In our study, vitamin D insufficiency and deficiency were found to be significantly higher in asthmatic children than in the control group, and these values were higher than values reported in other studies [6, 8, 11, 12, 14]. Vitamin D levels are classified as sufficient (≥ 30 ng/ml), insufficient (20–29 ng/mL) and deficient (< 20 ng/ml); and in our study these accounted for 9.5%, 20%, and 67% of the subjects (Figure 1), respectively. Studies conducted in Italy on children with a mean age close to those we studied,  and in the Middle East  showed similar results, whereas the percentages found in a study in Iran were respectively higher (84%, 12% and 4%) . Similar studies from another countries revealed variable results that can be explained by ethnicity, skin colour and maternal supplementation during pregnancy [6, 8, 11–13, 22, 23]. Examples of these discrepancies include residence in an urban environment and dark-skin.
The primary source of vitamin D is from the skin’s production upon exposure to sunlight; secondary sources are eating vitamin D-rich and vitamin D enriched foods . Our study showed that asthmatic children had less exposure to sunlight compared to control group (p < 0.001). This is explained by the fact that families wanted to keep their children at home for fear of an asthma attack if they went outdoors or engaged in physical activities. Bener et al. found similar results (p = 0.006) . In another study, no significant difference was found between vitamin D levels and their asthmatic children’s approximate three hours of exposure to sunlight per day (p = 0.49); however, the children in that study were dark-skinned and lived in an urban environment . In our study, finding no significant difference between exposure to sunlight and vitamin D levels led us to think that there were other factors and mechanisms determining serum vitamin D levels in asthmatic patients. A vitamin D insufficient diet was found significantly more often in the asthmatic group than in the healthy control group (p < 0.001). The fear of allergic reaction against diet enriched with vitamin D might be reason in this area.
In Turkey, vitamin D-rich foods are widely available but few people are familiar with this fact. We conclude that families need to be better informed about nutrition. Our findings were similar to a study from Puerto Rico, which examined the vitamin D levels in “high dietary intake of vitamin D” group and an asthmatic group .
Vitamin D supplements, which were given to the children during breastfeeding, did not make any significant difference in the serum vitamin D levels of the asthmatic and control groups. As a similar result, Bener et al. reported that there was no significant difference (p = 0.561) between asthmatic and control groups when the vitamin D supplements were given to the children during breastfeeding .
It is considered to be appropriate to increase the dosage of vitamin D and duration of vitamin D intake in Turkey (current vitamin D supplement level is 400 IU), according to the latest report of The Institute of Medicine of the National Academies in the United States of America. The vitamin D currently added to foods and the use of vitamin D as a replacement therapy have been shown not to eliminate vitamin D insufficiency  or vitamin D deficiency [25, 26]. The Institute of Medicine of the National Academies in the United States of America, in its latest report of 2011, recommended increasing the nutritional dose of vitamin D from 400 to 600 IU for children older than one year of age . We concur that it is necessary to increase the vitamin D dose, the period of exposure to sunlight, and the time of exposure to sunlight (preferably around midday). Equally, we must consider protective clothing, the use of sun blocks and the adverse effects of UVB in certain geographical regions. Further, we think that the relationship between vitamin D deficiency and asthma should be reinvestigated, taking into consideration the social eating habits, the dose and the duration of vitamin D supplementation.
When analysing the RTIs of the previous year, we observed that as vitamin D levels decreased, RTI frequency increased in both the asthmatic and the control groups, and there were frequent incidences of RTI in asthmatic patients with low vitamin D levels , leading to an increase in the severity of the asthma attack . The number of hospital admissions due to respiratory complaints increased as serum vitamin D decreased in the asthmatic group. Although some studies had similar results to ours [6, 12, 14], a study by Alyasin et al. found no relationship between vitamin D levels and hospitalizations . A negative correlation was found between serum vitamin D level and asthma attacks, which increased significantly in frequency as serum vitamin D decreased. Recent studies showed that vitamin D deficiency led to an increase in the risk of asthma attack [6, 14, 16]. Some studies indicated an inverse relationship between vitamin D levels and use of health services [6, 11, 12], while others showed no such relationship [9, 22].
In our study, the increase in IgE and eosinophil count in asthma patients (Table 1), RTI, asthma attacks, EU admissions and number of hospitalizations for treatment were observed in relation to vitamin D levels. This led us to believe that vitamin D deficiency increased the severity of asthma, complicating control of the disease. The relationship between vitamin D deficiency and severity of asthma has been investigated and it was reported that the severity of asthma increased with vitamin D deficiency [6, 8, 9, 11, 28]. The relationship between vitamin D concentration and control of asthma was investigated in some studies conducted generally in North American and Costa Rican populations. In these studies, the subjects belonged to certain ethnic groups and were dark-skinned, urban schoolchildren, and they reported that D vitamin insufficiency was associated with less time spent outdoors, increased total IgE concentrations, eosinophil counts, airway hyper-responsiveness, and increased symptoms and exacerbations [11, 28]. We should also bear in mind that there could be a decrease in vitamin D levels in children who go outdoors less, owing to asthma attack exacerbation and poor control of the illness.
Our study had some limitations. First, the sample size was relatively small (n = 170). A larger sample size would have increased our statistical power to detect associations. As asthma diagnosis can only be achieved after the age of six due to difficulties with applying spirometers and measurement of exhaled nitric oxide, our diagnoses mostly rely on physicians’ experiences [29, 30]. As with most other studies investigating the role of vitamin D in asthma, our design was cross-sectional, thus limiting our ability to establish a causal link between vitamin D and asthma morbidity. Future clinical trials are necessary to determine if vitamin D truly has effects upon asthma as suggested by the observational literature.