Many factors can affect vitamin D status in humans such as: season and latitude, sunlight exposure pattern and physical activity, skin colour, ethnicity, clothing style, dietary intake and body weight. Latitude, sunlight exposure and dietary intake were discussed previously in section 1.5.1 and section 1.5.2.
In general, Asian people have lower 25(OH)D levels than Caucasian people. In a study in the UK done by Smith (2010) among 200 healthy individuals Caucasian and Asian they found that during summer and winter, Asian males and females had significantly lower 25(OH)D levels than Caucasians. During winter 64% of Asian have 25(OH)D levels <10 ng/ml and 53% in summer while for Caucasians the percentage are 7.7% in winter and 3.5% in summer, Smith (2010) concluded that these different levels could be due to skin pigmentation and darker skin colour, lower dietary vitamin D intake and a covered clothing style. Glass et al (2009) stated that Asian and African American individuals have lower 25(OH)D levels and this difference between ethnicities could be related to genetic variation in vitamin D metabolism, different vitamin D dietary intake and dietary habits, darker skin colour and cultural differences (Glass et al., 2009; Mansour and Alhadidi, 2012).
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Previous studies have investigated the relationship between 25(OH)D status and skin colour in different ethnic groups. Generally, individuals with dark skin had lower serum 25(OH)D than individuals with fair skin and need more time exposed to sunlight to synthesis the same amount of 25(OH)D. Melanin pigmentation can reduce and limit 25(OH)D synthesis as it limits the penetration of UVR. However, a large study done in the UK investigated the relationship between 25(OH)D level and skin type using the Fitzpatrick skin type scale in the Caucasian population and found that females with fairer skin (type 1 and type 2) had lower vitamin D than females with darker skin. This finding could be due to avoidance of sunlight as a result of the primary care recommendations for individuals with light skin due to the risk of skin cancer (Glass et al., 2009).
Despite Saudi Arabia being a sunny country, vitamin D status is markedly low in the Saudi population in all age groups. A review by Christie and Mason (2011) reported the most important factors that may lead to low 25(OH)D in the Saudi population, especially in women. These factors are poor knowledge about vitamin D, low sunlight exposure because of the extreme hot weather, covering style clothing and low dietary vitamin D intake (Christie and Mason, 2011). Another study on adults in Saudi Arabia reported that a vitamin D intervention, including advice to eat more food sources of vitamin D and to increase exposure to sunlight before 10 am or after 3 pm (the time between 10 am to 3 pm is reported to be carcinogenic in Saudi Arabia) twice a week for 5 to 30 minutes led to a significant increase in serum 25(OH)D (Al-Daghri et al., 2012). However, it is estimated that exposing face and arms to sunlight between 10 am and 3 pm in the UK in summer, spring and autumn for 5 to 15 minutes can lead to production of an additional 1000 IU of vitamin D through skin (Glass et al., 2009). Being overweight or obese is correlated with lower serum 25(OH)D. In a large study included 2126 adult participants in Norway, the authors found that half of males and one third of females who had BMI of 40 or more were vitamin D deficient (Lagunova et al., 2009). It is also reported that UV
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radiation and oral supplementation interventions have less effect on obese individuals when compared to normal weight individuals (Wortsman et al., 2000). In a recent systematic review and meta-analysis including 15 trials, authors found that serum vitamin D was increased with weight loss (Mallard et al., 2016).
Possible explanations of the negative correlation between obesity and vitamin D levels could be because vitamin D can be stored in body fat and that leads to reduced vitamin D bioavailability in the body. Secondary hyperparathyroidism is also common in obese individuals and this can lead to lower vitamin D in addition to the slower release of vitamin D from fat to blood flow (Lagunova et al., 2009). Limited outdoor activity and lower mobility of obese people can also decrease vitamin D levels. Obese individuals tend to wear more clothes and cover a larger part of their body in the summer this is also can lead to a decrease in sunlight exposure (Lagunova et al., 2009; Wortsman et al., 2000).
From a large cohort study in 2007 undertaken by Forman, which included 613 males and 1198 females, they found that higher BMI and lower physical activity were correlated with lower 25(OH)D levels (Forman et al., 2007). In a large cross-sectional study in Kuwait, it was found that among 960 adults, 83% of them were vitamin D deficient or insufficient with 25(OH)D<20 ng/ml. They also found that people with low 25(OH)D had significantly higher BMI than people with adequate 25(OH)D level. However, their physical activity did not differ significantly (Zhang et al., 2016).
In a large study in the US, Wanner et al (2015) collected the data from NHANES 2003-2006 included 6370 adults. They aimed to investigate if there was a correlation between physical activity assessed by accelerometers and by self-reported physical activity and vitamin D levels. They found that a 10 minute increase in physical activity measured by the accelerometer was associated with increased 25(OH)D by 0.32ng/ml, and
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by 0.18 ng/ml for self-reported moderate or vigorous physical activity (Wanner et al., 2015). Another study from Saudi Arabia among 331 children and adolescents aimed to investigate the correlation between physical activity and 25(OH)D levels. Physical activity and sunlight exposure were measured by a questionnaire which found that children and adolescents who were moderately active or very active had higher serum 25(OH)D when compared to inactive participants (17.7±1.6 nmol/l vs 22.7±1.5 nmol/l). They also found that BMI was higher in children who were inactive. The authors concluded that outdoor exercise or physical activity during the daytime can increase sunlight exposure, which may result in increasing vitamin D synthesis and physical activity also can decrease calcium excretion and increase its absorption. Finally, exercise and physical activity can lead to weight loss and reduce adiposity resulting in increased vitamin D levels (Al-Othman et al., 2012).