The current study showed that low vitamin D status was prevalent in Lebanese adults during fall, with more than 30 and 60% of participants having serum 25(OH)D concentrations < 20 ng/mL and ≤ 30 ng/mL, respectively. BMI was not associated with vitamin D status; however PBF was associated with insufficient vitamin D status using the NOF cutoffs.
Low vitamin D status has become a major problem worldwide, even in sunny countries like Lebanon . The prevalence of low vitamin D status reported in our study (60%) was in line with the prevalence rates reported in neighboring countries ranging from 57.6% in Tehran  to up to 91% in Morocco . The range of prevalence rates varied among studies due to the different study populations, education levels, season, BMI, age, body composition, gender, and cutoffs for vitamin D status. Our results were concordant with the prevalence rate of 73.3% among 105 Lebanese adults (aged > 18 years) visiting outpatient clinics between April and July reported by Rachkidi and Aoun, 2015 . On the other hand, Gannage et al. 2014, reported a higher prevalence of 94.9% among employees in a university hospital (n = 329) . In the latter study, data was collected over a year; accordingly, some participants were recruited in the winter season, which could increase the likelihood of low vitamin D status. The seasonal variation in vitamin D concentration has been previously reported in the literature . Hoteit et al., 2014 reported that 25(OH)D concentrations were lower in the winter season (20.1 ± 9.2 ng/mL) among 9147 Lebanese outpatients compared to 24.8 ± 10.4 ng/mL in the fall season . The mean serum 25(OH)D concentration reported in our study was 28.2 ± 13.9 ng/mL, which was similar to the mean reported by Hoteit et al., 2014 during the same season.
In our study, in bivariate analyses, longer duration of sun exposure was associated with better vitamin D status in the total sample and in men but not in women. Due to highly pigmented skin, individuals of Middle Eastern origin might need to spend more time in the sun to synthesize sufficient amounts of vitamin D endogenously . This justification is backed up by our multivariate logistic analyses; since sun exposure > 1 h was protective against low vitamin D status, while sun exposure between 16 and 60 min was not protective. It is likely we did not observe this association in women in bivariate analyses, since around 54% of women were using sunscreen, while only 4% of men were using sunscreen. It is well established in the literature that sunscreen use can block UVB light and reduce vitamin D synthesis significantly . The association between sun exposure and vitamin D status is inconsistent in the literature, since several factors may affect the cutaneous synthesis and bioavailability of vitamin D, such as the time of day, the use of sunscreen, and skin pigmentation [6, 32, 33]. These factors could explain gender differences in our sample.
In bivariate analyses, vitamin D intake was associated with vitamin D status in the total sample, and in women but not in men. Women who had a sufficient status had a higher vitamin D intake compared to women who had an insufficient status; this same trend was observed in men but did not reach statistical significance. Vitamin D is obtained from two sources, diet and sun exposure . While sun exposure was not associated with vitamin D status in women due to the use of sunscreen, diet seems to be the most important factor affecting vitamin D status for women; however, the sun was the most important factor affecting vitamin D status for men . However, in multivariate logistic regression, once all confounders were controlled for, sun exposure, vitamin D intake, and vitamin D supplements became protective against low status, which is concordant with the literature [6, 25, 34].
Mean vitamin D intake (2.2 ± 3.2 μg/day) reported in our study was similar to that reported by Rachkidi and Aoun, 2015 (2.05 ± 1.69 μg/day). Both studies show low intakes of vitamin D compared to the DRI of 15 μg/day . It is important to point out that vitamin D is found only in a few foods. Furthermore, the fortification of milk in Lebanon is not mandatory compared to other countries, such as the US and Canada . In addition, the consumption of milk was low in our sample (0.97 ± 1.3 servings/day) which is below the recommendations set by the Lebanese Food-Based Dietary Guidelines of 3 cups/day . Old evidence suggested a high prevalence of lactose intolerance in the Lebanese population; however, new evidence is lacking .
The association between alcohol intake and vitamin D status was observed in women only; this has been previously reported in the literature [39,40,41]. In our sample, women who drank alcohol had a higher prevalence of sufficient vitamin D status compared to non-drinkers. The relationship between alcohol intake and vitamin D status is not well understood, and results are still inconclusive. It is likely that this relationship is affected by confounders; especially that alcohol intake was not associated with vitamin D status in multivariable analyses.
Measures of adiposity, including WC and PBF, were inversely associated with vitamin D status in all participants and in both genders, while BMI was inversely associated with vitamin D status in women only. The association between BMI and vitamin D status is not always consistent in the literature , since BMI has many limitations, as it does not necessarily reflect the PBF . The association of adiposity, measured by WC and PBF, with serum 25(OH)D is usually stronger than that with BMI . In concordance with our results, multiple studies showed that vitamin D concentrations were significantly lower in both males and females with high risk WC compared to low risk categories (p < 0.05) [9, 11, 17].
Our study found a positive association between low vitamin D status (25(OH)D ≤ 30 ng/mL) and PBF, which is also concordant with the literature [17, 25, 34, 44]. While this relationship is not fully understood, it is assumed that since vitamin D is a fat soluble vitamin, it might be sequestered in the excess adipose tissue of obese adults and would be slowly released at negative energy balance . Further, a potential confounder is that obesity is also linked to an unhealthy lifestyle, characterized by less physical activity, less sun exposure and, hence, lower vitamin D concentrations .
Using the IOM cutoffs to define vitamin D status as inadequate (25(OH)D < 20 ng/mL) the association between vitamin D status and PBF was observed but did not reach statistical significance (p > 0.05). It is possible that the association between vitamin D status and body composition is not observed unless vitamin D concentrations are high, since NOF cutoffs are higher than IOM cutoffs [2, 3].
The present study has some limitations that need to be acknowledged. First, the study design was cross-sectional, which does not allow drawing causal relationships between vitamin D status and measures of adiposity . Second, the study was performed on employees from a private university in Lebanon, which limits generalizability to the Lebanese population. In addition, BIA was used for the assessment of body composition. The dual-energy x-ray absorptiometry (DEXA), computed tomography (CT), and magnetic resonance imaging (MRI) are considered the gold standard imaging modality for the precise estimation of amount of adipose tissue in various body regions . However, DEXA, CT and MRI imaging are impractical for screening the general population, since they are expensive and invasive . For the assessment of vitamin D concentration, ELISA, the most commonly used technique, was used, since it is simple and inexpensive, yet it is less reliable than the gold standard technique, HPLC. HPLC is not commonly used because of its complexity and limitations including the need for specialized staff, the large volume of blood required, and the longer turnaround time . Data was collected between mid-October to mid-December, which means that we cannot compare our results to studies performed in different seasons. Vitamin D content of many foods listed in the FFQ were not available in the Lebanese food composition tables; instead, the Canadian Nutrient File was used to estimate the vitamin D content of these foods.
To our knowledge, it is the first study in Lebanon to assess the association between body composition and vitamin D status, while controlling for BMI and other important confounders .