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Vitamin D
When can vitamin D deficiency occur?
Nutrient deficiencies are usually the result of dietary inadequacy, impaired absorption and utilization, increased requirement, or increased excretion (loss). A deficiency of vitamin D can occur [9]:
when usual intake is below recommended levels
when there is limited exposure to sunlight
when the kidney cannot convert vitamin D to its active hormone form
when someone cannot adequately absorb vitamin D from the digestive tract
Vitamin D deficient diets are associated with milk allergy, lactose intolerance, and strict vegetarianism. Infants fed only breast milk also receive insufficient amounts of vitamin D unless they also receive appropriate levels of vitamin D supplementation [17].
The classic vitamin D deficiency diseases are rickets and osteomalacia. In children, vitamin D deficiency causes rickets. Rickets is a bone disease characterized by a failure to properly mineralize bone tissue. Rickets results in soft bones and skeletal deformities [15]. Rickets was first described in the mid-17th century by British researchers [15,18]. In the late 19th and early 20th century, German physicians noted that consuming 1 to 3 teaspoons (3 teaspoons is equal to 1 tablespoon) of cod liver oil per day could reverse rickets [18]. The most common causes of rickets are vitamin D deficiency from a vitamin D deficient diet, lack of sunlight, or both. The recommendation to fortify milk with vitamin D made rickets a rare disease in the U.S. for many years. However, rickets has recently reemerged, in particular among African American infants and children [15,18]. In 2003, a report from Memphis, Tennessee, described 21 cases of rickets among infants, 20 of whom were African-American [18].
Prolonged exclusive breastfeeding without vitamin D supplementation is one of the most significant causes of the reemergence of rickets. Additional causes include extensive use of sunscreens and increased use of day-care, resulting in decreased outdoor activity and sun exposure among children [15,18].
Rickets is more prevalent among immigrants from Asia, Africa, and Middle Eastern countries for a variety of reasons [15]. Among immigrants, vitamin D deficiency has been associated with iron deficiency, leading researchers to question whether or not iron deficiency may impair vitamin D metabolism [15]. Immigrants from these regions are also more likely to follow dress codes that limit sun exposure. In addition, darker pigmented skin converts UV rays to vitamin D less efficiently than lighter skin [15].
In adults, vitamin D deficiency can lead to osteomalacia, which results in muscular weakness in addition to weak bones [5-6,9]. Symptoms of bone pain and muscle weakness may indicate vitamin D deficiency, but symptoms may be subtle and go undetected in the initial stages. A deficiency is accurately diagnosed by measuring the concentration of a specific form of vitamin D in blood [9,14].
Who may need extra vitamin D to prevent a deficiency?
It can be difficult to obtain enough vitamin D from natural food sources. For many people, consuming vitamin D fortified foods and adequate sunlight exposure are essential for maintaining a healthy vitamin D status. In some groups, dietary supplements may be needed to meet the daily need for vitamin D.
Infants who are exclusively breastfed
In infants, vitamin D requirements cannot be met by human (breast) milk alone [4,19], which usually provides approximately 25 IU vitamin D per liter [20]. Sunlight is a potential source of vitamin D for infants, but the American Academy of Pediatrics (AAP) advises that infants be kept out of direct sunlight and wear protective clothing and sunscreen when exposed to sunlight [21]. The American Academy of Pediatrics (AAP) recommends a daily supplement of 200 IU vitamin D for breastfed infants beginning within the first 2 months of life unless they are weaned to receive at least 500 ml (about 2 cups) per day of vitamin D-fortified formula [20]. Children and adolescents who are not routinely exposed to sunlight and do not consume at least 2, 8-fluid ounce servings of vitamin D-fortified milk per day are also at higher risk of vitamin D deficiency and may need a dietary supplement containing 200 IU vitamin D [20].
Formula fed infants usually consume recommended amounts of vitamin D because the 1980 Infant Formula Act requires that infant formulas be fortified with vitamin D. The minimal level of fortification required is 40 IU vitamin D per 100 calories of formula. The maximum level of vitamin D fortification allowed is 100 IU per 100 calories of formula [22]. This range of fortification produces a standard 20 calorie per ounce formula providing between 265 and 660 IU vitamin D per liter.
Older adults
Americans age 50 and older are believed to be at increased risk of developing vitamin D deficiency [14]. As people age, skin cannot synthesize vitamin D as efficiently and the kidney is less able to convert vitamin D to its active hormone form [4,23-26]. It is estimated that as many as 30% to 40% of older adults with hip fractures are vitamin D insufficient [13]. Therefore, older adults may benefit from supplemental vitamin D.
Persons with limited sun exposure
Homebound individuals, people living in northern latitudes such as in New England and Alaska, women who wear robes and head coverings for religious reasons, and individuals working in occupations that prevent sun exposure are unlikely to obtain much vitamin D from sunlight. It is important for people with limited sun exposure to consume recommended amounts of vitamin D in their diets or consider vitamin D supplementation [27-29].
Persons with greater skin melanin content
Melanin is the pigment that gives skin its color. Greater amounts of melanin result in darker skin. The high melanin content in darker skin reduces the skin's ability to produce vitamin D from sunlight. It is very important for African Americans and other populations with dark-pigmented skin to consume recommended amounts of vitamin D. Some studies suggest that older adults, especially women, in these groups are at even higher risk of vitamin D deficiency [16,30]. Individuals with darkly pigmented skin who are unable to get adequate sun exposure and/or consume recommended amounts of vitamin D may benefit from a vitamin D supplement.
Persons with fat malabsorption
As a fat soluble vitamin, vitamin D requires some dietary fat for absorption. Individuals who have a reduced ability to absorb dietary fat may require vitamin D supplements [31]. Symptoms of fat malabsorption include diarrhea and oily stools [31]. Fat malabsorption is associated with a variety of medical conditions [9]:
Pancreatic enzyme deficiencyis characterized by insufficient secretion of pancreatic enzymes. Pancreatic enzymes are essential for fat absorption, and a deficiency of these enzymes can result in fat malabsorption.
Crohn's Disease is an inflammatory bowel disease that affects the small intestines. People with Crohn's disease often experience diarrhea and fat malabsorption.
Cystic Fibrosis (CF) is a hereditary disorder that causes the body to secrete a thick, sticky mucus. This mucus clogs the pancreas and lungs. People with CF often experience fat malabsorption.
Sprue, often referred to as Celiac Disease (CD), is a genetic disorder. People with CD are intolerant to a protein called gluten. In CD, gluten can trigger damage to the small intestines, where most nutrient absorption occurs. People with CD often experience fat malabsorption. They need to follow a gluten free diet to avoid malabsorption and other symptoms of CD.
Liver disease includes a wide variety of disorders that impair liver function. Some people with liver disease experience fat malabsorption.
Surgical removal of part or all of the stomach or intestines can impair digestion and absorption of many nutrients. Fat malabsorption can occur after this type of surgery.
What are the sources of vitamin D?
What is the recommended intake for vitamin D?
When can vitamin D deficiency occur?
What are some current issues and controversies about vitamin D?
