Latitude And Vitamin D
Abstract
Background: Recent data indicate a role for vitamin D in many health aspects including anthropometric measures and blood lipid profiles. Dermal vitamin D synthesis may be influenced by latitude. However, the contribution of latitude in vitamin D status and its association with anthropometric and blood lipid measures in Iranian children have not been studied to date.
Methods: We used data from the National Food and Nutritional Surveillance Program. In total, 667 apparently healthy children aged 5–18 years were randomly selected from six provinces of Iran with different latitudes, from 29 to 37°. Weight, height, circulating 25-hydroxycalciferol; calcidiol [25(OH)D] and blood lipids were measured.
Results: In total, 16.7 and 4.1% of children were overweight or obese, respectively. The mean 25(OH)D concentration was 27.3 ± 17.6 nmol/l (95% confidence interval: 26.0–28.7 nmol/l). Over 93% of all children had suboptimal circulating calcidiol concentrations. Undesirable status of vitamin D, serum triglyceride and low-density lipoprotein were all more prevalent in children living in regions >33° latitude than those in <33°, significantly. There was no significant difference in duration of sun exposure between children living in latitudes below and above 33° (p = 0.093). In multivariate regression model, sex, latitude, body mass index for age z-score and sun exposure duration were independently related to 25(OH)D concentrations, but age was not.
Conclusion: Despite significant association of latitude and vitamin D status, hypovitaminosis D is prevalent across latitude gradient in Iranian children. Our findings warrant immediate sustainable nutritional intervention, including supplementation, to protect children from hypovitaminosis D irrespective of the latitude of their residence.
INTRODUCTION
After several decades since its discovery and characterization, vitamin D is still a mysterious molecule whose deficiency is reported from almost all over the world [1]. Suboptimal vitamin D status has been associated with increased risk of multiple clinical outcomes including osteoporosis [2], type 1 diabetes mellitus [3], colorectal and prostate cancers [4], multiple sclerosis (MS) [5], insulin resistance [6], obesity [7] and cardiovascular disease [8].
Vitamin D can be acquired through cutaneous photosynthesis through sunlight exposure. There are limited dietary sources for vitamin D, which are not usually included in the Iranian food basket in appreciable amounts. Therefore, in the absence of supplementation, the most important source of vitamin D is exposure to solar beam. The main eco-environmental determinants of dermal photosynthesis of vitamin D3 are season, latitude, duration and time of sun exposure, the amount of air pollutants and the concentrations of atmospheric ozone [9]. There are also some individual factors, such as skin pigmentation, fat mass (FM), age and clothing [10].
Hypovitaminosis D is prevalent in Iran. The prevalence of undesirable vitamin D status in Iranian children varies from 41.3% in North Khorasan [11] to 85.2% in Zahedan [12] and 65–78% [13] in Tehran, which may reach to >90% in winter [14]. The effect of latitude on vitamin D status of the Iranian children has not been studied to date. This study aimed, therefore, to determine the effect of latitude on vitamin D status and its association with anthropometric and blood lipid measures in the Iranian healthy children.
SUBJECTS AND METHODS
We used data from National Food and Nutritional Surveillance Program (NFNSP), a population-based nutrition survey conducted periodically in Iran by National Nutrition and Food Technology Research Institute (NNFTRI) in collaboration with Nutrition Office of the Deputy of Health of Ministry of Health and United Nations Children's Fund.
Data were collected in mid-winter 2013 (20 January through 20 February). As a whole, 667 apparently healthy children aged 5–18 years were randomly selected from the registered households from six provinces of Iran with different latitudes including West Azarbaijan (37°), Semnan (36°), Lorestan (33°), South Khorasan (32°), Khoozestan (31°) and Fars (29°). We used multistage cluster random sampling method. The first stage included selection of a sample of area segments, comprising census blocks or combinations of blocks. The second stage of sample selection consisted of households in each block. Random selection of households was based on postal codes. A listing of all members in the selected households was prepared. From each household, one eligible child was enrolled. The inclusion criteria were age 5–18 years, no vitamin D, multivitamin or omega-3 supplement use within the past 3 months and absence of any clinical disease. Full description of the study objectives was given to the parents before they signed an informed consent. A questionnaire for demographic data and duration of direct sun exposure (the number of minutes/hours that they have spent in daylight) and sunscreen use was completed by interviewing with each participant. Height and weight were measured according to the standard procedures, to the nearest of 0.1 cm and 0.1 kg, respectively. Body mass index (BMI) was calculated as weight/height2 (kg/m2). Overweight and obesity were categorized using the BMI for age z-score (1–2 and >2, respectively). NFNSP is covered by the NNFTRI ethical committee.
Laboratory procedures
Blood samples were obtained after an overnight fast during a scheduled morning visit, at 8.00–9.00 am. Serums were immediately separated after centrifugation of blood samples and stored at −20 °C until analysis. All samples were analyzed at the central laboratory (Laboratory of Nutrition Research, NNFTRI). Serum lipid profile was determined using enzymatic methods (Pars Azmoon, Tehran, Iran) and an auto-analyzer (Selectra E; Vitalab, Holliston, the Netherlands). Circulating 25-hydroxycalciferol; calcidiol [25(OH)D] concentrations were measured by ELISA (Diasource, Belgium). The predefined cutoff points used for vitamin D status based on serum 25(OH)D concentrations were as follows: severe deficiency, <12.5 nmol/l; deficiency, <25 nmol/l; insufficiency, 25–50 nmol/l; and sufficiency, >50 nmol/l (conversion factor: nmol/l ÷ 2.5 = ng/ml) [15].
