Vitamin D: Functions, Deficiency, and Supplementation

Introduction to Vitamin D

Vitamin D is a group of fat-soluble molecules known as secosteroids. As a fat-soluble vitamin, it is not naturally present in many foods. However, it is produced photochemically when ultraviolet rays from sunlight strike the skin and trigger Vitamin D synthesis. Vitamin D obtained externally is biologically inert in the human body and must undergo two hydroxylations before it becomes active. The first occurs in the liver and converts Vitamin D to 25-hydroxyvitamin D. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D (VanDen Berg, 1997, S76).

Although called a vitamin, Vitamin D is not actually an essential dietary vitamin because it can be synthesized in more than adequate amounts from sunlight exposure. Typically, an organic chemical or group of chemicals is classified as a vitamin only when it must be obtained through diet or other external means and cannot be synthesized through bodily processes alone.

Vitamin D has many roles in the human body. It is an essential element in promoting calcium absorption from the gut and producing sufficient serum calcium and phosphate to keep the bone structure solid, support the formation of new bone cells, and facilitate the regeneration of bone after bruising, breaks, or aging. Without Vitamin D, bones become thin, brittle, misshapen, and unable to support the body. As the body ages, Vitamin D helps prevent osteoporosis, which is why many older individuals require calcium supplementation (Madrid, 2009). Vitamin D also modulates the relationship between nerves and muscles, reduces inflammation, and improves immune function (Eads, 1994). Emerging research further indicates that adequate sunlight exposure — and the resulting Vitamin D and calcium production — may help keep periodontal tissue healthy. Many periodontal diseases are inflammatory processes in which an overreaction to oral bacteria damages the surrounding bone; sufficient Vitamin D and calcium may help prevent this outcome (Dryden, 2005).

Vitamin D and the Body

Over the past two decades, a number of robust studies have demonstrated that Vitamin D has an even more integral effect on the human body than previously understood. The substance is now known to be critical at the enzymatic level, helping to keep neurotransmitters healthy and supporting the immune system. Low levels of Vitamin D in geriatric patients may contribute to premature aging and increased mortality, and this effect appears to be proportional to ethnicity.

Vitamin D is well established as critical for the formation and growth of healthy bone tissue in children and adolescents. Less widely recognized is the fact that these effects continue through menopause, and that Vitamin D remains important for both men and women throughout their adult life cycle — regardless of milk intake. Moderate to high doses of Vitamin D have, on occasion, been shown to decrease the risk of cardiovascular disease, depending on other variables. Low levels of Vitamin D are associated with multiple sclerosis, and while supplementation may have a protective effect, maintaining more optimal Vitamin D levels may reduce the chance of disease onset. Additionally, low Vitamin D levels are often associated with certain cancers, and supplementation appears beneficial in all cases except prostate cancer (Feldman et al., eds., 2005).

Serum concentration of 25(OH)D is one of the best indicators of Vitamin D status. This measurement reflects Vitamin D produced from food and supplements and serves as a biomarker of Vitamin D's effects, though it does not reflect the amount stored in the body's fat cells. Circulating Vitamin D tests are less reliable because the active hormone has a half-life of only approximately 15 hours and decreases detectably only in cases of severe or chronic deficiency. The recommended daily amounts of Vitamin D vary by age and represent minimum requirements to prevent deficiency:

Vitamin D Deficiencies

In children, an inadequate supply of Vitamin D results in impaired bone mineralization and bone softening. Rickets, for instance, is a childhood disease characterized by impaired growth of the long bones, primarily caused by a lack of Vitamin D and the resulting inability to use calcium and phosphorus for bone development. Rickets rarely appears in the developed world today, thanks to Vitamin D supplementation, but remains somewhat prevalent in low-income countries. The disease was described as early as 1650, and its dietary causes were confirmed during World War I. Notably, dietary rickets can occur even in regions with strong sunlight, particularly when animal protein is scarce in the diet.

In young infants in poorer countries, breast milk is naturally low in Vitamin D, and a combination of social customs and climate can prevent adequate sun exposure. In countries such as Nigeria and Bangladesh, rickets most often occurs among toddlers and young children who have limited access to dairy products and rely primarily on cereal-based diets. Similarly, rickets was endemic in some 19th-century immigrant populations in the United States, but with increased milk consumption after World War II and widespread food fortification with Vitamin D, it is now rare in the American population.

Osteomalacia is an adult disease characterized by bone thinning and muscle weakness. The condition stems from the body's inability to retain and utilize Vitamin D adequately, though research has not yet established a definitive correlation between musculoskeletal pain and reduced Vitamin D levels among sufferers.