In this cross-sectional study, we detected notably lower VD levels in children with DS, with a median value of 31.89 (3.77–102.68) ng/mL compared to controls (56.19 [19.05–173.19] ng/mL). El-Hawary et al. also reported a mean VD level of 30.65 ± 20.64 ng/mL in children with DS [11]. This finding was consistent with studies that found lower levels of VD in DS patients than controls.9–11 Our study showed that 14 out of 80 participants (17.5%) had VDD and 23 of 80 children with DS (28.7%) had VD insufficiency (VDI). In contrast, in the non-DS group, only 2/42 (4.5%) children had VDD and 6/42 had VDI. VD sufficiency in our study was much higher than in previously reported studies, both in the DS population.10,11 and in healthy Indonesian children.23,24 This difference may be due to the different cut-off values used to determine VDD. The lower level of VD in DS children is thought to be due to spending more time indoors and being less physically active.10 Our study also found that children with DS were significantly less exposed to sunlight and consumed less meat and milk than those in the non-DS group.
This study revealed that IFN-γ levels were descriptively higher in the DS group compared with the non-DS group; unfortunately, this difference was not significant. Several studies reported higher serum levels of IFN-γ in DS patients than controls.27–29 It is well known that four of the six interferon receptor (IFN-R) genes reside on the distal portion of the long arm of Chr21; thus trisomy-21 cells are more sensitive to the effects of IFN-γ30 in terms of mild interferonopathy.31 IFN-γ has been shown to have a prominent role in human systemic autoimmune responses. The excessive expression of inflammatory markers such as IFN-γ is considered to play key roles in inflammatory responses and is related with diseases such as Alzheimer’s dementia (AD).29 tumors.32 and some autoimmune diseases such as celiac disease, T1DM, and thyroid dysfunction, which are more pronounced in the DS population than the non-DS population.31 Studies building on RNA, protein, and functional data that provide insight into how IFN signaling contributes to the distinct characteristics of DS reveal potential pathways to treat the predominant diseases in children with DS.32 In a mouse model of DS, both anti-IFN therapy and genetic methods of reducing the number of IFN-Rs on the surface of cells were shown to improve growth and brain development.33 A case report on two trisomy-21 patients suffering from refractory HLH showed successful treatment with a gamma anti-IFN regimen. Since autoimmunity and HLH in trisomy 21 are thought to be driven by IFN- γ, targeting interferon signaling may have potential therapeutic benefits.34
This study showed a significant role of high VD levels in decreasing IFN-γ (p-value 0.039, R2 5.8%). VD plays an important role in regulating T helper cells as well as in the secretion of IFN-γ.35,36 VD inhibits the production of pro-inflammatory cytokines produced by Th1, such as IFN-γ, and induces the production of anti-inflammatory cytokines in cell culture without being affected by baseline VD status before supplementation.37 Another in vitro study reported that VD in the form of 1,25(OH)2D3 with concentrations of 1000 pmol/L was able to inhibit the production of IFN-γ, tumor necrosis factor (TNF)-𝛼, IL-6, IL-2, and IL-10 in whole blood culture38, while another study showed that VD did not reduce the concentration of IFN-γ produced by CD4 + T cells but reduced cytokine production from Th1 cells and differentiation into Th1 cells.39 Not many studies in humans have shown the effect of VD on IFN-γ levels. One study in patients with an autoimmune disease, namely systemic lupus erythematosus (SLE), reported that IFN-γ levels were 150% higher in patients who were deficient in VD compared to patients with higher levels of VD and the control group.40
Considering the importance of ensuring adequate VD levels in children with DS in terms of their ability to reduce IFN-γ levels, we evaluated several factors that could potentially influence the adequacy of VD levels in this group. In this study, younger age (less than 18 months) was a protective factor in univariate analysis. This result was in line with a large cohort study including 9795 children aged 0–12 years in northeast China, which reported that the highest mean level of serum VD was found at the 1–3-year stage and the lowest at the 6–12-year stage.41 Higher VD levels in infants and toddlers are thought to be due to their receiving breast milk and various types of oral supplements containing vitamins D and A in water or oil droplets; however, when children reach preschool and school age, parents pay less attention to supplementation because they think children’s growth has slowed down.42
This study also found that daily milk consumption of more than 500 cc/day was a consistent contributing factor for VD sufficiency in both univariate and multivariate tests in the DS group. The relationship between cow’s milk consumption and VD levels was assessed by a large study in Canada of 1311 children, showing that increasing cow’s milk consumption was associated with increasing 25-hydroxyvitamin D (p ≤ 0.0001). This study reported that two cups (500 mL) of cow’s milk per day maintained 25-hydroxyvitamin D > 75 nmol/L.43 Meanwhile, another study found modest associations between mean serum 25(OH)D concentrations and fresh milk consumption in children (r = 0.11; especially in girls (r = 0.12)44 and no significant correlation between dairy milk intake and VD levels in Indonesian children.39 Unfortunately, many children in the South East Asia (SEA) region consume < 1% of the recommended daily VD intake [45, 46]; this is consistent with our findings showing that the median consumption of milk was only 300 cc/day in the DS group and 380 cc/day in the non-DS group.
These data reflect the need for implementing education and monitoring of lifestyles that support VD adequacy in children with DS to reduce the risk of autoimmunity and other diseases caused by dysregulation of the immune system through the role of VD in reducing IFN-γ levels. However, this is the first study to explore vitamin levels in children with DS in Indonesia, which is a country rich in year-round sunshine but with a low daily intake of naturally VD-rich food. This study can be the basis for public health policies encouraging parents to meet the VD needs of children with DS by providing a diet with sufficient VD content, such as milk, to improve prognosis. However, this study has several weaknesses: 1. The cross-sectional design with a single-center approach cannot represent the condition of children with DS as a whole; 2. Several confounding variables that might play a role in influencing VD levels were not evaluated in more detail in this study.