The prevalence of vitamin D deficiency in this study was high, at 22% that is, 1 in 5 infants had VDD. Only 2/3 of the study population was found to have optimal level of serum vitamin D. The findings can be explained by the Arusha weather, which is cold several months of the year with temperature ranging from 14.5 ° C to 19° C(7). With this cold weather, most infants are kept indoors and are excessively swaddled with less sun exposure which predisposes them to vitamin D deficiency(7, 8). However, in this study most parents reported adequate sun exposure, but it was not objectively assessed. Susanna et al was able to associate cold weather with Vitamin D deficiency(11). Majority of the infants in our study population were dark skinned, which pigmentation can hinder the conversion of vitamin D by ultra violet light in the skin. Ann Prentice et al was able to associate Vitamin D deficiency and dark skin pigmentation(12).
The prevalence obtained in our study is similar to other studies carried out in near by regions (Dar es Salaam) Tanzania and (Nairobi) Kenya. This can be explained by the fact that they had similar study population, similar weather characteristics and socioeconomic status(4, 13). The prevalence of Vitamin D deficiency among infants was ranging from 21.2% to 83.7% in various studies done in East Africa (4, 5, 13, 14).
A similar finding in a study done in Nairobi, Kenya by Nusrat on infants aged ≤ 6 month and on exclusive breastfeeding is perhaps related to nearly similar weather characteristics as the one in our study site. Nairobi is located 1724 metres above sea level slightly higher than Arusha, which is 1400 metres above sea level(15). Both Nairobi and Arusha lie on the slopes of key mountains; mountain Kenya and Meru respectively(15, 16).
The discrepancy in these results compared to other studies, which found high prevalence of VDD, may be explained by age differences of study participants. Our Study had infants aged 6 week to one year, while the other studies included children less than 6 months only. In addition, the observed differences could be attributed to the fact that the studies were conducted at different times of the year and therefore the weather seasons could have been different(5). Furthermore, ALMC tends to have a more urban population that is potentially less likely to have high rates of malnutrition, a risk factor for vitamin D deficiency.
Surprisingly the prevalence obtained in our study is much lower than that in a study done by Nalunkuma et al in Uganda where it is sunny throughout the year and the country is at a lower altitude than my study site. This can be explained by the fact that Nalunkuma’s study used a different laboratory assay from the one I used in my study, and also her study included only children less than 6 months.
While the observed prevalence of vitamin D deficiency in my study was high, at 22%, it was less than what was expected. This can be explained by the dry season in which the study was done (this was November to January) as well as the fact that most of the infants that participated in the study were born during dry and hot seasons. During this time sun-exposure may be sufficient as a source of vitamin D. however the findings obtained were similar to those in studies done in the nearby regions.
In this study the factors that were significantly associated with VDD among the study participants included age of the infants, clinical signs of rickets and hypocalcemia. Infants who are older than 6 months were less likely to have VDD. This could be explained by the increase in physical activity above 6 months of age such as crawling that may increase the chances of sun exposure, but also they often will have started complementary feeds which are more likely to be an additional source of vitamin D. Ziegler et al also observed low rates of vitamin D deficiency among older children(17).
The commonly used complementary feeds among these infants included; eggs, porridge with vitamin D containing margarine and fish oils. The commonly used formula milk was Lactogen and NAN both of which contain adequate Vitamin D levels for daily intake(18). These could be the other reasons to explain why infants on complimentary milk and formula milk were less likely to develop VDD. Other studies reported similar findings. Gordon CM et al in a study among infants on breastfeeding had more than 10 fold increases risk of VDD compared to those who were on exclusive formula feeding(11).
Younger infants appeared more likely to have Vitamin D deficiency and may be explained by significant number of them being exclusively breastfed(17). Breast milk has low level of vitamin D yet mothers in Arusha are not supplemented with Vitamin D and plausibly they could also be low in Vitamin D. Said N et al made similar observations associating vitamin D deficiency with age less than six month and exclusive breastfeeding(13).
In addition, younger infants have less cutaneous synthesis of vitamin D and are likely to have received minimal transplacental transfer of vitamin D. Gordon C, Sudfeld C, and Nalunkuma Cissy were also able to associate vitamin D deficiency with age(4, 5, 11).
In our study, only 2 participants less than 6 months were on formula milk. This made it difficult to assess, if formula milk reduces the risk of vitamin D deficiency because of the inadequate sample size. However Gordon et al reported infants on fortified formula milk to have adequate vitamin D level thereby highlighting its protective effect on VDD(11).
Infants with signs of rickets usually tend to have a low level of Vitamin < 10 ng/ml. Therefore finding in our study that infants who had signs of rickets also had VDD is not surprising. These signs could have arisen as evidence rather than a cause of VDD. Holick MF et al made similar observations Holick MF (19). Resurrection of vitamin D deficiency and rickets.
In our study, there were few participants (11 infants) that had signs of rickets. However, it is important to note that these signs were picked only on clinical examination and no imaging was done, however those patients who found to have vitamin D deficiency were referred to the Paediatrics clinic where imaging was done. The finding of few patients only 11 with features of rickets is similar to that in a study done by Piloya et al(20).
In this study, laboratory investigation such as serum calcium concentration, serum phosphate, and serum alkaline phosphate were done; only the serum calcium level association with VDD was statistically significant associated with VDD. This can be explained by the role of vitamin D in the metabolism of calcium; vitamin D helps maintain calcium homeostasis by increasing absorption of ionized calcium in the intestines and in bone resorption. Thus, the major physiologic function of vitamin D is to maintain serum calcium and phosphorus levels within the normal physiologic range. Therefore low vitamin D Level can result in hypocalcemia. A study by Sukru Hatun et al also made similar observations between vitamin D deficiency and hypocalcemia among a cohort of Turkish children(21).
Strength of the study
This is the first study in Tanzania that measured serum vitamin D level in children aged 6 weeks to 12 months who attended RCH clinic for follow up so providing a baseline on the prevalence of vitamin D deficiency. This study was conducted at the RCH clinic in which services are free. The laboratory analysis used (electrochemilumniscense) has a good sensitivity and abroad dynamic range(22).
Limitations of the study
We obtained Information from the caregivers, which may be subject for recall and information bias. In addition, the cross-sectional study design applied may not exhaustively establish the causality. Equally we were unable to objectively assess the children’s exposure to sunshine since their responses were subjective. We also did not assess the effect of dietary intake of vitamin D, and or measure the PTH levels due to the difficulty in ascertaining the vitamin D and calcium levels in the diet, and financial limitations for the above two issue respectively.