In our study, 57% of women had VDD and only 24.5% of participants had optimal vitamin D status. Risk of VDD was associated with younger age, lower SES, winter season, while exercise and vitamin D supplement use were protective. A cross-sectional study among 351 patients at the out-patient department of General Medicine of a hospital in Islamabad, reported VDD present in 62.9% of females (mean age 46.03 +/- 16.18 years) and the mean vitamin D level was 14.09 +/- 12.93 ng/ml (12). In South Florida (U.S.), the mean (+/-SD) of 25(OH)D level was 22.4 +/- 8.2 ng/ml in 40% of women during winter(13). A study conducted in Lahore similarly reported high prevalence of Vitamin D deficiency among healthy women of child-bearing and was also associated with low education and lack of proper sun exposure and multivitamin intake (14).
In a study conducted at an urban hospital in Boston, VDD was associated with winter season, higher body mass index, and physical inactivity(15). In a study in Japan among 4,793 patients with rheumatoid arthritis, the mean (SD) serum 25(OH)D level was 16.9 ng/mL (6.1), and the prevalence of vitamin D deficiency was 71.8%. Predictors of VDD were female gender, younger age, among other factors that included low serum levels of total protein and total cholesterol, high serum ALP levels, and NSAID (16). VDD was also found high among pregnant women in Belgium (17). Similar to our study finding of association of VDD with younger age, vitamin VDD was found higher in younger women on other studies too (18-22).
Similar to our study, VDD is found to be more common among women during winter/spring compared with summer/autumn (23-25). A study in 3,327 pregnant Japanese women, VDD prevalence was 73.2% and it was higher in April after the winter season. Sun exposure of >15 mins for 1-2 days / week and usage of dietary vitamin D were protective against VDD (26).
A study conducted in Jinnah Postgraduate Medical Center, Karachi among students showed that VDD was more common in winter (27). A cross-sectional study of 14,302 Chinese participants aged 18-65 years from six major cities in China reported VDD was higher among females, in spring and winter from certain residential regions (28).
A study conducted among pregnant women in Malaysia reported that intake of vitamin D was protective against VDD. There was no association of VDD observed with age, educational, SES, employment, parity, body mass index, sun exposure(29). Another small cross-sectional study in Riyadh, Saudi Arabia, reported VDD among 60.2% of participants was associated with lack of usage of vitamin D, multi-vitamins (30). Another study in Riyadh, Saudi Arabia reported that absence of vitamin D supplements usage, younger age were factors associated with VDD among females(31). VDD was 75.1% in a study in France and was associated with no intake of vitamin D (32). A cross-sectional study of 634 healthy volunteers aged 18-50 years reported VDD associated with lack of multivitamin use (P<0.001 for each predictor)(33).
Our study showing lower SES and sedentary lifestyle associated with VDD was also reported by The National Health and Nutrition Examination Survey (NHANES) 2001-2010 (34). A cross-sectional study in Saudi Arabia, reported younger age, less exercise, less Vitamin D intake, as predictors of VDD (35).
Our study results are consistent with findings in few other studies conducted in Switzerland, China and New South Wales showing association of VDD with childbearing age, winter season and no usage of vitamin D supplement (36-38). A large study from the Korean National Health and Nutrition Examination Survey (KNHANES) 2008-2009 among 2062 adolescents also showed that VDD was higher in senior high school students and was associated with winter season and parental vitamin D deficiency (39).
Contrary to previous studies across different ethnic backgrounds, this study within Pakistani females showed that skin tone, obesity, etc were not associated with VDD. There was no association observed between VDD and sun exposure questionnaire. In a study in Brazil, the use of sun exposure questionnaire with 25OHD level showed low accuracy and its lack of discrimination between vitamin D sufficient and deficient individuals (40). A cross-sectional study among 254 university students in Shiraz also showed no association between VDD and sun exposure(41). Moreover, there was no association found between dietary sources of vitamin D and VDD. One reason could be the low consumption of fish and milk and vitamin D fortified food items in our population.
Our study has several strengths. The study collected comprehensive information on all potential factors associated with VDD. The data was collected by trained medical officers with expertise in both medical history taking and research. The main weakness of the study is the recall bias which is inherent in any cross sectional or case control study. Awareness programs and education about the importance of adequate vitamin D levels, are needed for sensible sunlight exposure and adequate nutritional intake of vitamin D-rich foods to prevent adverse health outcomes related to vitamin D deficiency. Supplement use of Vitamin D is particularly important especially during the current pandemic of Covid 19.