Effects Of Latitude, Skin Type, And Lifestyle On Vitamin D Levels

Geographical Influence on Vitamin D Levels

Question:

Describe about the Factors that may affect vitamin D?

Geographical Location

As the majority of vitamin D is produced in the skin and requires UVB radiation to initiate the process, latitude can have a pronounced effect vitamin D status. In countries with a latitude below 35°N the body can produce sufficient vitamin D all year round (Tsiaras & Weinstock 2011). At latitudes above 35°N, which includes much of Europe including Germany; Italy and Amsterdam, sunlight exposure is limited during the winter months and therefore there is higher risk of vitamin D₃ deficiency (Webb et al., 1988).  A study conducted in the USA, at high latitude 44°N, by Sullivan et al.(2005), found that   approximately 28% of younger girls had a serum 25(OH) D level below 75nmol/l in cold places. In contrast, hypovitaminosis D can be infected people who live in sunny countries that can produce a high amount of vitamin D because of their lifestyle (Horani et al. 2011).

In European countries, seasonal changes has significant impact on vitamin D as compared with countries located near equator, the concentration of 25(OH)D is higher in summer and gets lower in winter season (Levis et al., 2005 ). A cross sectional study was conducted by (Mavroeidi et al., 2010) to assess vitamin D status in 3000 postmenopausal women at different seasons, over a period of one year. Additionally the study assessed the incidence of hypovitaminosis D in participants from different cities in the UK (Aberdeen 57°N, Surrey 57°N). The data showed that vitamin D deficiency was higher in the north of the UK than in the south. In Surrey, 17.1% of Asian women were found to be deficient in vitamin D (These data’s were based on the dietary and predictors. Hypovitaminosis with the highest rates recorded among residents of regions in Aberdeen by 25-26% in postmenopausal women during winter and spring, and decreased to 4.2% in summer. Similarly, in a study carried out by (Hypponen and Power, 2007) showed consistent findings, confirming that the incidence of hypovitaminosis D is higher in Scotland compared to the rest of the UK.

In contrast, the survey conducted by Levis et al (2005) in Florida, the prevalence of vitamin D deficiency in 212 participants was 38% and 40% in male and female respectively, in the wintertime with mean serum 25 (OH) D was 24.9 ± 8.7 ng/cc, whereas, the mean serum 25 (OH) D concentrations of sub-optimal group (just 99 people) was (31.0 ± 11.0 ng/ml) in the end of summer.

Skin type and race considered as factors could effect on vitamin D status because the effectiveness of melanin to absorb UVB radiation could increase the effectiveness of cutaneous synthesis of vitamin D₃ (Kift et al., 2013). In 2007, a study carried out by Chen et al. (DATE) indicated that the skin pigmentation could have an effect on D₃ production in the skin. This study was conducted by measuring serum 25(OH) D concentrations in adults with different types of skin (Universal skin classification, II or White, fair, blue eyes, III or Mediterranean, blue or brown eyes , IV or Asian, brown eyes  and V or Light-skinned black, Indian. At the end of study, serum 25(OH)D concentrations were increased dramatically in all types of skin. Actual recorded mean levels for types II, III, IV and V were 210%, 187%, 125%, and 40% respectively. The authors concluded that the production of previtamin D₃ in Type II skin is (5-10 fold) more potent than the type V skin (highly pigmented skin).

Skin Type and Vitamin D Synthesis

Table 1: Skin type, skin reaction to sun exposure

From Lips et al (2014)

Shaw & Pal (2002) reported that the prevalence of vitamin D deficiency is increasing among minority groups living in Great Britain, particularly those are from India and Pakistan and this is due to their lifestyle or habit to stay indoors. Furthermore, studies carried out by de Roos et al, (2012) and Brough et al., (2010) state that skin pigmentation in those groups is not the only reason for reduced vitamin D production but it is s also due to wearing clothes that tend to cover their entire bodies and staying indoors for longer during the day thus limiting UVB exposure. ().  Kift et al. (2013) carried out a prospective cohort study in South Asian people aged 20-60 years to assess vitamin D level and lifestyle factors and compare the result with study conducted in Caucasian people with same condition. Demographic factors, vitamin D level, UV exposure and vitamin D intakes were analysed with same methodologies in study conducted on South Asian and Caucasian people. The authors found that there was no difference in the percentage of body area exposed to the sun. Also, they reported that white people reach a mean vitamin D level of 65.3nmol/l in summer, whilst south Asians only reach a mean level of 22.4nmol/l. During the wintertime, it was reported that 40% of South Asians were found to be deficient (15nmol/l), due to their low vitamin D intake and increased skin pigmentation. They further stated that it should be noted that skin pigmentation could affect pre vitamin D₃ production. The conclusions to this study state that future prospective studies need to find effective ways to address these apparent issues.