Statistical analyses
Continuous variables were expressed as mean ± SD, whereas categorical variables were presented as percentages. For between-group comparison of categorical and continuous variables, χ2 and analysis of variance followed by Tukey's post hoc analysis were used, respectively. Pearson's correlation coefficient was used for association between continuous variables. Risk factors associated with vitamin D deficiency (VDD) were assessed using multivariate logistic regression analysis, and odds ratio (OR) and 95% confidence interval (95% CI) were calculated for each variable included in the model. In this model, VDD was regarded as dependent variable, whereas age, gender, BMI z-score, latitude and duration of sun exposure were independent variables. All statistical analyses were performed using SPSS 21 (SPSS Inc, Chicago, IL). All tests were two-sided and p < 0.05 was considered significant.
RESULTS
General characteristics of the participants are shown in Table 1. On the whole, 16.7 and 4.1% of children were overweight and obese, respectively, and there was no significant difference between boys and girls (21.7 vs. 20.1%, p = 0.631).
Table 1
Some general characteristics and biochemical values of the participants
| Variable | Boys | Girls | p value | Total |
|---|---|---|---|---|
| n (%) | 322 (51.7) | 345 (48.3) | – | 667 |
| Age (years) | 10.9 ± 3.7 | 10.7 ± 4.0 | 0.598 | 10.8 ± 3.8 |
| Height for age z-score | 0.21 ± 1.3 | 0.13 ± 1.3 | 0.440 | 0.17 ± 1.3 |
| BMI for age z-score | −0.18 ± 1.3 | −0.06 ± 1.2 | 0.238 | −0.12 ± 1.3 |
| 25(OH)D nmol/l | 31.1 ± 17.4 | 23.8 ± 17.0 | <0.001 | 27.3 ± 17.6 |
| TG (mg/dl) | 84.8 ± 45.3 | 79.7 ± 35.9 | 0.114 | 82.2 ± 40.8 |
| TC (mg/dl) | 140.5 ± 27.4 | 140.6 ± 24.7 | 0.972 | 140.5 ± 26.0 |
| LDL-C (mg/dl) | 71.9 ± 22.1 | 74.2 ± 21.3 | 0.185 | 72.7 ± 25.1 |
| HDL-C (mg/dl) | 51.5 ± 11.9 | 50.4 ± 10.3 | 0.185 | 50.9 ± 11.1 |
| Sun exposure (%) | ||||
| <1 h/day | 56.2 | 74.6 | <0.001 | 65.7 |
| >1 h/day | 43.8 | 25.4 | 34.3 | |
| Time of day (%) | ||||
| 10 am to 3 pm | 57.8 | 42.9 | <0.001 | 50.1 |
| Others | 42.2 | 57.1 | 49.9 | |
| Sunscreen use (%) | ||||
| No | 92.8 | 65.2 | <0.001 | 78.6 |
| Yes | 7.2 | 34.8 | 21.4 |
| Variable | Boys | Girls | p value | Total |
|---|---|---|---|---|
| n (%) | 322 (51.7) | 345 (48.3) | – | 667 |
| Age (years) | 10.9 ± 3.7 | 10.7 ± 4.0 | 0.598 | 10.8 ± 3.8 |
| Height for age z-score | 0.21 ± 1.3 | 0.13 ± 1.3 | 0.440 | 0.17 ± 1.3 |
| BMI for age z-score | −0.18 ± 1.3 | −0.06 ± 1.2 | 0.238 | −0.12 ± 1.3 |
| 25(OH)D nmol/l | 31.1 ± 17.4 | 23.8 ± 17.0 | <0.001 | 27.3 ± 17.6 |
| TG (mg/dl) | 84.8 ± 45.3 | 79.7 ± 35.9 | 0.114 | 82.2 ± 40.8 |
| TC (mg/dl) | 140.5 ± 27.4 | 140.6 ± 24.7 | 0.972 | 140.5 ± 26.0 |
| LDL-C (mg/dl) | 71.9 ± 22.1 | 74.2 ± 21.3 | 0.185 | 72.7 ± 25.1 |
| HDL-C (mg/dl) | 51.5 ± 11.9 | 50.4 ± 10.3 | 0.185 | 50.9 ± 11.1 |
| Sun exposure (%) | ||||
| <1 h/day | 56.2 | 74.6 | <0.001 | 65.7 |
| >1 h/day | 43.8 | 25.4 | 34.3 | |
| Time of day (%) | ||||
| 10 am to 3 pm | 57.8 | 42.9 | <0.001 | 50.1 |
| Others | 42.2 | 57.1 | 49.9 | |
| Sunscreen use (%) | ||||
| No | 92.8 | 65.2 | <0.001 | 78.6 |
| Yes | 7.2 | 34.8 | 21.4 |
Values are presented as number or mean ± SD.
TC: total cholesterol; TG: triglycerides.
