Vitamin D is a crucial fat-soluble vitamin essential for children’s growth and long-term health. It aids in calcium and phosphorus metabolism, bone development, and various physiological functions, including immune response and cellular differentiation [1, 2]. Recently, vitamin D deficiency (VDD) has become a significant global health issue, contributing to diseases in children and adolescents such as dental caries, asthma, delayed growth, and rickets, while also increasing the risk of hip fractures and overall mortality later in life, thereby severely impacting quality of life [3–5].

Globally, the issue of VDD received widespread attention. Although regions with ample sunlight are typically considered to have a lower risk of VDD, studies show that the prevalence among children is still alarmingly high [6, 7]. Additionally, the VDD prevalence varies by geographic region, with about 15% of U.S. children aged 1–11 years [8] and 25.44% among children aged 6–18 years in India [9]. VDD is particularly common in countries where foods lack vitamin D fortification, such as Germany (45.6%), Australia (31%), Canada (61%), and Iran (81%) [4, 10, 11]. In China, a national cross-sectional study involving 10,696 children and teenagers aged 6–18 years from northern cities (Beijing, Changchun, Jinan, and Yinchuan) and southern cities (Shanghai, Chongqing, and Chengdu) found that 80% had VDD (25[OH]D < 50 nmol/L) [12]. A nationwide study reported VDD in 8.9% of 1,435 children aged 3–5 years [13].

The epidemiology of VDD is affected by various factors, including dark skin, southern latitude, limited sunlight exposure, impaired vitamin D metabolism, maternal VDD during pregnancy [14], and prolonged breastfeeding without supplementation [15]. Dietary habits, sunlight exposure, and socioeconomic factors related to physical activity are key contributors to VDD in children [16, 17]. Notably, VDD is prevalent among obese individuals [18], who may have lower dietary vitamin D intake and reduced sunlight exposure. Research suggests that adipose tissue acts as a primary storage site for vitamin D, leading to decreased bioavailability due to sequestration in excess fat [19]. Sunlight exposure and breast milk are primary natural sources of vitamin D for newborns [20]. Unfortunately, breast milk does not provide sufficient vitamin D, prompting experts to recommend supplementation within the first week of life to prevent rickets [21]. VDD has also been observed in sun-rich Asian countries like Malaysia, Indonesia, Thailand, and Vietnam, as well as in tropical Colombia [22]. Large-scale studies are needed to further investigate VDD status in tropical regions with high ultraviolet B (UV-B) radiation. Implementing targeted preventive and intervention strategies for VDD in these populations is essential.

Hainan Province, the largest tropical island in the South China Sea, is distinct due to its tropical monsoonal climate, geographical features, and diverse ethnicities [23]. However, there is limited data on vitamin D status and VDD prevalence and its related factirs in the region. To address this knowledge gap, we analyzed data across a broader age range to assess vitamin D status and VDD with strata of factors such as age and seasons in children in southern China.

This cross-sectional study found that lower serum levels of total vitamin D, as well as vitamins D2 and D3, were more common among older adolescents compared to young children, corresponding with a higher prevalence of VDD in older adolescents. High-risk groups for low vitamin D and elevated VDD included girls, Han individuals, urban residents, those with obesity, those breastfed for over 6 months, and families with annual incomes over 100,000 yuan. Lower vitamin D levels were mainly seen in humid and subhumid zones, while VDD prevalence was highest in semiarid and subhumid regions.

Numerous studies have shown that VDD is common among children and adolescents, particularly in older teens [25]. A multi-center study in Hainan Province found the highest serum 25(OH)D levels in the 0–1 year group and the lowest in the 14–18 years group [26]. Similarly, a cross-sectional study of 1,510 adolescents (12–18 years) from the Korea National Health and Nutrition Examination Survey (KNHANES) 2008–2009 reported an age-related decline in 25(OH)D [27]. Notably, similar results were also observed in South Africa, which possesses a tropical climate. Newborns exhibit the lowest prevalence of VDD, whereas youth aged 11–20 years show the highest prevalence [28].