Clothing and sunscreen are known as cutaneous factors, as they are a physical barrier in absorption of solar radiation by the skin and thus effective in less production of vitamin D level (Tsiaras and Weinstock, 2011). Most of sunscreens are having a sun protection factor (SPF) 8 that helps to protect the body against UV B radiation and decreases vit. D synthesis by less than 95 %, whereas (SPF) 15 decreases by less than 99% (Webb and Engelsen, 2006). According to Holick (2004), when the skin is exposed to the sunlight, the amount of UVB photons well absorbed by

Blocking out sunlight as a result of dress style is particularly associated with hypovitaminosis D amongst immigrant women in the UK and Europe (Gillie 2010). A previous study by (Glerup et al., 2000) indicated that the prevalence of vitamin D deficiency is higher in immigrant women (veiled and ethnic Muslims women) than in Danish women due to limited sunlight exposure. Another study conducted in Turkish women examined three groups of women with different dress styles (Alagöl et al., 2000). Group I wore a dress, which exposed the arms and lower legs to sunlight, group II covered whole body except hands and face, last group (III) wore a traditional Islamic style that cover whole body. The result reported that the serum 25(OH) D level was significantly high in group I than in groups II and III, where all of the women were under normal levels. 

Impact of Lifestyle on Vitamin D Levels

There are several diseases that can affect vitamin D level including kidney disease and obesity (Tsiaras &Weinstock 2011). The inverse relationship between obesity and vitamin D deficiency is well-established (Wortsman et al, 2000; Esteghamati et al., 2004; Holick & Chen 2008). In fact, those who are obese, living in high-risk regions such as Scotland are considered to be at twice the risk of those living in lower risk regions of Great Britain (Hyppönen & Power, 2007). A study was conducted by Wortsman et al (2000) to investigate the relationship between obesity and vitamin D production. It was shown that those who were obese participants (i.e. having a BMIover 30kg/m²) had 57% lower serum vitamin D concentrations than non-obese participants after exposure to ultraviolet D₃ or receiving oral D₂ supplements.. The authors suggested that this was due to increased vitamin D storage in adipose tissue. They further stated that although their findings are similar to another studies, they tend to believe that obesity did not affect vitamin D production but rather  the release of vitamin D₃ from the skin into the circulation . A recent cross–sectional study carried out by Turner, et.al. (2013)  has confirmed the latter hypothesis suggested by Wortsman et al.(2000), showing that the deposition of vitamin D₃ in body fat compartments results in decreased bioavailability of vitamin D₃ from cutaneous and dietary sources. Bischof et al (2006) indicated that the serum 25(OH) D concentrations associated negatively with BMI (body mass index) in a study of 483 adults.  Results reported that prevalence of hypovitaminosis (25(.OH)D <22.0 nmol/l) in participants with BMI less than 30kg/m² was increased from 8.8% to 15% in adults with BMI greater than 30kg/m². Lee et al.(2009) found in their study that the effectiveness of supplementation of vitamin D is dependent on BMI. The obese and vitamin D deficient patients may need a higher dose of supplement than non-obese to increase vitamin D levels.  95 subjects with (25(OH) D < 6 ng/mL) were given 10.000 IU (cholecalciferol) for 1 week, the authors reported that 25(OH) D concentrations correlated negatively with BMI.