Table 1
Some general characteristics and biochemical values of the participants
| Variable | Boys | Girls | p value | Total |
|---|---|---|---|---|
| n (%) | 322 (51.7) | 345 (48.3) | – | 667 |
| Age (years) | 10.9 ± 3.7 | 10.7 ± 4.0 | 0.598 | 10.8 ± 3.8 |
| Height for age z-score | 0.21 ± 1.3 | 0.13 ± 1.3 | 0.440 | 0.17 ± 1.3 |
| BMI for age z-score | −0.18 ± 1.3 | −0.06 ± 1.2 | 0.238 | −0.12 ± 1.3 |
| 25(OH)D nmol/l | 31.1 ± 17.4 | 23.8 ± 17.0 | <0.001 | 27.3 ± 17.6 |
| TG (mg/dl) | 84.8 ± 45.3 | 79.7 ± 35.9 | 0.114 | 82.2 ± 40.8 |
| TC (mg/dl) | 140.5 ± 27.4 | 140.6 ± 24.7 | 0.972 | 140.5 ± 26.0 |
| LDL-C (mg/dl) | 71.9 ± 22.1 | 74.2 ± 21.3 | 0.185 | 72.7 ± 25.1 |
| HDL-C (mg/dl) | 51.5 ± 11.9 | 50.4 ± 10.3 | 0.185 | 50.9 ± 11.1 |
| Sun exposure (%) | ||||
| <1 h/day | 56.2 | 74.6 | <0.001 | 65.7 |
| >1 h/day | 43.8 | 25.4 | 34.3 | |
| Time of day (%) | ||||
| 10 am to 3 pm | 57.8 | 42.9 | <0.001 | 50.1 |
| Others | 42.2 | 57.1 | 49.9 | |
| Sunscreen use (%) | ||||
| No | 92.8 | 65.2 | <0.001 | 78.6 |
| Yes | 7.2 | 34.8 | 21.4 |
| Variable | Boys | Girls | p value | Total |
|---|---|---|---|---|
| n (%) | 322 (51.7) | 345 (48.3) | – | 667 |
| Age (years) | 10.9 ± 3.7 | 10.7 ± 4.0 | 0.598 | 10.8 ± 3.8 |
| Height for age z-score | 0.21 ± 1.3 | 0.13 ± 1.3 | 0.440 | 0.17 ± 1.3 |
| BMI for age z-score | −0.18 ± 1.3 | −0.06 ± 1.2 | 0.238 | −0.12 ± 1.3 |
| 25(OH)D nmol/l | 31.1 ± 17.4 | 23.8 ± 17.0 | <0.001 | 27.3 ± 17.6 |
| TG (mg/dl) | 84.8 ± 45.3 | 79.7 ± 35.9 | 0.114 | 82.2 ± 40.8 |
| TC (mg/dl) | 140.5 ± 27.4 | 140.6 ± 24.7 | 0.972 | 140.5 ± 26.0 |
| LDL-C (mg/dl) | 71.9 ± 22.1 | 74.2 ± 21.3 | 0.185 | 72.7 ± 25.1 |
| HDL-C (mg/dl) | 51.5 ± 11.9 | 50.4 ± 10.3 | 0.185 | 50.9 ± 11.1 |
| Sun exposure (%) | ||||
| <1 h/day | 56.2 | 74.6 | <0.001 | 65.7 |
| >1 h/day | 43.8 | 25.4 | 34.3 | |
| Time of day (%) | ||||
| 10 am to 3 pm | 57.8 | 42.9 | <0.001 | 50.1 |
| Others | 42.2 | 57.1 | 49.9 | |
| Sunscreen use (%) | ||||
| No | 92.8 | 65.2 | <0.001 | 78.6 |
| Yes | 7.2 | 34.8 | 21.4 |
Values are presented as number or mean ± SD.
TC: total cholesterol; TG: triglycerides.
The mean 25(OH)D concentration was 27.3 ± 17.6 nmol/l (95% CI: 26.0–28.7 nmol/l) with Khoozestan (31°) and Fars (29°) having the lowest and highest mean concentrations (Table 2). Girls, as compared with boys, had lower 25(OH)D (23.8 ± 17.0 vs. 31.1 ± 17.4 nmol/l, p < 0.001). Accordingly, girls had also significantly shorter duration of sun exposure and more sunscreen use (Table 1).