This age-related decline in vitamin D status may be partly attributed to the increased utilization of vitamin D by soft tissue during growth [29], and a decreased intake of vitamin D among children with their age due to an unhealthy diet lacking dairy products and insufficient parental oversight [30, 31]. Moreover, the capacity for cutaneous to synthesize 25(OH)D3 decreases with age [32]. Another plausible explanation for the poorer vitamin D status observed in older children is the reduced time available for outdoor play as their educational commitments increase, leading to more time spent indoors [8]. This age-related decline in physical activity and sunlight exposure consequently impacts the levels of 25(OH)D3, the primary source of serum vitamin D synthesized naturally in human skin upon exposure to UVB rays from sunlight [4, 33]. Therefore, parents need to pay more attention to children’s vitamin D supplementation and increase outdoor activities to increase vitamin production. The Chinese Medical Association recommends a daily vitamin D supply of 400 IU for children aged 0–1 year, and 400–800 IU for children over 1 year of age [34]. In addition, scientific research has shown that short-term exposure to sunlight (e.g., 2-3 times a week, 15–30 min each time) is sufficient to achieve maximum vitamin D synthesis. Beyond this time, although vitamin D synthesis may continue, the increase is relatively limited and the risk increases accordingly [35].

Consistent with findings from other studies [25, 27, 36–39], we found that lower vitamin D levels and higher VDD prevalence were more pronounced in girls than boys. This gender disparity may be linked to lifestyle differences. Due to a preference for lighter skin, girls are more likely to use high sun protection factor (SPF) sunscreen, wear protective clothing, and limit outdoor activities during sun exposure to prevent tanning or sunburn. These practices may significantly reduce the cutaneous synthesis of 25(OH)D3. Sufficient UV radiation exposure necessary for generating sufficient serum 25(OH)D levels typically requires sun exposure of one-fourth of the body’s skin for 10–15 min without sunscreen protection [40].

Additionally, low vitamin D status appeared to be more prevalent among Han Chinese compared to other ethnic groups. This disparity may be partly due to genetic differences affecting vitamin D3 synthesis. Furthermore, compared to their urban counterparts, ethnic minorities in China predominantly reside in rural, underdeveloped, and mountainous regions [41]. These minorities may engage in outdoor activities more frequently due to their unique geographical locations, customs, and cultural practices. This could help explain the variations in vitamin D status observed across different ethnicities and living environments in Hainan.

Our study found a strong link between obesity and a high prevalence of VDD, aligning with meta-analysis findings that indicate a 35% increased risk of VDD in obese individuals compared to those with normal weight [42]. The reasons for lower vitamin D levels in obesity are unclear, but a key hypothesis is that fat-soluble vitamin D may be sequestered in adipose tissue, impairing absorption [43]. Excess body fat can also disrupt hormonal pathways essential for skeletal health. For instance, leptin, a hormone from fat cells, may inhibit the renal synthesis of active vitamin D [44]. A biologically plausible explanation for the correlation between low circulating vitamin D levels and elevated body fat is that vitamin D plays a role in regulating adipogenesis [45]. Vitamin D is a group of fat-soluble secosteroids, and its bioavailability is diminished due to sequestration in body fat and volumetric dilution [46, 47]. Furthermore, the social stigma associated with obesity may lead obese children to limit their exposure to sunlight and outdoor activities A more sedentary lifestyle, coupled with the tendency to wear darker clothing to cover their bodies, greatly restricts the synthesis of vitamin D in the skin [42].

