Vitamin D Insufficiency Due to Insufficient Exposure to Sunlight and Related Pathology

Vitamin D Insufficiency and Related Pathology

In overcast regions such as the Pacific Northwest region of the United States, there are certain diseases that have an increased prevalence as compared to other areas of the nation. The climate of the Pacific Northwest during the winter months may lead to a higher incidence of these diseases due to vitamin D insufficiency resulting from a lack of sunlight. Additionally, the widespread use of sunscreen coupled with an indoors-oriented lifestyle leaves many people vulnerable to vitamin D insufficiency. Here we will examine the nature of vitamin D, its importance in various body functions and disease prevention, and methods to improve an individual’s vitamin D level.

Properties and Uses of Vitamin D in the Body

 Vitamin D is not technically a vitamin. In its natural form, it is a prohormone that is created by the body; in contrast, vitamins are substances that are extracted from food and used as nutrients in the body.  However, the term “vitamin” is commonly used and will be used in this essay for ease of understanding.

Vitamin D is often called “the sunshine vitamin”, and for good reason. It is synthesized in the skin of humans and other mammals when exposed to sunlight. There are four forms of vitamin D. The first form, cholecalciferol, is made by the skin when it is exposed to direct sunlight containing UVB rays. Cholecalciferol is also called vitamin D₃, and is used in many supplements and in the fortification of food. The second form of vitamin D, calcidiol, is made from cholecalciferol in the liver and stored in the liver for future use. When blood is drawn to test for serum levels of vitamin D in a patient’s body, it is calcidiol that is being tested. The third form of vitamin D, calcitriol, is made from calcidiol in the kidneys. This is the active form of vitamin D, and it regulates calcium and displays properties that prevent cancer. The fourth form of vitamin D is not made in the body. It is called ergocalciferol, and is synthesized by irradiatng ergosterol with UV radiation. Ergosterol is derived from the ergot fungus. Ergocalciferol is often used in the supplementation of food, and once in the body it acts identically to cholecalciferol, and can be converted to calcidiol and calcitriol (Blake, 2007).

Traditionally, vitamin D has been viewed by researchers and physicians as functioning mainly to maintain bone density and prevent bone loss. Bone-softening diseases which have been attributed to vitamin D deficiency include rickets in children, and osteomalacia, osteopenia, and osteoporosis in adult patients (Moyad, 2009). While it is true that vitamin D plays a significant role in bone health, emerging research suggests that its presence or absence also contributes to the overall health of other body systems, including the immune system, the autoimmune system, the cardiovascular system, and the integumentary system (Hoffman, et al, 2010). However, according to Bordelon, et al (2009), “because the signs and symptoms of vitamin D deficiency are insidious or nonspecific, it often goes unrecognized and untreated.”

Factors Influencing Vitamin D Endogenesis

Humans evolved near the equator before migrating to distant corners of the prehistoric landmass. Our methods for synthesizing vitamin D were established at that time, and have not yet evolved to compensate for various factors that limit sun exposure.

Populations living at a latitude higher than 37° north of the equator in the Northern Hemisphere are at the greatest risk of developing vitamin D deficiency. In the United States, this translates roughly to a line drawn between San Francisco and Richmond, Virginia (Figure 1); anyone living north of that line has such reduced sun exposure in the autumn and winter months as to make vitamin D synthesis nearly impossible. Seattle is at a latitude of 47° north of the equator, which sets the population up for widespread vitamin D insufficiency. Garland (2003) explained that the storage form of vitamin D has a half life of approximately three weeks, so people in northern latitudes become deficient by December if they are relying solely on the sun for their vitamin D.

Figure 1: United States divided above and below 37th parallel

James Dowd, M.D. (2009) explained this concept in another way, saying that the ability to make vitamin D in the skin disappears in the autumn and does not return until the following spring. As the leaves fall from the trees, serum vitamin D levels fall as well. In regions that have a winter shorter than ten weeks, this decrease in serum vitamin D may not have any effect on overall health because the body’s stores will be enough to sustain the people who live there. In regions that have a longer winter, however, individuals’ vitamin D stores will be depleted by January. The only form of vitamin D available to these people in the winter comes from fat stores and dietary supplementation.

Industrialization has contributed to widespread vitamin D deficiency. Whereas humans used to spend many hours outside every day working in fields, on farms, and (pre-agricultural revolution) hunting and foraging for food, modern humans in industrialized nations work largely indoors. Time is spent commuting in cars, working all day in factories and office buildings, and leisure time is spent in windowless malls, theaters, or restaurants. When venturing out into the sun, the medical establishment has so forcefully encouraged sunscreen use, vitamin D is often not produced even while standing in direct sunlight if sunscreen is used as recommended.

In addition to widespread sunscreen use, clothing choices can predict an individual’s serum vitamin D levels. In populations whose cultural or religious garb covers most of their skin, the amount of vitamin D produced in the tiny areas of exposed skin is not enough to meet minimum serum vitamin D levels. Supplementation and dietary inclusion of vitamin D-containing foods is the most realistic recommendation for members of these populations, as increasing sun exposure would require breaking their own societal laws. Research has been done on hair-coated animals such as dairy cows in order to determine the effect of their hair and other coverings on vitamin D synthesis. The purpose of these studies is to examine the assumption that if the cows produce vitamin D in their hair much like humans produce vitamin D in our skin, clothing the cows would obstruct much of the vitamin D production. Hymøller, et al (2010) designed a study that examined dairy cows and varied their level of sun exposure using garments.  According to the concluding statements of this study, “the [serum vitamin D level] of the cows after 4 weeks of access to summer sunlight depended heavily on the size of the exposed body surface area. The larger the exposed body surface area of a given cow, […] the higher the serum vitamin D level of the cow at the end of that study.” These findings indicate that cows in their natural state synthesize vitamin D in their hair much like humans synthesize vitamin D in the skin, but adding clothing to the cow interferes with its vitamin D making ability. This experiment serves to underscore the importance of direct sun-to-skin exposure for humans, as clothing prevents or severely limits the amount of vitamin D produced in the skin.

Individuals with darker skin tones are naturally more susceptible to vitamin D insufficiency and deficiency. Progressively darker skin is progressively more susceptible to vitamin D insufficiency. Darker skin, which contains more melanin than relatively lighter skin, offers natural protection from UV radiation. This means that darker-skinned individuals need to spend more time in the sun before pre-vitamin D cells in the skin are prompted to produce vitamin D. According to Hall, et. al. (2010), “Melanin, the principal skin pigment, reduces but does not block cholecalciferol [vitamin D₃] synthesis. Thus, longer periods of sun exposure are required for equivalent vitamin D synthesis in people of African ancestry compared with those of European ancestry.” Also, “at all latitudes the skin must have enough melanin to block harmful solar UV while letting in enough UVB for vitamin D synthesis. Since the tropical zone has intense year-round sunlight, even very dark skin can produce sufficient vitamin D for the body’s needs. […] however, when dark-skinned humans move to the temperate zone […] their skin now screens out too much UVB and produces too little vitamin D.” (Frost, 2009). According to Cannell (2008), people with dark skin need to spend 5-10 times longer in direct sun exposure than do people with light skin to achieve similar vitamin D production.