Table 2
Mean values of lipid profile in the children according to the regions and sex
| Province | 25(OH)D | TG (mg/dl) | TC (mg/dl) | LDL-C (mg/dl) | HDL-C (mg/dl) | |
|---|---|---|---|---|---|---|
| West Azarbaijan (n = 111) | Boys | 32.1 ± 17.8 | 95.4 ± 56.5 | 147.4 ± 32.6 | 78.4 ± 25.4 | 49.9 ± 12.4 |
| Girls | 22.7 ± 11.8 | 91.3 ± 50.9 | 136.6 ± 21.5 | 70.7 ± 17.5 | 47.6 ± 11.7 | |
| p value | 0 .002 | 0 .696 | 0 .041 | 0 .066 | 0 .318 | |
| Total | 27.7 ± 15.9 | 93.5 ± 53.7 | 142.4 ± 28.3 | 71.8 ± 24.5 | 48.8 ± 12.1 | |
| Semnan (n = 74) | Boys | 26.0 ± 9.2 | 116.3 ± 65.4 | 147.7 ± 22.8 | 73.0 ± 16.6 | 51.4 ± 11.0 |
| Girls | 20.7 ± 7.9 | 86.3 ± 28.5 | 138.5 ± 19.8 | 69.8 ± 17.2 | 51.4 ± 9.9 | |
| p value | 0 .010 | 0 .012 | 0 .068 | 0 .414 | 1 .00 | |
| Total | 23.3 ± 8.9 | 100.9 ± 51.8 | 143.0 ± 21.7 | 73.0 ± 25.3 | 51.4 ± 10.4 | |
| Lorestan (n = 114) | Boys | 28.5 ± 11.8 | 88.7 ± 34.1 | 129.3 ± 22.7 | 64.1 ± 19.8 | 47.4 ± 11.5 |
| Girls | 23.0 ± 15.5 | 87.9 ± 32.8 | 139.1 ± 25.7 | 72.7 ± 22.2 | 48.8 ± 9.1 | |
| p value | 0 .038 | 0 .905 | 0 .034 | 0 .032 | 0 .495 | |
| Total | 25.5 ± 14.1 | 88.3 ± 33.3 | 134.5 ± 24.7 | 73.3 ± 25.0 | 48.1 ± 10.3 | |
| South Khorasan (n = 126) | Boys | 33.1 ± 23.5 | 72.4 ± 34.2 | 154.9 ± 25.6 | 82.0 ± 22.7 | 58.4 ± 10.4 |
| Girls | 23.5 ± 15.4 | 71.3 ± 31.2 | 150.6 ± 26.4 | 82.9 ± 23.7 | 53.4 ± 9.6 | |
| p value | 0 .009 | 0 .853 | 0 .361 | 0 .0838 | 0 .007 | |
| Total | 28.2 ± 20.3 | 71.9 ± 32.5 | 152.7 ± 26.0 | 81.0 ± 26.5 | 55.8 ± 10.3 | |
| Khoozestan (n = 121) | Boys | 24.9 ± 7.1 | 63.0 ± 32.1 | 134.3 ± 25.9 | 69.4 ± 20.3 | 52.2 ± 12.6 |
| Girls | 21.6 ± 8.3 | 69.2 ± 32.3 | 142.5 ± 26.1 | 78.0 ± 20.6 | 50.7 ± 9.6 | |
| p value | 0 .027 | 0 .302 | 0 .089 | 0 .027 | 0 .467 | |
| Total | 22.9 ± 8.0 | 66.7 ± 32.3 | 139.2 ± 26.2 | 73.1 ± 26.2 | 51.3 ± 10.9 | |
| Fars (n = 121) | Boys | 38.2 ± 20.4 | 82.4 ± 33.0 | 130.5 ± 22.1 | 64.4 ± 19.0 | 49.6 ± 10.5 |
| Girls | 30.6 ± 30.4 | 78.8 ± 31.5 | 133.2 ± 22.5 | 67.3 ± 20.3 | 50.1 ± 11.6 | |
| p value | 0 .107 | 0 .543 | 0 .0505 | 0 .420 | 0 .791 | |
| Total | 34.6 ± 25.8 | 80.7 ± 32.2 | 131.8 ± 22.2 | 65.3 ± 21.1 | 49.9 ± 11.0 | |
| Province | 25(OH)D | TG (mg/dl) | TC (mg/dl) | LDL-C (mg/dl) | HDL-C (mg/dl) | |
|---|---|---|---|---|---|---|
| West Azarbaijan (n = 111) | Boys | 32.1 ± 17.8 | 95.4 ± 56.5 | 147.4 ± 32.6 | 78.4 ± 25.4 | 49.9 ± 12.4 |
| Girls | 22.7 ± 11.8 | 91.3 ± 50.9 | 136.6 ± 21.5 | 70.7 ± 17.5 | 47.6 ± 11.7 | |
| p value | 0 .002 | 0 .696 | 0 .041 | 0 .066 | 0 .318 | |
| Total | 27.7 ± 15.9 | 93.5 ± 53.7 | 142.4 ± 28.3 | 71.8 ± 24.5 | 48.8 ± 12.1 | |
| Semnan (n = 74) | Boys | 26.0 ± 9.2 | 116.3 ± 65.4 | 147.7 ± 22.8 | 73.0 ± 16.6 | 51.4 ± 11.0 |
| Girls | 20.7 ± 7.9 | 86.3 ± 28.5 | 138.5 ± 19.8 | 69.8 ± 17.2 | 51.4 ± 9.9 | |
| p value | 0 .010 | 0 .012 | 0 .068 | 0 .414 | 1 .00 | |