Although breast milk is considered the best nutrition for infants, excessive breastfeeding without vitamin D supplementation poses a risk for VDD and rickets in infants [48]. This may explain why children exclusively breastfed for more than 6 months exhibited lower serum vitamin D levels and a higher prevalence of VDD in our study. The median concentration of vitamin D in human milk and colostrum is only 15.9 ± 8.6 IU/L [49]. However, according to the expert consensus on clinical application of vitamin D in Chinese children, vitamin D supplementation should be started within the first week after birth until the age of 18 to ensure the growth and fertility needs of infants, young children, and adolescents [34]. Thus, breastfeeding alone is insufficient to provide adequate circulating levels of precursor hormone 25(OH)D for infants [50]. Due to the limited transfer rate, breast milk contains only 5–136 IU/L vitamin D, despite maternal intake of 600–700 IU/D [51]. A randomized controlled trial (RCT) involving 334 mother-infants pairs further demonstrated that maternal vitamin D supplementation of 6400 IU/day could facilitate sufficient vitamin D transfer in breast milk to meet the needs of her nursing infants [52]. Our findings suggest that appropriate vitamin D supplementation for breastfeeding infants is crucial and should be advised for lactating mothers.

While it may seem intuitive that higher-income families would have better vitamin D levels due to greater access to nutrient-rich foods, evidence is mixed regarding the actual vitamin D status across different income groups. Our study indicates that low circulating vitamin D levels and high prevalence of VDD are more common in children and adolescents from affluent families compared to those from lower-income households, though the impact of income on vitamin D status remains inconclusive. For example, Weng FL et al. [53] reported that low vitamin D status was more prevalent among family members with lower annual incomes. But studies have shown that high-income families may be more inclined to consume fast food and ready to eat foods, which often lack necessary nutrients, including vitamin D [54]. In addition, high-income families may reduce their time spent outdoors due to work and life pressures. Especially in urban environments, many high-income families may live in high-rise buildings with relatively fewer opportunities for outdoor activities. This lifestyle change may make them more susceptible to vitamin D deficiency issues [55]. Further research is needed to assess whether family income is an independent predictor of vitamin D status.

Vitamin D production can vary significantly by latitude, despite differences in participant age. A comprehensive study in China found that only Hainan Province had a serum 25(OH)D concentration exceeding 30 ng/mL, with a low prevalence of VDD at 7.3% [56]. Hainan, located near the equator, receive more sunlight than regions closer to the poles; however, the amount of UVB radiation varies by region [32]. Ozone in the atmosphere absorbs more UVB radiation as the path length increases due to the sun’s oblique angle at zenith, further diminishing the amount of UVB rays that reach the Earth’s surface [4]. Consequently, the energy of UVB radiation may be insufficient to promote the conversion of 7-dehydrocholesterol to previtamin D3 on rainy or foggy days, thereby limiting the transformation of previtamin D3 into vitamin D3 in the skin [57]. Particularly, sunlight exposure can also be limited in regions characterized by high humidity and a sultry climate, where sun-seeking behavior is uncommon, even during the summertime in Hainan. Local residents often avoid the heat and sun by remaining indoors or adopting specific clothing practices. phenomenon may help explain the vitamin D status in some humid, subhumid, and humid zones. This phenomenon is not unique to China; it is also observed in countries blessed with abundant sunlight, like Malaysia and numerous nations across the African continent, and South Asia [58, 59], VDD remains common, suggesting that sunlight alone is insufficient to ensure adequate vitamin D levels. Factors such as social culture, lifestyle, and dietary habits also play a significant role in determining vitamin D status [60].

Our study has several important limitations. First, the generalizability of our findings may be limited, as our sample consisted solely of Chinese children and adolescents. However, this provincial-level sample was substantial and representative, encompassing healthy children and adolescents across a broad age range (0–18 years), suggesting that the results could be relevant in similar settings. Second, blood samples were collected during the autumn and winter months, which may have influenced 25(OH)D levels, potentially resulting in higher vitamin D concentrations than would be expected during other times of the year. Third, due to the cross-sectional design of this study, we were unable to fully control for all potential confounding factors. While we adjusted for several demographic and health-related covariates, such as age, sex, nationality, BMI, prenatal vitamin D supplementation, and exclusive breastfeeding duration, we did not adequately account for other possible confounders like dietary intake and individual sunlight exposure habits. These factors could significantly influence serum vitamin D levels, and their lack of direct adjustment in our analysis may have introduced bias. Finally, as this is an observational study, causal inferences cannot be made, and our findings should be interpreted with caution.