Introduction

Growing attention has been given to vitamin D (VD) and children's health over the past two decades. VD deficiency has been described as one of the most significant impact health problems in the pediatric population¹-³. The prevalence of VD deficiency ranges from 60 to 80% worldwide in children¹,²,⁴,⁵. In Mexico, deficiencies in up to 20% have been reported in this population³,⁶-⁸.

Several factors have been associated with VD levels in humans, such as nutritional status, skin color, geographic localization, and exposure to sunlight⁹,¹⁰. VD deficiency can result in severe health problems, such as abnormalities in calcium and phosphorus uptake and bone metabolism¹¹-¹³.

Besides VD's importance for calcium homeostasis and bone growth, its deficiency has also been associated with the risk of other pathologies, such as type 1 diabetes in children and type 2 diabetes in adults¹⁴-¹⁶. Moreover, VD deficiency has been related to increased systolic pressure, low serum calcium levels, and HDL cholesterol in the adult population¹⁷,¹⁸. In children, frequent infections can identify VD deficiency since it has been associated with low immunity¹⁹,²⁰.

VD has another essential role in calcium metabolism. Only 10-15% of dietary calcium is absorbed through the intestinal tract when low VD levels are present²¹. Moreover, when calcium and 25-(OH)-D serum levels are low, PTH is activated to stimulate 1,25-(OH)-D production and increase calcium absorption through the gut, causing an inverse correlation between PTH and VD levels¹³,²².

In a previous report⁸, we found a prevalence of VD deficiency > 40% and an inverse relation with PTH levels in Mexican adults. Although some studies have reported VD concentrations in Mexican children, their association with PTH levels³,⁷ has not been addressed, although this association has been demonstrated in children in other countries²³,²⁴.

Therefore, this study aimed to report VD concentrations in a group of healthy Mexican children between 2-17 years of age and their correlation with PTH levels with a methodology previously used in Mexican adults⁸.

Methods

Results

From a total of 275 participants, 55% were male. The median age for each group was 4 years in young children, 8 years in school children, and 13 years in adolescents.

The median weight was 16.0 kg in the group of young children, 25.3 kg in school children, and 47.8 kg in adolescents. According to the BMI Z-score, 68% of the sample was in a normal weight range, and 26.9% with overweight or obesity (Table 1).

The median (CI 95%) concentration levels of 25-(OH)-D in young children were 27.4 ng/ml (24.7-31.0 ng/ml); in school children, 25.6 ng/ml (23.2-31.3 ng/ml); and adolescents, 24.7 ng/ml (21.2-29.4 ng/ml). Only 10.5% of the participants showed 25-(OH)-D levels < 20 ng/ml. None of the participants showed 25-(OH)-D levels < 11 ng/ml. Most of the participants (62.2%) showed levels between 20 and 29 ng/ml, while 27.3% showed levels > 30 ng/ml. PTH was found in normal ranges in 95.6% of the sample (Table 2).

When we analyzed the frequency in each 25-(OH)-D levels subgroup, we found 11% of the participants in the 12-19 ng/ml group, 61% in the 20-30 ng/ml group, and 28% in the > 30 ng/ml group. When performing the same analysis according to age groups, we found statistically significant differences in the 12-19 ng/ml subgroup in which only 3% of the young children were included. In contrast, 10% of school children and 21% of adolescents were found in this subgroup (p ≤ 0.05). We found no statistically significant differences in either the 20-30 ng/ml or the > 30 ng/ml subgroups by age.

Most of the children and adolescents showed PTH levels according to 25-(OH)-D concentrations groups within the normal range. No significant differences were found between age groups (Table 3). When analyzing the association between 25-(OH)-D concentration and PTH levels, a weak inverse association was observed although not statistically significant (rho = -0.112, p = 0.063).

We also analyzed the concentration of 25-(OH)-D according to the BMI (Table 4). We found BMI in a normal range in over half of the population (64.2%) with 25-(OH)-D levels between 20 to 29 ng/ml.

When we analyzed the association between 25-(OH)-D and PTH levels by age group, no statistically significant differences were found. We also analyzed the association between BMI and VD levels. 25-(OH)-D levels > 20 ng/ml were observed in 74.2% of participants in the normal or low BMI groups and 25.8% of participants with overweight or obesity (p = 0.009).

Finally, we analyzed dietary calcium intake in this population and found that 49% of the population consumed < 700 mg/day, with the adolescents group being the one with the lowest intake (Table 5). Besides, only 18% of adolescents reported calcium intake levels > 1300 mg/day (p = 0.001) (Figure 1).

Figure 1 Calcium intake (mg/day) per age group. 

