Prolonged Vitamin D deficiency leads to metabolic bone diseases like osteoporosis and osteomalacia, its role in autoimmune diseases such as type 1diabetes mellitus(11), SLE (12), multiple sclerosis (13) and malignancy has been confirmed (14). Vitamin D enrich diet is necessary whenever sun exposure is not adequate (15). So vitamin D fortified foods have been used in different countries yearly (16, 17). Because of higher rate of growth velocity in youth, their micronutrients requirement such as vitamin D is higher too. A routine balanced diet may provide all necessitated nutrients except vitamin D.
Iranian diet has no significant source of vitamin D and food products didn’t fortified with vitamin D (18). Hence, the role of vitamin D in growth of Iranian teens hasn’t been explored ,this study was conducted to investigate the efficacy of vitamin D supplements on anthropometric indices in sixth grade school girls of Jahrom ; a city located at south west of Fars province.
The results showed that the prevalence of vitamin D deficiency among participants was 95%, which is higher than previous studies. Saki et al. (19), Alizadeh et al.(20) and Faraji et al. (21) also reported the prevalence of vitamin deficiency in their study as 81.3%, 70% and 68% respectively indicating a high prevalence of vitamin D deficiency in this age range. Even though Jahrom is located in the south west of Iran where the weather is almost always warm and sunny ,blood sampling for this study was performed in September and October to reduce the bias of low sun exposure in winter,nevertheless the prevalence of vitamin D deficiency was still high. Khashayar et al. (22) also observed that there is a weak linear relationship between the increase in body mass index and vitamin D levels. Another study, based on data from 21 cohort studies with 42024 adult participants by Vimaleswaran et al., revealed that an increase in BMI is associated with an increased incidence of vitamin D deficiency, but a low level of vitamin D has little effect on an increase in body mass index (23).
However, contrary to the results of this study, Motlaghzadeh et al.(24) observed that serum vitamin D level had a significant correlation with body mass index before and after treatment. Sakki et al. also observed that serum levels of (OH) D 25 have a significant reverse correlation with body mass index and puberty, and exposure to sunlight has a significant direct relationship (18). Najarzadeh et al. (25) also did not explain significant differences in the anthropometric indices after 3 months of supplementation with vitamin D (mean BMI, BMI, waist to hip ratio). Bonakdaranet al. also perceived that vitamin D deficient patients had a clear difference in body mass index (p = 0.003) compared to patients with normal levels of vitamin D. in other words, There is a reverse and significant relationship between serum vitamin D levels and body mass index(26). In the study of McGill et al., There was a significant and inverse relationship between the weight, body mass index and abdominal circumference (27).
However, in the study of Geng S et al.(28), there was no significant relationship between the levels of (OH) D 25 and the anthropometric indexes (body mass index). Khor et al. observed that there was a significant correlation between serum vitamin D level and body mass index (p = .16) among boys (p = .16) (9).
However, according to cross sectional studies’ limitations, confirmation of cause and effect is not clear. Due to obesity, vitamin D is absorbed and seized in excess adipose tissue which causes decreasing the availability of vitamin D, leading to reduction in serum vitamin D levels. On the other hand, obesity and vitamin D deficiency can both happen due to low exposure to sunlight due to the lack of physical activity outside of the home. Another hypothesis is that vitamin D deficiency increases the risk of becoming overweight.
It was also observed that there was no significant difference in waist circumference between the two groups after intervention (P = 0.6). Significant differences in waist circumferences of cases and controls were observed before and after intervention (P = 0.001). However, since observed differences remained significant in both groups, it cannot be attributed to the effect of vitamin D.
Najarzadeh et al. also observed that the mean waist circumference in the treatment group was significantly decreased throughout the study period (p = 0.05) (24). In another study, Faraji et al. also found that there are a reverse and significant relationship between serum vitamin D levels with waist circumference (p < 0.02), height (p < 0.001) and Waist-hip ratio(p < 0.007), but has not significantly changed with hip circumference and BMI variables(20). The results of Al-Mulhim et al. study, showed that waist circumference in the intervention group significantly decreased compared to the pre-treatment group However, no significant differences were observed in other anthropometric indices after intervention. Vitamin D supplementation decreased women’s waist circumference significantly while not affecting other anthropometric indices (29).
N. Phetkrajaysang et al. also observed that increased waist circumference could increase the risk of vitamin D deficiency (7). In another study, GILBERT-DIAMOND et al. showed that serum vitamin D levels have a significant and reverse relationship with the waist circumference and body mass index. Thus, it can be concluded that the serum level of vitamin D has a significant and reverse relationship with obesity in children at school age (10). In another study, Gonca Tamer et al. showed that body mass indexes, waist circumference, and hip circumference, the rate of obesity and abdominal obesity were significantly lower in vitamin D sufficient subjects. It was also observed that the Serum (OH) D 25 level had significant reverse relationship with body mass index (r = -0.481, p < 0.0001), waist circumference (r = -0.480, p < 0.0001), and Waist-hip ratio (8).
Other results of the present study showed that there is a significant and reverse relationship between height and serum vitamin D levels (r = -0.266), and also the height index after intervention in the two groups did not significantly change (P = 0.55), which indicates that the intervention was not effective during this time interval on the height index. Also, intra-group comparisons showed a significant difference between measured height before and after the treatment in both groups (p = 0.001) which as we mentioned, it cannot be attributed to vitamin D.
Khalaji et al. observed that serum levels of D3 (OH) 25 had a significant and direct relation with exposure to sunlight, but had a significant and reverse relation with weight, height, and body mass index. Therefore, the high prevalence of vitamin D deficiency in Primary school children (especially girls) requires urgent intervention and appropriate nutritional support (30). Other results of this study also showed that the prevalence of vitamin D deficiency among tall children is about 61%. Considering the short stature growth disorder in the case group compared to the control group, it seems that the eight weeks follow up is not sufficient for evaluation of vitamin D effects; so we suggest further studies with prolonged follow ups for better observation.
Other results of the present study showed that there is a reverse, and significant relationship between weight and serum vitamin D levels (r = -0.205 and p = 0.001), and also the weight index after intervention in both groups was significantly different (P = 0.61), which indicates that the intervention is not affected the weight. There is a significant difference in measured weight between before and after the treatment results in the control group (P = 0.001). Also in the case group, the difference in weight was significant before and after treatment (P = 0.001). So the observed difference cannot be as a result of vitamin D treatment. To consider the ethical issue, after the study time, the control group received the vitamin d supplement as it was helpful.
E Rodríguez-Rodríguez et al. also showed that only weight and body mass index could be dependent on the level of vitamin D, and it was also observed that children with higher BMI, weight, and waist circumference have a higher risk of vitamin D deficiency. Therefore, it can be concluded that BMI and abdominal obesity have a significant effect on vitamin D deficiency in children (31).