To the best of our knowledge, this is the first ever population-based study conducted to estimate the prevalence of VDD among adults studied in a relatively large sample in high-risk urban areas in Sri Lanka.

The study revealed a striking finding: despite the abundant sunshine in this tropical region, a significant public health concern persists, with a crude prevalence estimate of low vitamin D levels at 93.9% (95% CI: 92.5, 95.2%). The finding signifies the potential risk the adults have for both skeletal and extra-skeletal health effects. This low vitamin D level is primarily driven by a substantial prevalence of VDD, which stands at 67.5% (95% CI: 64.9, 70.1%), accompanied by some degree of insufficiency at 26.4% (24.0, 28.9%). Encouragingly, the majority of VDD cases fall into the ‘mild’ severity category, accounting for 53.9% of the cases. Being a tropical country, Sri Lanka typically has sunlight and warm weather throughout the year with minimal seasonal variation, relative to countries in the temperate zone. Further, despite variations in rain pattern, cloud cover and UV index, results showed that there was no significant difference in the median serum 25(OH)D level in the samples collected during January, May–July (monsoon rains) and other months (p > 0.05). Therefore, the estimates of low vitamin D emphasize the importance of addressing it in this population, where sunlight availability does not seem to guarantee sufficient vitamin D levels.

Comparatively, the reported VDD prevalence in developed countries using the same cut-off value (< 20.0 ng/mL) ranged from 20 to 32%. For instance, in the USA, the prevalence was 24% in 2016, 32% in Canada in 2012/2013, and 31% in Australia in 2012 [5,6,7]. Further, in Northern European populations, it averaged below 20%, and in Western, Eastern and Southern European countries, it ranged from 30 to 60% [34]. This relatively lower burden of VDD in developed countries, despite the inadequate exposure to its major determinant- sunlight, may be attributed to well-established national vitamin D supplementation and food fortification programs that enabled such countries to control VDD encountered in the past [8]. On the contrary, when considering Asian populations living in developed countries, VDD prevalence appeared to remain high (82% to 94%) compared to their native counterparts, also suggesting a genetic vulnerability to VDD among Asians [9].

The present study's findings align more closely with VDD and VDI prevalence estimates from Asia including South Asian countries. India, for example, reports some of the highest VDD and VDI prevalence rates in the South Asian region, with 98% VDD in North India, 85% in Eastern India (Odisha coastal region) and 97% in South India [10,11,12]. Similar trends have been observed in Pakistan and Thailand as well [13, 14].

Several factors are likely to contribute to the high prevalence of low vitamin D including VDD among Sri Lankan adults. In contrast to ancient times, contemporary sun-avoidance practices such as wearing hats, using umbrellas and adopting protective clothing, especially in hot climates, have become widespread in modern societies of Asia, including Sri Lanka. This trend is particularly prominent in densely urbanized areas, where occupational preferences, transportation choices and recreational activities often favour indoor settings, as part of their sedentary lifestyles shaped by socio-cultural perspectives. Additionally, Sri Lankans typically have Fitzpatrick skin types 4–5, which require longer sun exposure than Caucasians to produce the same amount of vitamin D [16, 35]. Low consumption of vitamin D-rich dairy products in Sri Lanka may also exacerbate the issue [36]. Additionally, the staple diet in Sri Lanka, rice, contains phytate, which reduces calcium absorption from the intestines. This reduction in calcium levels could trigger the parathyroid gland to release parathyroid hormone, leading to increased bone resorption and decreased circulating 25(OH)D levels.

Furthermore, the study reveals that women are significantly more likely to have low vitamin D (predominantly in relation to VDD) than men (p < 0.001). However, there was no significant gender difference noted in relation to the severity of VDD (0.7 vs. 0.9). This gender disparity is in concurrence with global VDD prevalence and trends of Asian women’s clothing pattern such as saris which cover most of the body. Further, it could be due to intentional sun avoidance behaviours of women, often linked to cultural and body image perceptions where fair skin of women is deemed a beauty standard [16]. Recent climate change leading to increased ambient temperature has also made venturing outdoors uncomfortable for women, due to increased sweating, premature skin aging and skin darkening [37, 38].

Surprisingly, the study also indicates that younger age groups (35–44 years) have higher VDD prevalence compared to older age groups (65–74 years), contrary to the expected norm. This may be due to lower rates of vitamin D supplementation [39] or less opportunity or preference for outdoor sun exposure among younger individuals. A recent study conducted among young adults in Sri Lanka has shown that more than 60% adopt at least one sun avoidance behaviour when outdoors [40].

In terms of ethnicity, the study found the highest prevalence of VDD among Tamils (82.0%). This trend may be attributed to their predominantly vegetarian diet and conservative clothing choices. In contrast, Muslims, who also adhere to traditional clothing styles, exhibited improved vitamin D levels in the study. Although there is no clear explanation, to some extent, this may be owing to their higher consumption of fats, oils and spreads, meat and poultry. Interestingly, effect of long-term fasting conducted for religious purpose has also been implicated in this regard [41, 42].

Previous community-based studies in Sri Lanka have also reported high VDD prevalence rates among adults in relatively urban areas, such as Kandy Municipal Council area (47.4%) [27] and in one underserved settlement area in Colombo District (58.8%) [43], conducted almost 10–15 years ago, reinforcing the current study's findings. Notably, the highly urban area exhibited the highest VDD prevalence; however, rural dwellers did not have the lowest prevalence. This is contrary to what was expected, i.e., occupational sun exposure due to agriculture-based outdoor occupations of rural dwellers, leading to a lower prevalence of VDD. In the present-day context, even rural farmers commonly adopt protective measures like wearing long-sleeved clothing and brimmed hats to shield themselves from pesticides during farming activities, resulting in reduced sunlight exposure among this group. This may explain this discrepancy to some extent.

Finally, the study identified that 9.1% of participants had serum total calcium levels below the reference range, with the highest observed among the elderly (65–74 years). Low calcium levels showed no significant gender disparity, although females had a slight preponderance. The majority of those with VDD maintained normal serum calcium levels, possibly due to the predominance of mild VDD cases. Further, among those with VDD, 6.9% had high calcium levels, possibly an incidental finding or due to secondary hyperparathyroidism where low calcium levels lead to increased stimulation of the parathyroid glands, which could lead to bone resorption and adverse consequences [44]. However, further clinical and biochemical tests are needed to interpret these results accurately. Further, as 22.4% (n = 17) of those who had normal vitamin D levels showed low serum calcium, other causes of hypocalcaemia such as inadequate dietary intake, pancreatitis, lactose intolerance and disorders of the parathyroid gland should be explored [44].

There were several strengths in the methodology of this study related to adequate sample size, representativeness of the sample of urban settings in Sri Lanka and high response rate. Sample was stratified by the age- and sector-distribution of Colombo District. Although stratification by sex distribution of the district was not considered during sampling, selection bias was unlikely, as the sample showed a slight female preponderance compatible with census data of the district. Further, sampling was done on weekends and Sundays, so that both working adults and males were available at the time of sampling. Standard protocols were adopted during sample collection, transportation, storage and processing. The use of chemiluminescence assay method with a wide functional sensitivity range (4–150 ng/mL) and 100% detection of both serum 25(OH)D2 and 25(OH)D3 presented as a single value enabled an accurate estimation of true positives for VDD, minimising misclassification. Quality control indicators of vitamin D and calcium tests were within acceptable limits indicating satisfactory precision and reliability; however, gold standard methods would have further improved the measurements.