Several studies have linked low vitamin D intake with low economic status (Dealberto, 2006). In many of these associations, the authors cite issues such as poor nutrition, poor lifestyle and inability to afford supplements to treat the deficiency. For instance, poor dietary intake is prevalent in regions with a high poverty rate, mostly affecting middle-aged women of childbearing age (Brough et al., 2010). According to Brough et al. (2010) a socially deprived population cannot afford some of the basic nutrients such as vitamin D, which are essential for normal metabolic function. Therefore, some resort to what have been described as ‘shortcuts of life’ (means shortage of essential nutrients); the impact of this is exposing their immune system to chronic diseases. A report released by the Greater Manchester Poverty Commission (GMPC) in 2002, identified Manchester as one of the regions with the highest incidence of extreme poverty with approximately 25% of its population living in abject poverty (GMPC, 2012). It also revealed that those who are socio economically deprived couldn’t efficiently protect themselves from low winter temperatures, causing these individuals to stay indoors longer than other UK residents, compared to those with an average annual income. According to Grimes (2011) those who have a low income and are socioeconomically deprived are also burdened with a higher risk of vitamin D deficiency. Several campaigns such as the Glasgow campaign introduced free vitamin D supplements for the ethnic groups to improve their vitamin D status (Shaw and Pal 2002). According to Dunnigan et al (1985), the campaign was started in 1979 and ran for 5 years. The intervention gave Asian schoolchildren, up to 18 years, a low daily dose of vitamin D (100 IU). After supplementation, it showed that the prevalence of rickets decreased. This effort led to significant improvement amongst these communities, particularly those targeted cities in Northern England. However, the identification of a large number of deficient people in study conducted by Roy et al., (2007) suggests that the gains made 40 years ago are no longer visible, and more people have been diagnosed with vitamin D deficiency among the minority population than ever before.

Relationship Between Obesity, Kidney Disease, and Vitamin D Deficiency

Tedstone (2014) has published the most recent survey about food consumption, which showed that many of the UK’s population are still suffering from vitamin D insufficiency or deficiency with 24% of adults aged 19 years and older and 22% of children not having sufficient vitamin D levels. In wintertime, the prevalence of vitamin D deficiency was found to increase to 40% in both groups.  The reason for the hypovitaminosis D was attributed to the reduction in sunlight exposure (longer nights, less external activity and weather pattern), which gives the body 90% of its vitamin D requirement (O’Connor and Benelam 2011).

A startling statistic is that vitamin D dietary intake is much lower in Great Britain as compared to other western nations including United States and Canada (Calvo et al., 2005). Variance in dietary intake of vitamin D between Britain, the United States and Canada has been attributed to the differing extents to which mandatory fortification food occurs in these countries.  In the UK only specific foods are fortified with vitamin D, these include margarine, breakfast cereals and infant milk (O’Connor and Benelam 2011).

According to Sinha et al (2013) there is still a debate between whether the vitamin D intake from food is adequate and enough to maintain serum 25(OH) D concentrations at an optimal level. Some of the most common food sources, which are rich in vitamin D, are fish, liver, fortified margarine and fortified cereals (see table 2.4).

Table 2: Dietary source of vitamin D in the UK

Adopted from O’Connor & Benelam (2011)

Clinical nutritional assessments of natural food items suggest that with the exception of fish and cod liver oil, most natural food stuffs contain minimal vitamin D, if any (Brough et al., 2010; Sinha et al., 2013).  According to Schmid and Walther (2013) although there are several sources of vitamin D, it is still difficult for people to meet their recommended intake of vitamin D through consumption of natural food alone.  Conversely, Hill et al. (2004) stated that in countries with low levels of sunlight, vitamin D deficiency could be treated by ensuring individuals have an abundance of food that is rich in vitamin D. Additionally, a recent study carried out by Rizzoli (2014) has shown that an improvement in bone health and a reduction in the risk of fracture in later life could be achieved through dietary intervention. It is proposed that vitamin D deficiency can be addressed by consuming 3 servings of dairy products a day, which include milk and yogurt, both of which are rich sources of essential nutrients and include a substantial amount of vitamin D. Significantly, it is important to note that insufficient natural sources for vitamin D is a risk factor in itself, and should be taken into consideration when plans are put into place to tackle the problem.

There are a huge number of intervention studies that have considered vitamin D supplementation, taking into account factors such as in the different forms of the vitamin used and in the dosage levels applied (Sinha et al., 2013). The expression of the amount of vitamin D in food or supplements is micrograms (μg) or International Units (IU).  μg is most common used by Europe (1 μg is equivalent to 40 IU) (O’Mahony et al., 2011) A summary of these studies is given in Table 2.3.

