VDD is a significant public health concern in Bulgaria, with a notable portion of the population affected. The general Bulgarian population is estimated to have a 21.3% prevalence of vitamin D deficiency, which is defined as serum 25(OH)D levels below 25 nmol/L. Additionally, 54.5% of the population experiences vitamin D insufficiency, with levels between 25–50 nmol/L, while only 24.2% have sufficient levels above 50 nmol/L.^[5] Globally, VDD affects a substantial portion of the population. A comprehensive analysis of 7.9 million participants from 81 countries found that 15.7% had severe VDD (<30 nmol/L), 47.9% had moderate deficiency (<50 nmol/L), and 76.6% had insufficient levels (<75 nmol/L).^[5] The prevalence of VDD is notably higher in regions with high latitudes and during winter-spring seasons, reflecting limited sunlight exposure.^[6]

Neurodevelopmental disorders like cerebral palsy (CP) and autism spectrum disorder (ASD) are significantly impacted by VDD. Research indicates that developmental VDD is a risk factor for autism, causing behavioral changes and gut health issues in animal models.^{[7, 8]} VDD during pregnancy may contribute to ASD by causing structural and functional nervous system abnormalities, potentially affecting speech development.^[9]

The prevalence of VDD and insufficiency in children with CP and ASD is significantly high worldwide, with studies consistently reporting a substantial portion of these populations being affected. Studies in India^[10] and around the world^[11] have shown that the prevalence of VDD in children with cerebral palsy varies between 32 and 42 percent. Additional findings highlight that 61% of children with CP in India experience vitamin D insufficiency.^[10] Factors contributing to this high prevalence include limited sunlight exposure, feeding difficulties, and the use of antiepileptic drugs.^[10] Motor impairments, as measured by the Gross Motor Function Classification System (GMFCS), are also significantly correlated with lower vitamin D levels.^[12]

Similarly, children with ASD exhibit a high prevalence of VDD, with studies reporting rates of up to 57%.^[13] A systematic review and meta-analysis encompassing 870 ASD patients found significantly lower serum 25(OH)D levels in individuals with ASD compared to healthy controls.^[14] Low vitamin D levels have been linked to altered brain development, immune system dysregulation, and imbalances in neurotransmitter function, all of which are common pathological features of ASD.^[15–17] Although some studies suggest that vitamin D supplementation may improve behavioral symptoms in children with ASD, findings remain inconsistent, necessitating further investigation.^{[13, 17]}

Cerebral palsy (CP) and autism spectrum disorder (ASD) are two of the most prevalent neurodevelopmental disorders, each with unique etiological and clinical characteristics. CP often involves significant physical disabilities that restrict mobility and sunlight exposure, predisposing individuals to a higher risk of vitamin D deficiency (VDD). In contrast, ASD is characterized by behavioral challenges, sensory sensitivities, and dietary selectivity, which can also influence vitamin D status. These distinctions make CP and ASD ideal conditions for exploring the interplay of biological and social factors contributing to VDD.

Examining VDD in these two populations allows for a deeper understanding of whether biological factors, such as mobility limitations in CP or dietary selectivity in ASD, or shared social factors, such as limited healthcare access, social stigma, or nutritional deficits, play a more significant role. Identifying these determinants is critical for developing targeted interventions to address vitamin D insufficiency and improve health outcomes in these vulnerable groups.

Participants included children with cerebral palsy or autism, recruited through the Karin Dom Foundation in Varna and the Equilibrium Association in Ruse. Parents were contacted via phone or email, with initial screenings and intake interviews conducted to determine eligibility. Informed consent was obtained, ensuring confidentiality.

Data were securely and anonymously collected. Descriptive statistics summarized demographic data. Independent t-tests and chi-square tests compared continuous and categorical variables, respectively, with statistical significance set at p<0.05.

The study included 95 children: 59 with cerebral palsy (CP) and 36 with autism. Significant gender differences were observed, with 49.2% of the CP group being male compared to 83.3% in the autism group (p<0.001). Ethnicity also varied significantly, with 83.3% of the autism group being Bulgarian compared to 60.3% in the CP group (p=0.006). No significant differences were found in the education levels of the parents between the two groups (Table 1).

Vitamin D levels in children with CP and autism are summarized below. Among the CP group, 29.4% were deficient, 17.6% insufficient, and 52.9% had normal levels. In the autism group, 15.8% were deficient, 26.3% insufficient, and 57.9% had normal levels. There were no significant differences in vitamin D status between the groups (p=0.492) (Table 2).

