of individuals had a higher intake of vitamin D due to supplementation, which may explain the differences observed. In those who did not consume supplements, serum 25-OH-D average levels were 62.5 nmol/L (CI95% 60.4-64.5), which are more similar to those found in the present study.

As in Mexico, multiple countries in Europe report low vitamin D intake on a population level, usually below 200 IU/d; this is observed as well in Eastern Europe. On the other hand, Nordic countries report higher levels of intake of this vitamin.²¹ In Europe, vitamin D deficiency (25-OH-D <50 nmol/L) has been shown to affect 27-61.4% of the population. In Germany, 54.5% of the children aged 1-17 years were shown to have this deficiency, and mean serum 25-OH-D levels were 54 nmol/L, while in Greece, the problem of vitamin D deficiency is more pronounced than in the area covered by the present study, and serum vitamin D levels are even lower.

Among the potential causes of the persistence of vitamin D deficiency in children as observed in two national-level surveys in Mexico (2012 and 2018), inadequate intake of vitamin D has been suggested, mainly due to the fact that the main food sources providing vitamin D are animal-based, and, although milk is a widely consumed food product among young children, its fortification with 200 IU/L is considered to be insufficient. Furthermore, consumption of vitamin D supplements is low, and Mexico does not have supplementation programs targeted towards vulnerable groups. Together, these factors, as well as indoor lifestyles, environmental contamination and excess body weight, may be contributing to vitamin D deficiency. In addition, genetic variations (SNPs) related to vitamin D metabolism have been documented to be widely present in the Mexican population and may be associated with up to 50% of the deficiencies.²²

Two of the strengths of the present study are the inclusion of a representative sample of pre-school-age and school-age Mexican children, and the use of standardized methods and personnel to perform data collection. Furthermore, the laboratory method used for 25-OH-D measurements (CMIA) has been shown to have a high rate of reactivity towards 25-hydroxyvitamin-D3 (approximately 100%) and an adequate performance when compared to the gold standard (tandem mass spectrometry).⁷

The limitations of this study relate to inherent characteristics of the use of the FFQ method, which may underestimate nutrient intake, as well as the potential over- or under-reporting by the informant associated with age, sex and nutritional state of the child participant and which introduces potential biases in the estimation of the intake. Nonetheless, our findings are consistent with previous studies, not only in Mexico but in other populations as well.

Recently, a modification to the Official Mexican Standard (NOM, for its acronym in spanish) 051 was approved by the Senate of the Republic,²³ made official in October of 2020, in which the daily intake recommendation of vitamin D was increased from 200 IU/d to 400 IU/d (10 μg/d). This constitutes an important advancement in building legislation that supports improved vitamin D intake in Mexico.

In conclusion, vitamin D deficiency continues to be a population nutrition problem in Mexican children, being most important in pre-school-age children, female children and school-age children with a high BMI. The fight against vitamin D deficiency in vulnerable groups requires concrete actions and supplementation programs, as well as efforts towards improving the fortification of widely consumed food products (for example, increasing the fortification of milk, while also including other potential vehicles such as eggs).²⁴