| Total | 23.3 ± 8.9 | 100.9 ± 51.8 | 143.0 ± 21.7 | 73.0 ± 25.3 | 51.4 ± 10.4 | |
| Lorestan (n = 114) | Boys | 28.5 ± 11.8 | 88.7 ± 34.1 | 129.3 ± 22.7 | 64.1 ± 19.8 | 47.4 ± 11.5 |
| Girls | 23.0 ± 15.5 | 87.9 ± 32.8 | 139.1 ± 25.7 | 72.7 ± 22.2 | 48.8 ± 9.1 | |
| p value | 0 .038 | 0 .905 | 0 .034 | 0 .032 | 0 .495 | |
| Total | 25.5 ± 14.1 | 88.3 ± 33.3 | 134.5 ± 24.7 | 73.3 ± 25.0 | 48.1 ± 10.3 | |
| South Khorasan (n = 126) | Boys | 33.1 ± 23.5 | 72.4 ± 34.2 | 154.9 ± 25.6 | 82.0 ± 22.7 | 58.4 ± 10.4 |
| Girls | 23.5 ± 15.4 | 71.3 ± 31.2 | 150.6 ± 26.4 | 82.9 ± 23.7 | 53.4 ± 9.6 | |
| p value | 0 .009 | 0 .853 | 0 .361 | 0 .0838 | 0 .007 | |
| Total | 28.2 ± 20.3 | 71.9 ± 32.5 | 152.7 ± 26.0 | 81.0 ± 26.5 | 55.8 ± 10.3 | |
| Khoozestan (n = 121) | Boys | 24.9 ± 7.1 | 63.0 ± 32.1 | 134.3 ± 25.9 | 69.4 ± 20.3 | 52.2 ± 12.6 |
| Girls | 21.6 ± 8.3 | 69.2 ± 32.3 | 142.5 ± 26.1 | 78.0 ± 20.6 | 50.7 ± 9.6 | |
| p value | 0 .027 | 0 .302 | 0 .089 | 0 .027 | 0 .467 | |
| Total | 22.9 ± 8.0 | 66.7 ± 32.3 | 139.2 ± 26.2 | 73.1 ± 26.2 | 51.3 ± 10.9 | |
| Fars (n = 121) | Boys | 38.2 ± 20.4 | 82.4 ± 33.0 | 130.5 ± 22.1 | 64.4 ± 19.0 | 49.6 ± 10.5 |
| Girls | 30.6 ± 30.4 | 78.8 ± 31.5 | 133.2 ± 22.5 | 67.3 ± 20.3 | 50.1 ± 11.6 | |
| p value | 0 .107 | 0 .543 | 0 .0505 | 0 .420 | 0 .791 | |
| Total | 34.6 ± 25.8 | 80.7 ± 32.2 | 131.8 ± 22.2 | 65.3 ± 21.1 | 49.9 ± 11.0 | |
TC: total cholesterol; TG: triglycerides.
Table 2
Mean values of lipid profile in the children according to the regions and sex
| Province | 25(OH)D | TG (mg/dl) | TC (mg/dl) | LDL-C (mg/dl) | HDL-C (mg/dl) | |
|---|---|---|---|---|---|---|
| West Azarbaijan (n = 111) | Boys | 32.1 ± 17.8 | 95.4 ± 56.5 | 147.4 ± 32.6 | 78.4 ± 25.4 | 49.9 ± 12.4 |
| Girls | 22.7 ± 11.8 | 91.3 ± 50.9 | 136.6 ± 21.5 | 70.7 ± 17.5 | 47.6 ± 11.7 | |
| p value | 0 .002 | 0 .696 | 0 .041 | 0 .066 | 0 .318 | |
| Total | 27.7 ± 15.9 | 93.5 ± 53.7 | 142.4 ± 28.3 | 71.8 ± 24.5 | 48.8 ± 12.1 | |
| Semnan (n = 74) | Boys | 26.0 ± 9.2 | 116.3 ± 65.4 | 147.7 ± 22.8 | 73.0 ± 16.6 | 51.4 ± 11.0 |
| Girls | 20.7 ± 7.9 | 86.3 ± 28.5 | 138.5 ± 19.8 | 69.8 ± 17.2 | 51.4 ± 9.9 | |
| p value | 0 .010 | 0 .012 | 0 .068 | 0 .414 | 1 .00 | |
| Total | 23.3 ± 8.9 | 100.9 ± 51.8 | 143.0 ± 21.7 | 73.0 ± 25.3 | 51.4 ± 10.4 | |
| Lorestan (n = 114) | Boys | 28.5 ± 11.8 | 88.7 ± 34.1 | 129.3 ± 22.7 | 64.1 ± 19.8 | 47.4 ± 11.5 |
| Girls | 23.0 ± 15.5 | 87.9 ± 32.8 | 139.1 ± 25.7 | 72.7 ± 22.2 | 48.8 ± 9.1 | |
| p value | 0 .038 | 0 .905 | 0 .034 | 0 .032 | 0 .495 | |
| Total | 25.5 ± 14.1 | 88.3 ± 33.3 | 134.5 ± 24.7 | 73.3 ± 25.0 | 48.1 ± 10.3 | |
| South Khorasan (n = 126) | Boys | 33.1 ± 23.5 | 72.4 ± 34.2 | 154.9 ± 25.6 | 82.0 ± 22.7 | 58.4 ± 10.4 |
| Girls | 23.5 ± 15.4 | 71.3 ± 31.2 | 150.6 ± 26.4 | 82.9 ± 23.7 | 53.4 ± 9.6 | |
| p value | 0 .009 | 0 .853 | 0 .361 | 0 .0838 | 0 .007 | |