Table 2.3: vitamin D intervention trials

Calcifediol supplement 25(OH) D₃ is a vitamin D metabolite used to treat vitamin D deficiency, it is hydrophilic and has a shorter half-life than vitamin D₃ (Jetter et al., 2014). Supplementation of calcifediol is a simple, safe and economic treatment to increase 25(OH) D concentrations more rapidly than vitamin D₃ (Bischoff-Ferrari et al., 2012).

Jetter et al (2014) evidenced in their study that the (HyD₃) single or bolus increases the plasma 25 (OH) D concentrations more rapidly than vitamin D₃. Where the plasma 25(OH)D was between 8 to 24ng/ml at baseline, after 15 weeks  70% of those taking vitamin D₃ reached a 25(OH)D concentration of 30ng/ml whilst all  of the women given HyD₃ reached a plasma concentration >30ng/ml.  Similar findings were reported by Cashman et al (2012), 58 old age people received vitamin D₃ and calcifediol in doses equivalent to 20μg or 7μg HyD₃ for 10 weeks. At the end of the study, only 29% of the vitamin D₃ group increased their 25(OH) D₃ concentrations after 5 weeks and no further increases by week 10. In contrast, the groups given HyD₃ showed increased serum 25(OH)D3 concentrations within the 5 week period with further increases by week 10. The above studies have a limitation in that they examine the effect of calcifediol on increasing plasma 25(OH) D concentrations in old aged (50 to 70 years) and therefore the previous findings may not be generalizable to all adults. Further studies are therefore required in order to confirm these results in younger adults.

References

Alagöl, F., Shihadeh, Y., Boztepe, H., Tanakol, R., Yarman, S., Azizlerli, H., & Sandalci, Ö. (2000) ‘Sunlight exposure and vitamin D deficiency in Turkish women.’ Journal of Endocrinological Investigation, 23(3) pp. 173-177.

Aloia, J. F., Patel, M., DiMaano, R., Li-Ng, M., Talwar, S. A., Mikhail, M., ... & Yeh, J. K. (2008) ‘Vitamin D intake to attain a desired serum 25-hydroxyvitamin D concentration.’ The American journal of clinical nutrition, 87(6) pp. 1952-1958

Ann M. Coulston, Carol J. Boushey, Mario Ferruzzi (2012) Nutrition in the Prevention and Treatment of Disease. Academic Press.

Bischof, M. G., Heinze, G., & Vierhapper, H. (2006) ‘Vitamin D status and its relation to age and body mass index.’ Hormone Research in Paediatrics, 66(5) pp. 211-215.

Bischoff‐Ferrari, H. A., Dawson‐Hughes, B., Stöcklin, E., Sidelnikov, E., Willett, W. C., Edel, J. O., ... & Egli, A. (2012) ‘Oral supplementation with 25 (OH) D3 versus vitamin D3: effects on 25 (OH) D levels, lower extremity function, blood pressure, and markers of innate immunityl.’ Journal of Bone and Mineral Research, 27(1) pp. 160-169.

Bischoff-Ferrari, H. A., Giovannucci, E., Willett, W. C., Dietrich, T., & Dawson-Hughes, B. (2006) ‘Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes.’ The American Journal of Clinical Nutrition, 84(1) pp.18-28

Blair. M. (2012) 'Action needed on vitamin D levels'. [Online] [Accessed on 16th May 2014] https://www.bbc.co.uk/news/health-20710026.

Brough, L., Rees, G. A., Crawford, M. A., Morton, R. H. and Dorman, E. K. (2010) 'Effect of multiple-micronutrient supplementation on maternal nutrient status, infant birth weight and gestational age at birth in a low-income, multi-ethnic population.' British Journal of Nutrition, 104(3) pp. 437-445.

Calvo, M. S., Whiting, S. J. and Barton, C. N. (2005) 'Vitamin D intake: A global perspective of current status.' Journal of Nutrition, 135(2) pp. 310-316.

