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At the proper levels, vitamin D is necessary for the human body to function. Too little or too much stresses cells and tissues and may lead to serious, life-threatening diseases. The diseases related to overdoses or deficiencies of vitamin D largely reflect the relationship between this vitamin and the mineral calcium.

Diseases related to vitamin D deficiency are often characterized by weakened bones. They include:

• Rickets. This disease slows growth and causes skeletal abnormalities in children. Bones soften and bend when they support weight, leading to bowed legs and arms, and spinal deformities. In addition, the rib cage may be lined with bumps that resemble beads. The child’s head may also become large and abnormally shaped. Rickets may lead to permanent disfigurement without treatment.
  
  
• Osteomalacia. The adult form of vitamin D deficiency causes soft, painful bones and muscle weakness. In many patients, this leads to a stooped posture as the spine bends and legs become bowed. Patients also face an increased risk of fractures. It is most likely to occur in older women with low vitamin D intake and little exposure to the sun.
  
  
• Osteoporosis. This loss of bone density increases the risk of fractures. It is often associated with insufficient calcium intake, but may also result from vitamin D deficiency which reduces calcium absorption. Osteoporosis occurs most commonly among postmenopausal women.

These diseases can usually be treated by providing vitamin D and in some cases, additional calcium and phosphorus.

There are several factors that can increase the risk of developing vitamin D deficiency. Any factor that reduces exposure to sunlight limits the skin’s ability to synthesize vitamin D and can lead to deficiency. Possible contributors include:

• Dark skin. Melanin is a pigment that protects the skin from the harmful effects of the sun, but it can also reduce the rate of vitamin D synthesis, especially in northern climates. Studies have shown that African-American children are at greater risk than their peers for having low levels of vitamin D.
  
  
• Geographic location. During the winter months, sunlight is not strong enough to cause skin to create vitamin D in people living above about 35 degrees north latitude (approximately a line connecting Los Angeles and Columbia, SC). People who live at higher latitudes also may be at risk of vitamin D deficiency because the angle of the sun’s rays may not be appropriate for producing adequate levels of vitamin D in the skin.
  
  
• Urban environment. The smoke and smog often present in large cities blocks the ultraviolet light necessary for vitamin synthesis.
  
  
• Skin cover. Clothing and sunscreen (SPF 8 or higher) used to reduce exposure to the sun also interfere with the process of vitamin D production. However, for many people the skin only requires sun exposure on the extremities for several minutes a day several times per week for adequate vitamin D. Protecting the skin from UV radiation with sunscreen is important for skin cancer prevention.

Other risk factors include:

• Age. People who are 50 and older are believed to be at increased risk of developing vitamin D deficiency. As people age, the ability of the skin, liver and kidneys to make and activate vitamin D decreases. This may occur in conjunction with reduced exposure to the sun and limited dietary intake of vitamin D.
  
  
• Breastfeeding. Infants fed a diet consisting exclusively of breast milk after six months of age may develop vitamin D deficiency, particularly during winter in a northern climate. This can be prevented by including fortified formula or vitamin supplements in the infant’s diet.
  
  
• Fat malabsorption. Vitamin D is a fat-soluble nutrient. Any condition that limits fat absorption may also reduce the amount of vitamin D usable from foods. Diseases that reduce the body’s ability to absorb fats include Crohn’s disease, cystic fibrosis, pancreatic enzyme deficiency, liver disease and gluten intolerance. Fat absorption may also be limited by surgical removal of parts of the stomach or intestines and medications to treat obesity.
  
  
• Liver and kidney disease. These organs are critical to converting inactive forms of vitamin D to forms the body can use. Any disease that impacts liver or kidney function can limit the amount of active vitamin D in the body.
  
  
• Gastric bypass. Surgery performed to reduce the size of the stomach (bariatric surgery) can increase the risk of vitamin D deficiency.
  
  
• Hereditary vitamin D resistance. An extremely rare genetic disease that prevents the body from using vitamin D. It causes deficiency symptoms to occur within the first years of life.

While vitamin D deficiency can lead to serious diseases, taking too much of the vitamin from supplements can also cause significant problems. Excessive levels of vitamin D in the body can cause too much calcium to enter the bloodstream, a condition known as hypercalcemia. Initial symptoms include:

• Dehydration
• Nausea
• Vomiting
• Anorexia
• Constipation
• Muscle weakness
• Weight loss
• Confusion
• Lethargy
• Fatigue

Over time, toxic levels of vitamin D in the body can cause serious disease. Calcium may leave the blood and build up in soft tissues. Kidney stones are one result of this process. Damage may occur in the kidneys, blood vessels, heart and lungs.

Overuse of supplements is the primary cause of vitamin D toxicity, but this rarely occurs. Staying in the sun too long does not lead to vitamin D toxicity. Excess exposure to sunlight degrades the early, inactive forms of vitamin D in the skin, limiting production of the active form. However, too much sun increases your risk for skin cancer.