| Total | 28.2 ± 20.3 | 71.9 ± 32.5 | 152.7 ± 26.0 | 81.0 ± 26.5 | 55.8 ± 10.3 | |
| Khoozestan (n = 121) | Boys | 24.9 ± 7.1 | 63.0 ± 32.1 | 134.3 ± 25.9 | 69.4 ± 20.3 | 52.2 ± 12.6 |
| Girls | 21.6 ± 8.3 | 69.2 ± 32.3 | 142.5 ± 26.1 | 78.0 ± 20.6 | 50.7 ± 9.6 | |
| p value | 0 .027 | 0 .302 | 0 .089 | 0 .027 | 0 .467 | |
| Total | 22.9 ± 8.0 | 66.7 ± 32.3 | 139.2 ± 26.2 | 73.1 ± 26.2 | 51.3 ± 10.9 | |
| Fars (n = 121) | Boys | 38.2 ± 20.4 | 82.4 ± 33.0 | 130.5 ± 22.1 | 64.4 ± 19.0 | 49.6 ± 10.5 |
| Girls | 30.6 ± 30.4 | 78.8 ± 31.5 | 133.2 ± 22.5 | 67.3 ± 20.3 | 50.1 ± 11.6 | |
| p value | 0 .107 | 0 .543 | 0 .0505 | 0 .420 | 0 .791 | |
| Total | 34.6 ± 25.8 | 80.7 ± 32.2 | 131.8 ± 22.2 | 65.3 ± 21.1 | 49.9 ± 11.0 | |
| Province | 25(OH)D | TG (mg/dl) | TC (mg/dl) | LDL-C (mg/dl) | HDL-C (mg/dl) | |
|---|---|---|---|---|---|---|
| West Azarbaijan (n = 111) | Boys | 32.1 ± 17.8 | 95.4 ± 56.5 | 147.4 ± 32.6 | 78.4 ± 25.4 | 49.9 ± 12.4 |
| Girls | 22.7 ± 11.8 | 91.3 ± 50.9 | 136.6 ± 21.5 | 70.7 ± 17.5 | 47.6 ± 11.7 | |
| p value | 0 .002 | 0 .696 | 0 .041 | 0 .066 | 0 .318 | |
| Total | 27.7 ± 15.9 | 93.5 ± 53.7 | 142.4 ± 28.3 | 71.8 ± 24.5 | 48.8 ± 12.1 | |
| Semnan (n = 74) | Boys | 26.0 ± 9.2 | 116.3 ± 65.4 | 147.7 ± 22.8 | 73.0 ± 16.6 | 51.4 ± 11.0 |
| Girls | 20.7 ± 7.9 | 86.3 ± 28.5 | 138.5 ± 19.8 | 69.8 ± 17.2 | 51.4 ± 9.9 | |
| p value | 0 .010 | 0 .012 | 0 .068 | 0 .414 | 1 .00 | |
| Total | 23.3 ± 8.9 | 100.9 ± 51.8 | 143.0 ± 21.7 | 73.0 ± 25.3 | 51.4 ± 10.4 | |
| Lorestan (n = 114) | Boys | 28.5 ± 11.8 | 88.7 ± 34.1 | 129.3 ± 22.7 | 64.1 ± 19.8 | 47.4 ± 11.5 |
| Girls | 23.0 ± 15.5 | 87.9 ± 32.8 | 139.1 ± 25.7 | 72.7 ± 22.2 | 48.8 ± 9.1 | |
| p value | 0 .038 | 0 .905 | 0 .034 | 0 .032 | 0 .495 | |
| Total | 25.5 ± 14.1 | 88.3 ± 33.3 | 134.5 ± 24.7 | 73.3 ± 25.0 | 48.1 ± 10.3 | |
| South Khorasan (n = 126) | Boys | 33.1 ± 23.5 | 72.4 ± 34.2 | 154.9 ± 25.6 | 82.0 ± 22.7 | 58.4 ± 10.4 |
| Girls | 23.5 ± 15.4 | 71.3 ± 31.2 | 150.6 ± 26.4 | 82.9 ± 23.7 | 53.4 ± 9.6 | |
| p value | 0 .009 | 0 .853 | 0 .361 | 0 .0838 | 0 .007 | |
| Total | 28.2 ± 20.3 | 71.9 ± 32.5 | 152.7 ± 26.0 | 81.0 ± 26.5 | 55.8 ± 10.3 | |
| Khoozestan (n = 121) | Boys | 24.9 ± 7.1 | 63.0 ± 32.1 | 134.3 ± 25.9 | 69.4 ± 20.3 | 52.2 ± 12.6 |
| Girls | 21.6 ± 8.3 | 69.2 ± 32.3 | 142.5 ± 26.1 | 78.0 ± 20.6 | 50.7 ± 9.6 | |
| p value | 0 .027 | 0 .302 | 0 .089 | 0 .027 | 0 .467 | |
| Total | 22.9 ± 8.0 | 66.7 ± 32.3 | 139.2 ± 26.2 | 73.1 ± 26.2 | 51.3 ± 10.9 | |
| Fars (n = 121) | Boys | 38.2 ± 20.4 | 82.4 ± 33.0 | 130.5 ± 22.1 | 64.4 ± 19.0 | 49.6 ± 10.5 |
| Girls | 30.6 ± 30.4 | 78.8 ± 31.5 | 133.2 ± 22.5 | 67.3 ± 20.3 | 50.1 ± 11.6 | |
| p value | 0 .107 | 0 .543 | 0 .0505 | 0 .420 | 0 .791 | |
| Total | 34.6 ± 25.8 | 80.7 ± 32.2 | 131.8 ± 22.2 | 65.3 ± 21.1 | 49.9 ± 11.0 | |
TC: total cholesterol; TG: triglycerides.