Cashman, K. D., Seamans, K. M., Lucey, A. J., Stöcklin, E., Weber, P., Kiely, M. and Hill, T. R. (2012) 'Relative effectiveness of oral 25-hydroxyvitamin D3 and vitamin D3 in raising wintertime serum 25-hydroxyvitamin D in older adults.' American Journal of Clinical Nutrition, 95(6) pp. 1350-1356.

Cashman, K. D., Hill, T. R., Lucey, A. J., Taylor, N., Seamans, K. M., Muldowney, S., & Kiely, M. (2008) ‘Estimation of the dietary requirement for vitamin D in healthy adults.’ The American journal of clinical nutrition, 88(6) pp. 1535-1542.

Dawson, C., (2009) Introduction to Research Methods: A Practical Guide for Anyone Undertaking a Research Project. How to Books, Oxford.

de Roos, B. Sneddon, A. and Macdonald, H. (2012) ' Fish as a dietary source of healthy long chain n-3 polyunsaturated fatty acids (LC n-3 PUFA) and vitamin D'. Food & Health Innovation Service, available at https://www.abdn.ac.uk/rowett/documents/fish_final_june_2012.pdf.

Dealberto, M.J. (2006) 'Why immigrants at increased risk for psychosis? Vitamin D insuffiency,epigenetic mechanisms, or both?' Medical Hypothesis, Vol. 68, pp. 259- 267.

Dehghan, M., Del Cerro, S., Zhang, X., Cuneo, J. M., Linetzky, B., Diaz, R., & Merchant, A. T. (2012) ‘Validation of a semi-quantitative food frequency questionnaire for Argentinean adults.' PloS one, 7(5) pp. e37958.

DeLuca, H. F., Prahl, J. M., & Plum, L. A. (2011) ‘1, 25-Dihydroxyvitamin D is not responsible for toxicity caused by vitamin D or 25-hydroxyvitamin D’. Archives of biochemistry and biophysics, 505(2) pp. 226-230.

Dunnigan, M. G., Glekin, B. M., Henderson, J. B., McIntosh, W. B., Sumner, D., & Sutherland, G. R. (1985) 'Prevention of rickets in Asian children: assessment of the Glasgow campaign.' British Medical Journal (Clinical research ed),  291(6490) pp. 239.

Esteghamati, A., Aryan, Z. and Nakhjavani, M. (2004) 'Differences in vitamin D concentration between metabolically healthy and unhealthy obese adults: Association with inflammatory and cardiometabolic markers in 4391 subjects. Diabetes & Metabolism, [Online] [Accessed on 5 May 2014] https://www.sciencedirect.com/science/article/pii/S1262363614000469

Gillie, O. (2010) 'Sunlight robbery: A critique of public health policy on vitamin D in the UK.' Molecular Nutrition and Food Research, 54(8) pp. 1148-1163.

Glerup, H., Mikkelsen, K., Poulsen, L., Hass, E., Overbeck, S., Thomsen, J., ... & Eriksen, E. F. (2000) 'Commonly recommended daily intake of vitamin D is not sufficient if sunlight exposure is limited.' Journal of Internal Medicine, 247(2) pp. 260-268.

Greater Manchester Poverty Commission (GMPC) (2012) 'Research Report’. The Centre for Local Economic Strategies. [Online] [Accessed on 16th May 2014] https://www.cles.org.uk/publications/greater-manchester-poverty-commission-evidence-and-recommendations-report/.

Grimes, D. S. (2011) 'Vitamin D and the social aspects of disease.' QJM Oxford University Press, 104(12) pp. 1065-1074.

Hill, T. R., O'Brien, M. M., Cashman, K. D., Flynn, A. and Kiely, M. (2004) 'Vitamin D intakes in 18-64-y-old Irish adults.' European Journal of Clinical Nutrition, 58(11) pp. 1509-1517.

Hoffmann, M. R., Senior, P. A., & Mager, D. R. (2015). Vitamin D Supplementation and Health-Related Quality of Life: A Systematic Review of the Literature’. Journal of the Academy of Nutrition and Dietetics.

Holick, M. F. (2004a) 'Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease.' The American Journal of Clinical Nutrition, 80(6 Suppl) pp. 1678S-1688S.

Holick, M. F. (2004b) 'Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis.' The American Journal of Clinical Nutrition, 79(3) pp. 362-371.

