Most people have not considered that a primate could experience a vitamin D deficiency and be stricken with rickets, a condition typically associated with bone malformations in children. However, many people know that vitamin D is often called the “sunshine vitamin,” since it can be produced in the skin from cholesterol with exposure to ultraviolet (UV) light. Most of us meet at least some of our vitamin D needs this way, but with limited sun exposure or impaired synthesis it becomes important to consume sufficient vitamin D through our diet. But, whether through synthesis in the skin or through foods, vitamin D must be activated in the kidneys and liver to fulfill its biological functions in the body.

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And that, Holick realized, is ultimately what had sickened Kimani—as an animal in captivity, she hadn’t been getting enough sunlight or enough dietary sources of vitamin D, so she required supplements to help her recover. Holick was well aware of the potential benefits of supplemental vitamin D in rectifying deficiency issues, so although Kimani’s recovery was gratifying, it was not surprising.

Although sunlight can be a significant source of vitamin D, the RDAs—established to maintain bone health and normal calcium metabolism in healthy people—are set on the basis of minimal sun exposure. In 2010, the Institute of Medicine increased the RDA for vitamin D for all age groups—it was increased threefold to 600 IU (15 mcg), for example, for adults in the 19 to 50 years age range. However, the recent appreciation of the importance of vitamin D for functions beyond that of bone health has caused some scientists to question whether the new RDAs are actually high enough to provide for optimal health.

It can be difficult to meet those RDAs from food alone, because vitamin D is present in very few foods naturally. Some of the best sources include fatty fish (such as salmon, tuna, and mackerel) and fish liver oils. Beef liver, cheese, eggs, and some mushrooms (particularly those exposed to UV light) contain small amounts of vitamin D. Alternatively, most Americans get dietary vitamin D from fortified foods—most of the U.S. milk supply is fortified with vitamin D (100 IU, or 2.5 mcg, per cup). This is the result of a milk fortification program that was instituted in the 1930s in the United States to reduce the incidence of rickets, a significant public health issue at the time. (INFOGRAPHIC 10.3)

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Question 10.2

Vitamin D has many important known functions in the body. Mainly, as illustrated by the case of Kimani, vitamin D plays a key role in the growth and maintenance of bone by maintaining blood concentrations of minerals involved in bone development, such as calcium and phosphorus. In addition, vitamin D works in combination with other nutrients and hormones in bone growth and maintenance. But beyond bone health, research continues to explore and expand vitamin D’s role in human health.

Vitamin D actually functions as a hormone, since, like hormones, it is made in one part of the body, but carries out its regulatory effects elsewhere. Since it is made from cholesterol, vitamin D is a member of the steroid family of hormones (which includes estrogen, testosterone, and cortisol, among others).

Vitamin D also plays an important role in the regulation of calcium. Most calcium in the body is stored in bones (see Chapter 13), with less than 1% circulating in the blood; however, this circulating calcium is critical for normal muscle and nerve functioning and must be maintained. (INFOGRAPHIC 10.4)

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Question 10.3

The regulation of blood calcium requires the interaction of bones, kidneys, and intestines. For example, when the concentration of calcium in blood falls, vitamin D is converted into its fully active hormone form (calcitriol) in the kidneys. This active hormonal form of vitamin D then increases the absorption of calcium in the gastrointestinal tract—for this reason, you will often find calcium supplements that also contain vitamin D. Activated vitamin D also works together with parathyroid hormone to decrease calcium excretion by the kidneys, and increase its release from bone to raise blood calcium concentrations.

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It is estimated that calcitriol is involved in regulating the synthesis of 5% of all proteins in the body. Consequently, it is required for normal cell development and immune function and is critical for other organs and body systems, including the brain, heart, and nervous system, reproductive organs, skin, and muscle. Recent findings suggest that maintaining vitamin D levels in the elderly increases muscle strength and function, and appears to reduce the risk of falls. Evidence is also accumulating that vitamin D is necessary for regulating cardiovascular function. Research now provides evidence that sufficient intake and stores of vitamin D may reduce the risk of a number of important diseases, including cancer, autoimmune diseases, kidney disease, type 2 diabetes, and even cardiovascular disease.

Vitamin D is listed as a nutrient of concern in the 2015 Dietary Guidelines for Americans. According to data from the 2007-2010 National Health and Nutrition Examination Survey, 94% of all Americans consume less than adequate intakes of vitamin D increasing risk of deficiency particularly in vulnerable populations. People most at risk are those who avoid vitamin D–fortified dairy products (because of allergies, intolerance, or a vegan diet); people with dark skin (since the melanin that darkens skin can interfere with the body’s ability to make vitamin D from sunlight); and those with little sun exposure, or who regularly use sunscreen. Although sunscreen can block UV rays and reduce the risk of skin cancer, it also prevents vitamin D synthesis. Other groups at risk include infants who are exclusively breast-fed, since breast milk is not a rich source of vitamin D, and the elderly, who have a reduced ability to synthesize vitamin D when exposed to sunlight. Many elderly individuals are also more likely to stay indoors and therefore have less opportunity to synthesize vitamin D.

Having extremely low levels of vitamin D can, over time, cause serious bone diseases. Starting in the mid-1600s, researchers began describing mysterious symptoms in children that Holick instantly recognized in Kimani— “bowed” legs, soft bones, and other skeletal malformations. Today we know that rickets is caused by low vitamin D levels, leading to impairments in the maturation and mineralization of cartilage in regions of the bone where growth is occurring, causing the characteristic “bowed” legs or “knocked” knees. In adults, the same deficiency can cause osteomalacia, in which the bone mineral is being depleted, causing the bones to become soft or weak and putting people at risk of fractures or falls, as well as creating pain in the pelvis, lower back, and legs.

But you can also get too much vitamin D—called hypervitaminosis D—which is likely the result of supplement use rather than food intake or sun exposure. The established UL for vitamin D is set at 4,000 IU (100 mcg) for males and females 9 years and older. The symptoms of hypervitaminosis D vary, but may include loss of appetite, weight loss, irregular heartbeat, and frequent urination. Having too much vitamin D can also increase levels of calcium in the blood, which leads to calcification of the soft tissue and damage to the heart, blood vessels, and the formation of kidney stones.