When children were divided into three age subgroups, i.e. 5–9, 9–13 and 13–18 years, serum 25(OH)D levels were found to decrease progressively with age. However, differences were significant only in girls (p = 0.019). Higher levels of 25(OH)D were found in the girls aged 5–9 years. Nevertheless, duration of sun exposure in girls did not show any significant difference among the age subgroups.
The distribution of vitamin D status in the children according to the regions is illustrated in Fig. 1. Undesirable vitamin D status [25(OH)D < 50 nmol/l] was found in 56.0% of all children, with more occurrence in girls than in boys (96.2 vs. 90%, p = 0.002). Vitamin D insufficiency was detected in 37.2% of the children, with more occurrence in boys than in girls (48.6 vs. 26.4%, p < 0.001), whereas VDD was more prevalent in girls than in boys (69.8 vs. 41.4%, p < 0.001). Severe VDD was found in 66 children (9.9%), including 52 girls and 14 boys. The highest occurrence of severe VDD was in Semnan (13.7%) followed by West Azarbaijan (13.5%), South Khorasan (12.3%), Lorestan (8.8%), Khoozestan (7.4%) and Fars (5.8%), respectively.
Fig. 1.
Vitamin D status of the studied children according to the regions. Numbers are in percent.
Fig. 1.
Vitamin D status of the studied children according to the regions. Numbers are in percent.
There was a weak but significant inverse correlation between BMI z-score and 25(OH)D (r = −0.077, p = 0.049). Serum 25(OH)D was higher in normal (<1 z-score BMI for age) than in overweight/obese children (>1 z-score BMI for age) (27.9 ± 18.6 vs. 23.9 ± 10.3 nmol/l, p = 0.001).
The lipid profile based on sex and region is shown in Table 2. The prevalence of undesirable vitamin D status, serum triglyceride and low-density lipoprotein cholesterol (LDL-C) was higher in children living in >33° latitude than those in <33° significantly (Table 3). There was no significant difference in duration of sun exposure between regions with latitudes below and above 33° (p = 0.093).
Table 3
The mean of 25(OH)D and lipid profile in low and high latitudes
| Variable | Under 33° latitude | Above 33° latitude | p value |
|---|---|---|---|
| 25(OH)D nmol/l | 28.6 ± 20.0 | 25.8 ± 13.9 | 0.035 |
| TG mg/dl | 73.1 ± 32.7 | 93.3 ± 46.6 | <0.001 |
| TC mg/dl | 141.3 ± 26.3 | 139.5 ± 25.7 | 0.380 |
| LDL-C mg/dl | 74.3 ± 22.3 | 71.6 ± 20.9 | 0.113 |
| HDL-C mg/dl | 52.3 ± 11.0 | 49.2 ± 11.0 | <0.001 |
| Variable | Under 33° latitude | Above 33° latitude | p value |
|---|---|---|---|
| 25(OH)D nmol/l | 28.6 ± 20.0 | 25.8 ± 13.9 | 0.035 |
| TG mg/dl | 73.1 ± 32.7 | 93.3 ± 46.6 | <0.001 |
| TC mg/dl | 141.3 ± 26.3 | 139.5 ± 25.7 | 0.380 |
| LDL-C mg/dl | 74.3 ± 22.3 | 71.6 ± 20.9 | 0.113 |
| HDL-C mg/dl | 52.3 ± 11.0 | 49.2 ± 11.0 | <0.001 |
TC: total cholesterol; TG: triglycerides.
Table 3
The mean of 25(OH)D and lipid profile in low and high latitudes
| Variable | Under 33° latitude | Above 33° latitude | p value |
|---|---|---|---|
| 25(OH)D nmol/l | 28.6 ± 20.0 | 25.8 ± 13.9 | 0.035 |
| TG mg/dl | 73.1 ± 32.7 | 93.3 ± 46.6 | <0.001 |
| TC mg/dl | 141.3 ± 26.3 | 139.5 ± 25.7 | 0.380 |
| LDL-C mg/dl | 74.3 ± 22.3 | 71.6 ± 20.9 | 0.113 |
| HDL-C mg/dl | 52.3 ± 11.0 | 49.2 ± 11.0 | <0.001 |
| Variable | Under 33° latitude | Above 33° latitude | p value |
|---|---|---|---|
| 25(OH)D nmol/l | 28.6 ± 20.0 | 25.8 ± 13.9 | 0.035 |
| TG mg/dl | 73.1 ± 32.7 | 93.3 ± 46.6 | <0.001 |
| TC mg/dl | 141.3 ± 26.3 | 139.5 ± 25.7 | 0.380 |
| LDL-C mg/dl | 74.3 ± 22.3 | 71.6 ± 20.9 | 0.113 |
| HDL-C mg/dl | 52.3 ± 11.0 | 49.2 ± 11.0 | <0.001 |
TC: total cholesterol; TG: triglycerides.
When children were categorized based on their serum high-density lipoprotein cholesterol (HDL-C) concentrations, mean serum 25(OH)D was found higher in optimal HDL-C subgroup than in the suboptimal subgroup (28.8 ± 18.2 vs. 24.1 ± 15.6 nmol/l, p = 0.001).
Linear regression showed that 25(OH)D was negatively affected by age (B = −0.3, CI = −0.7 to −0.047, p = 0.026) and BMI for age z-score (B = −1.19, CI = −2.21 to −0.18, p = 0.021).
