Vitamin D: The Sunshine Vitamin

Vitamin D is more than just a nutrient — it’s a fat-soluble vitamin that acts as a hormone. It’s one of the few vitamins our bodies can both make and store. Vitamin D is essential for promoting calcium absorption in the intestines, maintaining calcium and phosphate balance in the blood, and building strong bones [1]. Beyond skeletal health, vitamin D plays a role in gene regulation, influencing the expression of more than 200 genes involved in immunity, hormone balance, and metabolism [2].

How Vitamin D Works in the Body

Most people think we get vitamin D directly from the sun. In reality, the body stores vitamin D in the skin in an inactive form, which is then activated by exposure to UVB rays. This process produces vitamin D3 (cholecalciferol). To become fully active, vitamin D undergoes further conversion in the liver and kidneys, where it is transformed into calcitriol, the active hormone form [1].

Vitamin D is also found in foods such as fatty fish (salmon, sardines, mackerel), cod liver oil, and fortified dairy products. However, its half-life is relatively short. This means that vitamin D added to fortified foods may degrade before reaching consumers, and dietary intake alone is rarely sufficient [3].

Why Supplementation Is Common

Doctors often recommend vitamin D supplements during winter months when sun exposure is limited. However, people who are active outdoors in summer store vitamin D in their fat cells, where it can be activated as needed by the liver and kidneys [1].

For many people, supplementation is beneficial. Statistics highlight the scope of the problem:

• Over 30% of doctors and medical students are vitamin D deficient [4].

• 40% of Americans overall are deficient [5].

• More than 75% of pregnant women have inadequate levels [6].

• The elderly population is particularly vulnerable, as skin synthesis decreases with age [7].

Certain medications (such as glucocorticoids and anticonvulsants) and chronic conditions that impair fat absorption — including Crohn’s disease, ulcerative colitis, celiac disease, and cystic fibrosis — also raise the risk of deficiency [8].

Vitamin D Deficiency and Its Consequences

Skeletal Disorders

• Rickets – softening and demineralization of bones in children, leading to bowed legs and skeletal deformities.

• Osteomalacia – thinning and weakening of bones in adults, producing musculoskeletal pain, weakness, and fragility fractures [1].

Misdiagnosis as FibromyalgiaBecause vitamin D deficiency causes muscle aches, weakness, and pain, it is often mistaken for fibromyalgia or other musculoskeletal disorders [9].

Other Health Risks Low vitamin D levels have been linked to:

• Depression and seasonal affective disorder (SAD) [10]

• Cognitive decline in older adults [11]

• Increased risk of multiple cancers, including colorectal and breast cancers [12]

• Autoimmune disorders such as multiple sclerosis and type 1 diabetes [13]

Who’s at Risk?

• People in northern latitudes or who spend little time outdoors

• Individuals with darker skin pigmentation (higher melanin reduces vitamin D synthesis)

• Older adults with reduced skin efficiency

• Pregnant women and infants

• People with digestive or absorption disorders

• Those taking medications that interfere with vitamin D metabolism

Everyday Uses & Supplementation

• Aim for 10–30 minutes of midday sun exposure a few times weekly.

• Include vitamin D–rich foods such as fatty fish, cod liver oil, and fortified products.

• Consider a supplement — preferably vitamin D3 (cholecalciferol) — especially in winter or if blood tests confirm deficiency.

• Optimal blood levels are generally 40–60 ng/mL; consult your provider for individualized recommendations.

✅ Wellness Tips

• Take vitamin D3 with meals containing fat for best absorption

• Pair vitamin D3 with vitamin K2 to optimize calcium utilization and bone protection

• Get your levels tested at least once yearly

• Support vitamin D metabolism with a nutrient-dense diet (magnesium is a key cofactor)

So get outdoors and start enjoying the sun!

References

1. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281. https://doi.org/10.1056/NEJMra070553

2. Carlberg C, Haq A. The concept of the personal vitamin D response index. J Steroid Biochem Mol Biol. 2018;175:12–17. https://doi.org/10.1016/j.jsbmb.2016.12.011

3. Calvo MS, Whiting SJ, Barton CN. Vitamin D fortification in the United States and Canada: current status and data needs. Am J Clin Nutr. 2004;80(6 Suppl):1710S–1716S. https://doi.org/10.1093/ajcn/80.6.1710S

4. Thomas MK, Lloyd-Jones DM, Thadhani RI, et al. Hypovitaminosis D in medical inpatients. N Engl J Med. 1998;338(12):777–783. https://doi.org/10.1056/NEJM199803193381201

5. Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011;31(1):48–54. https://doi.org/10.1016/j.nutres.2010.12.001

6. Aghajafari F, Nagulesapillai T, Ronksley PE, Tough SC, O’Beirne M, Rabi DM. Association between maternal serum 25-hydroxyvitamin D level and pregnancy and neonatal outcomes: systematic review and meta-analysis. BMJ. 2013;346:f1169. https://doi.org/10.1136/bmj.f1169

7. Lips P. Vitamin D status and nutrition in Europe and Asia. J Steroid Biochem Mol Biol. 2007;103(3–5):620–625. https://doi.org/10.1016/j.jsbmb.2006.12.076

8. Holick MF. High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc. 2006;81(3):353–373. https://doi.org/10.4065/81.3.353

9. Plotnikoff GA, Quigley JM. Prevalence of severe hypovitaminosis D in patients with persistent, nonspecific musculoskeletal pain. Mayo Clin Proc. 2003;78(12):1463–1470. https://doi.org/10.4065/78.12.1463

10. Anglin RE, Samaan Z, Walter SD, McDonald SD. Vitamin D deficiency and depression in adults: systematic review and meta-analysis. Br J Psychiatry. 2013;202(2):100–107. https://doi.org/10.1192/bjp.bp.111.106666

11. Llewellyn DJ, Lang IA, Langa KM, et al. Vitamin D and risk of cognitive decline in elderly persons. Arch Intern Med. 2010;170(13):1135–1141. https://doi.org/10.1001/archinternmed.2010.173

12. Garland CF, Gorham ED, Mohr SB, Garland FC. Vitamin D for cancer prevention: global perspective. Ann Epidemiol. 2009;19(7):468–483. https://doi.org/10.1016/j.annepidem.2009.03.021

13. Munger KL, Levin LI, Hollis BW, Howard NS, Ascherio A. Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis. JAMA. 2006;296(23):2832–2838. https://doi.org/10.1001/jama.296.23.2832