Holick, M. F. and Chen, T. C. (2008) 'Vitamin D deficiency: A worldwide problem with health consequences.' American Journal of Clinical Nutrition, 87(4) pp. 1080S-1086S.

Horani, M., Dror, A., Holland, D., Caporaso, F., Sumida, K. D. and Frisch, F. (2011) 'Prevalence of vitamin D3 deficiency in Orange County residents.' Journal of Community Health, 36(5) pp. 760-764.

Hyppönen, E. and Power, C. (2007) 'Hypovitaminosis D in British adults at age 45 y: Nationwide cohort study of dietary and lifestyle predictors.' American Journal of Clinical Nutrition, 85(3) pp. 860-868.

Janet Houser (2011) Nursing Research: Reading, Using, and Creating Evidence.  2nd ed., Jones & Bartlett.

Jetter, A., Egli, A., Dawson-Hughes, B., Staehelin, H. B., Stoecklin, E., Goessl, R., Henschkowski, J. and Bischoff-Ferrari, H. A. (2014) 'Pharmacokinetics of oral vitamin D3 and calcifediol.' Bone, 59 pp. 14-19.

Kift, R., Berry, J. L., Vail, A., Durkin, M. T., Rhodes, L. E. and Webb, A. R. (2013) 'Lifestyle factors including less cutaneous sun exposure contribute to starkly lower vitamin D levels in U.K. South Asians compared with the white population.' British Journal of Dermatology, 169(6) pp. 1272-1278.

Lee, P., Greenfield, J. R., Seibel, M. J., Eisman, J. A., & Center, J. R. (2009) 'Adequacy of vitamin D replacement in severe deficiency is dependent on body mass index.' The American Journal of Medicine, 122(11) pp. 1056-1060.

Levis, S., Gomez, A., Jimenez, C., Veras, L., Ma, F., Lai, S., & Roos, B. A. (2005) 'Vitamin D deficiency and seasonal variation in an adult South Florida population.' The Journal of Clinical Endocrinology & Metabolism, 90(3) pp. 1557-1562.

Lips, P., van Schoor, N. M. and de Jongh, R. T. (2014) 'Diet, sun, and lifestyle as determinants of vitamin D status.' 92-98-page Handout, distributed in lecture 1317 for module

Mavroeidi, A., O’neill, F., Lee, P. A., Darling, A. L., Fraser, W. D., Berry, J. L., ... & Macdonald, H. M. (2010)' Seasonal 25-hydroxyvitamin D changes in British postmenopausal women at 57° N and 51° N: A longitudinal study'. The Journal of Steroid Biochemistry and Molecular Biology, 121(1) pp. 459-461.

Maxwell, S. M., Salah, S. M., & Bunn, J. E. G. (2006) 'Dietary habits of the Somali population in Liverpool, with respect to foods containing calcium and vitamin D: a cause for concern?.' Journal of Human Nutrition And Dietetics, 19(2) pp. 125-127.

O’Mahony, L., Stepien, M., Gibney, M. J., Nugent, A. P., & Brennan, L. (2011) ' The potential role of vitamin D enhanced foods in improving vitamin D status.' Nutrients, 3(12) pp. 1023-1041.

O'Connor, A. and Benelam, B. (2011) 'An update on UK Vitamin D intakes and status, and issues for food fortification and supplementation.' Nutrition Bulletin, 36(3) pp. 390-396.

Pal, B. R., Marshall, T., James, C. and Shaw, N. J. (2003) 'Distribution analysis of vitamin D highlights differences in population subgroups: Preliminary observations from a pilot study in UK adults.' Journal of Endocrinology, 179(1) pp. 119-129.

Pearce, S.H. and Cheetham, T.D. January, (2010)' Diagnosis and management of Vitamin D deficiency'. BMJ, 11: 340.

Peat, J., Mellis, C., Williams, K. and Xuan W (2002) Health Science Research: A

Handbook of Quantitative Methods. London: Sage.

Preece, M. A., McIntosh, W. B., Tomlinson, S., Ford, J. A., Dunnigan, M. G. and O'Riordan, J. L. (1973) 'Vitamin-D deficiency among Asian immigrants to Britain.' Lancet, 1(7809) pp. 907-910.