In multivariate regression model, sex (female vs. male, OR: 2.39, 95% CI: 1.2–4.76, p = 0.013), latitude (>33° vs. <33°, OR: 3.12, 95% CI: 1.45–6.71, p = 0.003), BMI for age z-score (overweight/obese vs. normal weight, OR: 3.34, 95% CI: 1.0–11.1, p = 0.049) and sun exposure duration (10 am to 3 pm vs. others, OR: 0.45, 95% CI: 0.22–0.91, p = 0.026) were related to 25(OH)D concentrations, but age was not.
DISCUSSION
VDD was detected in 49 to >70% of the children, whereas the range of severe VDD was from 5.8 to 13.7%. VDD has been reported in 8% of the Great Britain [16] and in 18% of the US children [17]. In three studies reported from Spain [18], France [19] and Finland [20], about 80% of children and adolescents had 25(OH)D concentrations <50 nmol/l in winter. It is reasonable that increased urbanization, air pollution, increased time spent indoors, sun-protective behaviors, age and being overweight/obese could all play a role in VDD in children and adolescents [21].
The occurrence of hypovitaminosis D was higher among children living in 33–37° latitude than those in 29–33°. It is believed that insufficient intensity of sunlight at higher latitudes especially from October to March is the cause of decreasing circulating 25(OH)D concentrations with increasing latitude [22]. However, despite significant inverse association of latitude and circulating calcidiol in Iranian children, the prevalence of VDD even in low latitudes still was remarkable, supporting other reports that living in sunny regions does not necessarily protect children from VDD [23]. The efficiency of sunlight for dermal vitamin D synthesis during winter in areas between 18 and 34° latitude has been evaluated years ago [24]. However, a meta-regression analysis failed to show the influence of latitude on circulating 25(OH)D globally, but reported a decrement in 25(OH)D with increasing latitude just in Caucasians [25]. Lower risk of hip fracture [26], MS [27], tuberculosis [28], childhood food allergy and anaphylaxis [29], eczema [30] and lymphomas [31] in low latitude areas has been reported and related to ultraviolet B exposure and vitamin D status.
We found undesirable vitamin D status more prevalent among girls compared with boys, confirming the previous reports from children and adolescents living in Tehran [13, 14]. However, in some studies, VDD was found more prevalent among males than in females [32] and in some others no association was found with gender at all [33]. Higher occurrence of VDD in females may be because of their lesser sun exposure because of type of clothing. This can especially be the case in Islamic populations wherein females from the age 9 years are veiled. The other explanation could be higher body FM in females. The already reported negative association of FM and vitamin D status [34] might be resulted from decreased bioavailability of vitamin D because of its deposition in body fat [7]. Along the same line of evidence, we found that overweight/obesity was negatively correlated with serum 25(OH)D. Several studies have demonstrated that increased body FM per se can be an independent risk factor for poor vitamin D status in children [35], and there is also evidence that decreased circulating 25(OH)D could be related to the increment in BMI during childhood [36].
Our data demonstrated that the circulating 25(OH)D decreased when the girls (but not boys) became older. The negative association of age and serum 25(OH)D in children has been already reported [17], which may be because of the difference in duration of sun exposure. A recent study on fair-skinned women reported that only 30% variation in vitamin D status can be explained by sun exposure duration [37]. Nevertheless, we did not find any significant difference in duration of sun exposure between younger (<9 years) and older (≥ 9 years) girls. The change in serum calcidiol concentrations in our subjects might be because of the type of clothing after the age of 9 years and accumulation of more body fat as the child becomes older.
Higher prevalence of hypertriglyceridemia as well as suboptimal HDL-C in the children living in latitudes above 33° is noteworthy. A recent meta-analysis reported the association of desirable vitamin D status and favorable blood lipid profile in Iranian children [38]. However, the actual effect of improvement of vitamin D status on blood lipids is still controversial and needs more large-scale clinical trials [39].
One of the limitations of this study was that only children in urban areas were enrolled, and children in rural areas, where there may be more outdoor activities, were not studied. However, data from two national reports on micronutrients with a 10 year interval [National Investigation of Micronutrients Status (NIMS) I, 2002 and NIMS II, 2012] demonstrated that VDD is as much problematic in rural areas as in urban ones. The effect of skin pigmentation on circulating calcidiol was not evaluated either.
CONCLUSION
Despite significant association of latitude and vitamin D status, VDD is prevalent across latitudinal gradient in Iranian children. Our findings warrant immediate sustainable nutritional interventions, including supplementation, to protect children from VDD irrespective of the latitude of their residence.
ACKNOWLEDGEMENTS
All laboratory bench works were performed at the Laboratory of Nutrition Research, NNFTTRI. We wish to thank the children and their parents for taking part in this project. We also appreciate our provincial contributors, their teams and the provincial deputies of health for their assistance, especially: Somayeh Asghari, Fariba Babai, Fariboz Bojdi, Mostafa Hosseini, Razieh Shenavar, Mahnoosh Sahebdel, Ma asoomeh Moradi, Sekine Noori.
FUNDING
The National Food and Nutrition Surveillance (NFNS) is funded by UNICEF, Community Nutrition Office of the Iran Ministry of Health, and NNFTRI in order of their financial contributions.
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