Children with VDD had significantly lower height-for-age Z scores (HAZ) (−3.22±2.47) compared to those with normal or insufficient levels (−0.99±1.89) (p=0.002). Weight-for-age Z scores (WAZ) were also lower in the deficient group (−2.99±2.72) compared to the normal/insufficient group (−1.25±1.83) (p=0.021). Although body mass index-for-age Z scores (BMIAZ) were lower in the deficient group, this difference was not statistically significant (p=0.067).

Biochemical tests revealed similar hemoglobin levels across groups, but the vitamin D deficient group had significantly lower lymphocyte counts (1.26±1.25×10⁹/L vs. 4.38±3.18×10⁹/L, p=0.011) and lower total iron-binding capacity (TIBC) (38.79±35.13 µmol/L vs. 60.09±14.03 µmol/L, p=0.018). Serum iron levels were lower in the deficient group, approaching significance (p=0.060). Additionally, albumin levels were significantly lower in the deficient group (43.33±3.79 g/L) compared to the normal/insufficient group (47.04±2.86 g/L) (p<0.001). No significant differences were found in ferritin, total protein, calcium, or magnesium levels (Table 3).

These results highlight significant differences in demographic characteristics, vitamin D status, and several biochemical and anthropometric measures between children with low and normal vitamin D levels, underscoring the need for tailored nutritional and medical interventions in these populations.

This study is the first of its kind to examine VDD in children with ASD and CP in Bulgaria. Although most of the research that has been done so far has concentrated on populations that live in extreme climates, such as those that are located in very high or very low latitudes, there is a noticeable scarcity of data from countries with moderate climates, like Bulgaria. The findings from our study not only provide valuable insights into the prevalence and characteristics of VDD in these neurodevelopmental conditions but also contribute to the global understanding of the interplay between biological and socio-environmental factors in moderate climate regions.

The observed gender distribution aligns with existing literature, which shows that ASD is significantly more common in males, with a typical male-to-female ratio of 4:1.^[1] In this study, 83.3% of the children with autism were male, compared to 49.2% in the CP group. This gender disparity in autism may be attributed to biological factors, such as sex-linked genetic susceptibility, as well as diagnostic biases that under-identify autism in females with subtler symptoms.^{[1, 18]} The relatively balanced gender distribution in CP reflects its lack of a gender-based predisposition.^[3] These findings underscore the necessity of gender-specific diagnostic and therapeutic strategies, particularly for autism, to address these unique health disparities.

Ethnic differences were notable, with 83.3% of the autism group being Bulgarian compared to 60.3% in the CP group. Conversely, children from other ethnic backgrounds (Roma, Turkish, and others) were more prevalent in the CP group. These variations likely stem from a combination of genetic predispositions, socio-economic factors, and disparities in access to healthcare services. Prior research has suggested that ethnic minority groups may face barriers to early diagnosis and specialized interventions, particularly for autism, due to cultural stigma and limited resources.^{[5, 19]} Understanding these ethnic disparities is critical for designing equitable and culturally sensitive healthcare strategies.

Parental education levels showed no significant differences between the CP and autism groups in this study. While previous research has highlighted the influence of parental education on access to healthcare and intervention outcomes, the lack of disparity in this study may reflect relatively equal access to care in the Bulgarian context.^[3] Nevertheless, parental education remains an important factor to consider in broader settings, as it shapes awareness and advocacy for specialized care.

VDD was observed at comparable rates in children with CP and ASD, suggesting that it is a common and significant issue across these neurodevelopmental disorders. In our study, 29.4% of children with CP and 15.8% of children with ASD were vitamin D deficient. These findings align with prior research reporting high prevalence rates of VDD in neurodevelopmental populations. For instance, severe VDD in youth with ASD has been linked to limited outdoor activity and dietary restrictions.^[4] Similarly, neurodevelopmental disorders like CP have shown strong associations with VDD due to challenges such as reduced mobility, feeding difficulties, and antiepileptic drug use.^[7]

The observed prevalence in our study mirrors findings from India, where 32% of children with CP were vitamin D deficient^[8] and global reports documenting even higher rates (42.18%) of deficiency in children with CP.^[9] For ASD, research indicates rates as high as 57%^[10], significantly higher than in the general population.

The shared prevalence of VDD in both CP and ASD populations underscores the multifaceted challenges these children face. Reduced outdoor exposure, selective eating habits, and chronic medication use are key contributing factors.