Discussion

In the present study, we described VD concentrations and their association with PTH levels in a group of healthy children and adolescents from the Mexico City metropolitan area and Toluca City. This study is the first to explore this association in Mexican children to the extent of our knowledge. This study follows the same methodology that we previously followed to report VD levels and their association with PTH in a sample of Mexican adults⁸. As the present group of children and adolescents came from the same cities as the adults studied previously, and the same techniques and laboratories were used in both studies (LC/MS for 25-(OH)-D and RIA for PTH), the results of both groups can be complemented and compared with each other⁸.

We found VD deficiency among Mexican children/adolescents aged from 2 to 17 years: 10% of the participants showed < 20 ng/ml values. We found < 10 ng/dl levels in one case.

No statistically significant association was found between PTH and 25-(OH)-D levels, although an inverse relationship trend (rho= -0.112, p = 0.063) was identified. Possibly, we need a larger sample to find a statistically significant difference since the correlation between PTH and 25-(OH)-D in the pediatric population has been demonstrated by Clark et al., Sahin et al., and Asghari et al.⁸,²³,²⁴.

Only a few studies have described VD levels in children and adolescents in Mexico. Toussaint et al.³ reported VD deficiency in 24.7% of children aged 3 to 8 years, whereas our results showed VD deficiency in only 5% of children at this age range. The observed differences could be related to the participants' recruitment procedure. While Toussaint et al.³ participants were recruited in four different hospitals within the metropolitan area of Mexico City, our study recruited healthy children from day-care centers, schools, and the waiting room of dental offices. Moreover, a different technique was performed to measure 25-(OH)-D in both reports: our study used LC/MS, while Toussaint's used quantitative electrochemiluminescence analysis (ECLIA).

A similar situation occurred with the study by Flores et al.⁷, which included a randomized sample of children (2 to 12 years old) recruited at the 2006 National Health and Nutrition Survey (ENSANUT). They reported VD levels < 20 ng/dl in 30% of young children (2-5 years old) and 18% of school children (6-12 years old). Like the previous study³, we cannot compare the population recruited by Flores et al.⁷ with the present population because they included a broader range of regions of the country. Several of these areas are known for having high food insecurity and malnutrition³². Also, the representation of children coming from Mexico City and the metropolitan area was small. Finally, different techniques were used to analyze VD levels (enzyme-linked immunosorbent assay vs. LC/MS).

It is known that overweight and obesity are associated with low levels of 25-(OH)-D. Consistent with this association, we observed that 25.8% of children and adolescents with a weight-for-age Z-score > 1 presented VD levels < 20 ng/ml, similar to those reported previously by Elizondo et al.⁶, who found 27.3% of children with overweight and obesity with low levels of VD in the north of Mexico.

We found 25-(OH)-D levels < 20 ng/ml in 17.8% of adolescents. Indeed, this group presented the highest prevalence of deficiency in our study. This finding may be explained by lifestyle changes related to adolescence. Ekelund et al. and Mitchell et al.³³,³⁴ reported that sedentary behavior (i.e., screen time, among others) increases with age, a pattern already described in Mexican adolescents³⁵. Such trends include unhealthy dietary habits with potentially harmful effects on long-term health.

Our study found a trend toward less calcium intake as age increased. The group of adolescents reported dietary calcium intake below the minimum recommended (700 mg) by the Institute of Medicine³⁰. The low daily calcium intake has been reported among adolescents in other studies as well³⁶,³⁷. However, this finding must be taken cautiously since measuring nutrients intake by questionnaires has sometimes been proven inaccurate. For example, children and adolescents tend to under-report their intake, as much as 67% below the actual intake, and females are more prone to under-report the dietary intake³⁸,³⁹. Conversely, we found calcium intake deficiency in 28% of adolescent females compared to 17% in males.

One limitation of our study is that the overall population in Mexico was not represented since our sample was limited to Mexico City and its surroundings. Therefore, data extrapolation to the rest of the country was not possible. Future studies with a broader representation of the country's different geographical areas should be conducted using the same methodology, including 25-(OH)-D and PTH measurement techniques to improve the knowledge of VD deficiency in other regions within Mexico.

In the present study, we found a low prevalence of VD deficiency in a sample of healthy children and adolescents in Mexico, where only 10% of the total sample showed VD levels < 20 ng/ml. A higher prevalence was observed as age increased. These results are lower than those previously reported in a Mexican population, probably due to heterogeneity in the sample and the different techniques used to quantify 25-(OH)-D. We also observed VD levels < 20 ng/dl in 26% of subjects with overweight or obesity. Therefore, particular attention should be paid to this risk group due to the clinical implications associated with VD deficiency. Finally, although the correlation between the decrease in VD and the proportional increase in PTH was not significant in this study, more studies with a larger sample should be conducted on Mexican children. Recommendations for a healthy diet and lifestyle should be emphasized during this critical period of life because of future adulthood and old age implications.