Rizzoli, R. (2014) 'Dairy products, yogurts, and bone health.' The American Journal of Clinical Nutrition, 99(5), 1256S-1262S.

Roy, D. K., Berry, J. L., Pye, S. R., Adams, J. E., Swarbrick, C. M., King, Y., Silman, A. J. and O'Neill, T. W. (2007) 'Vitamin D status and bone mass in UK South Asian women.' Bone, 40(1) pp. 200-204.

Ruston D, Hoare J, Henderson L et al. (2002) 'The National Diet and Nutrition Survey: adults aged 19 to 64 years.' Volume 4:Nutritional status (anthropometry and blood analytes), blood pressure and physical activity. London: TSO.

Schmid, A. and Walther, B. (2013) 'Natural vitamin D content in animal products.' Advances in Nutrition (Bethesda, Md.), 4(4) pp. 453-462.

Shaw, N. J. and Pal, B. R. (2002) 'Vitamin D deficiency in UK Asian families: Activating a new concern.' Archives of Disease in Childhood, 86(3) pp. 147-149.

Sinha, A., Cheetham, T. D. and Pearce, S. H. S. (2013) 'Prevention and treatment of vitamin D deficiency.' Calcified Tissue International, 92(2) pp. 207-215.

Sullivan, S. S., Rosen, C. J., Halteman, W. A., Chen, T. C., & Holick, M. F. (2005) 'Adolescent girls in Maine are at risk for vitamin D insufficiency'. Journal of the American Dietetic Association, 105(6) pp. 971-974.

Tedstone, A. (2014) ‘New National Diet and Nutrition Survey shows UK population is eating too much sugar, saturated fat and salt’. https://www.gov.uk/government/news/new-national-diet-and-nutrition-survey-shows-uk-population-is-eating-too-much-sugar-saturated-fat-and-salt.

Thuesen, B., Husemoen, L., Fenger, M., Jakobsen, J., Schwarz, P., Toft, U., Ovesen,T. and Linneberg, A. (2012) 'Determinants of vitamin D status in a general population of Danish adults.' Bone, 50(3) pp. 605-610.

Tsiaras, W. G. and Weinstock, M. A. (2011) 'Factors influencing vitamin D status.' Acta Dermato-Venereologica, 91(2) pp. 115-124.

Turner, C. B., Lin, H., & Flores, G. (2013) 'Prevalence of vitamin D deficiency among overweight and obese US children.' Pediatrics, 131(1) pp. e152-e161.

Utiger, R. D. (1998) 'The need for more vitamin D.' New England Journal of Medicine, 338(12) pp. 828-829.

Van Teijlingen, E., & Hundley, V. (2002) ‘The importance of pilot studies’. Nursing Standard, 16(40) pp. 33-36.

Vieth, R. (2009)’ How to optimize vitamin D supplementation to prevent cancer, based on cellular adaptation and hydroxylase enzymology.’ Anticancer research, 29(9) pp. 3675-3684.

Webb, A. R. and Engelsen, O. (2008) 'Ultraviolet exposure scenarios: Risks of erythema from recommendations on cutaneous vitamin D synthesis'.72-85-page handout, distributed in lecture 624 for module.

Webb, A. R., Kline, L., & Holick, M. F. (1988) 'Influence of Season and Latitude on the Cutaneous Synthesis of Vitamin D3: Exposure to Winter Sunlight in Boston and Edmonton Will Not Promote Vitamin D3 Synthesis in Human Skin'. The Journal of Clinical Endocrinology & Metabolism, 67(2) pp.373-378.

Welman, C., Kruger, F. & Mitchell, B. (2005) Research Methodology. 3rd ed., Oxford: University Press

WHO. (2009) ‘Global Database on Body Mass Index. World Health Organization, An interactive surveillance tool for monitoring nutrition transition’. https://www.who.int/bmi/index.jsp?introPage=intro_3.html , Geneva (accessed 17, May 2010).

Wortsman, J., Matsuoka, L. Y., Chen, T. C., Lu, Z., & Holick, M. F. (2000) ‘Decreased bioavailability of vitamin D in obesity.' The American journal of clinical nutrition, 72(3) pp. 690-693.