Comparatively, the global prevalence of moderate-to-severe VDD is 47.9% and 15.7%, respectively.^[6] The lower prevalence in the ASD group (15.8%) compared to some reports^[10] may reflect regional differences, dietary habits, or supplementation practices in Bulgaria.

Children with VDD exhibited significantly poorer growth outcomes, including lower height-for-age Z scores (HAZ), body mass index-for-age Z scores (BMIAZ), and weight-for-age Z scores (WAZ). These findings are consistent with other studies demonstrating a correlation between VDD and impaired growth in children with neurodevelopmental disorders.^{[8, 11]}

Biochemically, lower lymphocyte counts and serum iron levels were observed in vitamin D-deficient children, highlighting potential immune and nutritional deficits. The significantly lower albumin levels further emphasize the systemic impact of VDD on protein metabolism and immune function.^[20] These findings underscore the need for comprehensive nutritional support to address the multifaceted challenges associated with VDD.

Our study sheds light also on the complex interactions between VDD, iron metabolism, and protein synthesis. Notably, in our cohort, normal ferritin levels alongside low transferrin levels were observed, suggesting that protein synthesis deficits rather than depleted iron stores underlie these findings. Importantly, C-reactive protein (CRP) levels were normal in all participants, ruling out significant inflammation as a confounding factor.

Ferritin, an acute-phase reactant, can remain normal or even elevated in inflammatory states, while transferrin, a negative acute-phase reactant, often decreases.^[21] However, in the absence of inflammation, as indicated by normal CRP levels in our study, the discrepancy between ferritin and transferrin levels likely reflects the selective impact of VDD on protein synthesis. Transferrin, a smaller protein with a molecular weight of approximately 80 kDa, is more sensitive to changes in protein metabolism compared to ferritin, a larger storage protein (450 kDa) capable of binding up to 4500 Fe atoms in its central cavity.

This observation aligns with previous reports where nutritional status rather than inflammation plays a pivotal role in influencing transferrin levels.^[22] Malnutrition, often linked to VDD, affects proteins like transferrin more acutely due to their transport functions, whereas storage proteins like ferritin remain relatively stable. For instance, transferrin levels correlate positively with markers of nutritional status such as albumin, which was also lower in the vitamin D deficient group.

The findings of this study about VDD in children with neurodevelopmental disorders are consistent with other studies conducted worldwide. Studies have consistently reported a high prevalence of VDD in children with ASD and CP, attributing it to factors such as reduced sunlight exposure, dietary restrictions, and antiepileptic drug use.‌^{[4, 7]} While this study observed no significant differences in VDD prevalence between CP and ASD groups, the overall rates underscore the widespread nature of this deficiency among children with neurodevelopmental conditions.

Interestingly, the lack of disparity in vitamin D status between CP and ASD groups contrasts with findings that reported higher deficiency rates in CP due to more severe mobility impairments limiting outdoor activity.^[11] This discrepancy may be explained by regional differences, variations in supplementation practices, or the demographic characteristics of the study population.

The prevalence of ASD in this study shows significant disparities between Bulgarian (Bg) and non-Bulgarian (non-Bg) ethnic groups. Among children with ASD, 83.3% were Bulgarian, compared to 60.3% in the CP group. Conversely, children from non-Bulgarian ethnic backgrounds (e.g., Roma, Turkish, and others) were more prevalent in the CP group, comprising 39.7%. These findings highlight critical sociocultural dynamics influencing the diagnosis and reporting of ASD in Bulgaria.

Cultural attitudes towards autism heavily influence its recognition and diagnosis. Greater societal awareness and better access to diagnostic services among Bulgarian families may explain the higher prevalence of autism in this group. In contrast, minority communities may face cultural stigma, economic challenges, and limited healthcare outreach, all contributing to underreporting or delayed diagnosis.^{[5, 23]}

Freeth et al.^[19] explain that societal attitudes towards autism shape the likelihood of seeking diagnosis and the resources available for intervention. For non-Bulgarian communities, cultural norms often interpret autism-related behaviors as personality traits rather than signs of a disorder. This finding aligns with Montanez^[24], who observed similar patterns in minority populations globally.

Diagnostic frameworks for autism, often rooted in Western behavioral standards, may not fully apply to non-Western or minority populations.^[25] Language barriers and a lack of culturally adapted diagnostic tools in non-Bulgarian communities further delay identification and treatment. Cultural stigma also discourages medical consultation, perpetuating underdiagnosis in these groups.^[26]