light from the sun’s rays. In addition, many Europeans worked in sweatshops devoid of sunlight. Rickets became an epidemic.
From the 19th to 20th centuries, there were several major breakthroughs in research on rickets. These included the discovery that city dwellers were more likely to be afflicted by the disease compared to people who lived in the countryside, that the lack of sun exposure caused it, and that fish liver oil could cure it. In 1922, Elmer McCollum confirmed that the lipid-soluble substance in fish liver oil that cured rickets was not vitamin A but an unknown substance he named vitamin D. In 1936, Adolf Windaus confirmed that the chemical structure of vitamin D was calciferol.
Upon sun exposure, human skin produces vitamin D. Strictly speaking, the body produces vitamin D; therefore, vitamin D is not an essential nutrient. Nevertheless, vitamin D produced by the skin is not sufficient for the needs of most people. Intake from foods or supplementation is necessary to ensure optimal health. Vitamin D produced by the skin or from foods is activated in the liver to produce 25-hydroxyvitamin D. Also known as “25(OH)D,” 25-hydroxyvitamin D is the major circulating form of vitamin D in the bloodstream, and it is commonly used to assess the blood level of vitamin D. It in turn is further activated in the kidneys to become 1,25-hydroxyvitamin D, the active form of vitamin D, which is involved in hundreds of gene expressions and numerous physiological functions.
What Are the Major Functions of Vitamin D?
• Calcium homeostasis. Concentrations of calcium are closely regulated in the body, and calcium homeostasis is related to normal physiological functions in the nervous system, bones, muscles, and many other organs. To maintain good health, it is of foremost importance to carefully control calcium concentration in the body. Vitamin D enhances the deposition of calcium in the bones, while parathyroid hormones release calcium from the bones. The interplay between vitamin D and parathyroid hormones controls calcium concentration in the body.
• Phosphorus homeostasis. Similarly, concentrations of phosphorus ions influence many normal physiological functions in muscles, bones, and other organs in the body. When phosphorus levels are low in the blood, vitamin D instructs the intestines to increase absorption of phosphorus ions, and when the phosphorus ion levels are too high, parathyroid hormones instruct the kidneys to excrete excessive phosphorus ions in the urine.
• Cell proliferation and differentiation. Proliferation is a self-replication process by which a cell multiplies and produces more cells, and differentiation is like assigning a given cell specific tasks to perform. When proliferation is dominant, differentiation will be slow, and vice versa. Uncontrollable proliferation can lead to cancer. Vitamin D promotes cell differentiation and prevents cell proliferation to curtail the risk of tumor formation.
• Immune system. Many immune cells—including monocytes, macrophages, and T cells—have their own vitamin D activation processes through which the active form of vitamin D influences the gene expression of immune cells. Vitamin D enhances innate immunity (general immune defense) and inhibits autoimmunity (misdirected immune responses). Vitamin D deficiency increases susceptibility to autoimmune disorders.
• Insulin secretion. The active form of vitamin D enters pancreatic cells, regulates gene expression in pancreatic cells, and stimulates secretion of insulin from pancreatic cells.
What Are the Symptoms of Vitamin D Deficiency?
Vitamin D deficiency causes rickets in children (103) and osteomalacia in adults.
• Rickets. Vitamin D promotes the mineralization of calcium phosphate in the bones, increasing bone density and strength. The bones of children with vitamin D deficiency may grow in length but lack mineralization of calcium phosphate, resulting in soft and porous bones. Such bones cannot support the weight of the body and become bent and deformed. This is known as rickets.
• Osteomalacia. Calcium in bones exists in a homeostatic state—an equilibrium of deposition and removal. Vitamin D deficiency causes removal rather than deposition of calcium in the bones, triggering loss of bone density and, in severe cases, resulting in osteomalacia, or soft bone disease, which is characterized by aching pain in the lower back, pelvis, hips, legs, and ribs in adults. Osteomalacia can lead to osteoporosis.
What Are the Risk Factors for Vitamin D Deficiency?
• Geography. The closer a place is to the equator, the stronger the intensity of ultraviolet light from the sun. Therefore, people who live closer to the equator have a lower risk of vitamin D deficiency compared to those who live farther away from the equator. Indeed, people who live at the 43rd parallel north and farther are more susceptible to vitamin D deficiency. In those regions, every year from October to April, ultraviolet light from the sun cannot reach the surface of the Earth, so the skin ceases to produce vitamin D, increasing the risk of deficiency.
• Sun protection. Except beachgoers and those who love outdoor activities, most people try to avoid sun exposure. Sun umbrellas, hats, long sleeves, long pants, and sunscreen are common remedies to shun sun exposure. Excessive avoidance of the sun can cause vitamin D deficiency.
• Infants fed with breast milk. The vitamin D content of breast milk is relatively low. Infants who are born to Asian and African families in the northern hemisphere are at especially high risk of vitamin D deficiency. A daily dose of 400 IU is recommended for infants to avoid vitamin D deficiency.
• Obesity. Vitamin D is a lipid-soluble vitamin. Upon absorption in the intestines, vitamin D dissolved in chylomicrons (fat globules) is transported to the liver and other organs in the body. In the bloodstream, vitamin D binds to a vitamin D–binding protein, which is then absorbed by fat tissue. In other words, in obese individuals, vitamin D is mostly stored in fat tissue, while the rest of the body is devoid of vitamin D, causing vitamin D deficiency. Weight loss is the only way to release it from fat tissue and increase vitamin D levels.
• Chronic kidney disease. An enzyme in the kidneys is required for final activation of vitamin D in the body, which creates the active form of vitamin D. Chronic kidney disease hinders the activation of vitamin D in the kidneys, resulting in vitamin D deficiency.
• Enteritis (inflammation of the small intestine). Inflammatory bowel diseases—such as ulcerative colitis, Crohn’s disease, and fat malabsorption—may lower the absorption of vitamin D in the intestines, aggravating the risk of vitamin D deficiency.
Do I Need Sun Exposure?
Vitamin D has a host of health benefits, including maintaining healthy bones and lowering the risk of heart disease, stroke, asthma, inflammation, and autoimmune diseases. Vitamin D also may extend life expectancy.
The amount of sun exposure required to produce an adequate amount of vitamin D depends on the color of your skin and where you live. For example, to produce 1,000 IU of vitamin D in Chicago, Caucasians require 15 minutes, Asians and Hispanics 30 minutes, and Africans 2 hours. The best time for sun exposure is on a sunny day when your shadow is shorter than your height, during which time the skin is most efficiently producing vitamin D. People who live in places at or above the 43rd parallel north can only achieve this from May through September, so in the winter, they may have to rely on intake through vitamin D–rich foods or supplements to obtain adequate amounts of vitamin D.
What Are the Health Reference Values for Vitamin D?
Sufficient blood levels
