Understanding Vitamin D: Science and Sunlight

We all talk about how important vitamins and minerals are for health. But if we knew more about why they are so important, we would be more likely to eat the foods that contain these vital vitamins and minerals. To start, I have chosen to explain the why of vitamin D.

Part 1: The Science Behind Vitamin D

Vitamin D is a fat-soluble vitamin (binds with fat in the body to be absorbed) that has an inactive form under the skin. When sunlight hits the skin, it actually activates this vitamin so it can do its various functions in the body. Vitamin D can also be found in its active form in some foods such as fatty fish, milk, and eggs, but it is in small quantities. This really makes you see that sunlight is necessary to maintain adequate vitamin D in the body. Moreover, vitamin D is stored in the body. So, when winter comes around and there is minimal sunlight, most of our bodies rely on the vitamin D stored in fat. If you find that you are not receiving adequate exposure to sunlight or a blood test reveals you are vitamin D deficient, it would be wise to take a vitamin D supplement to ensure you get adequate amounts of it.

Vitamin D’s main job in the body is to maintain adequate calcium levels in the body. For example, when calcium levels are low, the kidneys increase the production of vitamin D, so that vitamin D can then travel to the small intestine to increase intestinal absorption of calcium.

However, vitamin D is now found to have other important functions (besides maintaining calcium balance) as well. It has been discovered that other organs and tissues in the body possess vitamin D receptors (structures on the surface of a cell that selectively receive and bind a specific substance), such as lungs, colon, prostate, and breast. Bone and epidermal (skin) cells also have vitamin D receptors. This is important because vitamin D actually functions as a hormone (a hormone is a compound that exerts a biological response when it binds to its target tissue). So, when vitamin D binds with its receptor it causes a biological reaction in the tissue.

Furthermore, vitamin D is responsible for causing a very important biological reaction called antiproliferation. That is, it acts to decrease cell growth and cell division, which makes vitamin D an important anti-cancer agent. Vitamin D has been found to be an antiproliferative agent for cultured tumor cells (a tumor is an abnormal lump or growth of cells) such as tumor breast, colon, lung, prostate, and melanoma cells that possessed a vitamin D receptor.

Another type of cell with a vitamin D receptor is a promyeloid leukemic cell (a type of white blood cell that functions in the immune system). An interesting finding was that when vitamin D binded to its receptor in these leukemic cells, the biological reaction caused the leukemic cells to become macrophages (a type of white blood cell that works with the immune system to rid the body of toxic substances). The macrophages engulf various toxins and foreign substances in the body that are then to be excreted.

Vitamin D has also been linked to certain skin conditions such as psoriasis. Psoriasis is a hyperproliferative (an abnormally high rate of cell growth and cell division) skin disorder and the anti-proliferative action of vitamin D is now being recognized as a safe and effective treatment for psoriasis.



Part 2: Recommendations For Getting Adequate Vitamin D

Daily exposure to sunlight provides most people with their vitamin D requirement. However, due to the increased awareness about the relationship between long-term exposure to sunlight, and skin cancer and wrinkling, it is important to establish guidelines for sun exposure, so that the body can get its vitamin D requirement.

It is generally recommended for children and young adults to be outdoors without sunscreen for approximately 5-15 minutes, at least 2-3 times per week, in order to get the required amount of vitamin D. The elderly population however, is recommended to have higher vitamin D requirements. This is because elderly persons actually have a decreased capacity to produce vitamin D in their skin. Furthermore, the number of vitamin D receptors decrease with aging, so vitamin D has fewer receptors both to bind to, and do its functions in the body. Therefore, it is important for the elderly population to make sure they get sun exposure for 5-15 minutes at least 3 times per week.

The Institute of Medicine recommends that the adequate intake of vitamin D is 200 IU for 1-50 year olds, 400 IU for 51-70 year olds, and 600 IU for 71 years and older. However, these recommendations are said to be outdated, and new research suggests that higher intakes are necessary. In times of inadequate sun exposure, it is recommended that elderly persons consume 1000 IU/day of vitamin D, either through dietary sources that naturally contain vitamin D (e.g. dairy products, whole eggs, fatty fish) and fortified foods (e.g. juices, milk, and cereals), or a vitamin D supplement. These recommendations are crucial to all of the elderly population, but especially to those living in nursing and rehabilitation homes.

It has been observed that after a healthy person is exposed to 20-30 minutes of sunlight, their circulating vitamin D levels (within the body) are comparable to an intake of 10,000 IU of vitamin D. This shows you how powerful the influence of sunlight is on increasing vitamin D levels. In the absence of sunlight, such as in the winter months, it has been noted that 1000 IU/day of vitamin D is required to maintain a sufficient level of vitamin D in the blood of at least 30 to 40 ng/mL (the normal range of blood levels of vitamin D is approximately 30 ng/mL to 74 ng/mL). One study examined the effects of vitamin D fortified orange juice over a 12-week period. The subjects in the study drank one 8-ounce glass of orange juice per day that was fortified with 1000 IU of vitamin D. The results suggested that 1000 IU/day of vitamin D maintained adequate blood levels of vitamin D.

If you get a blood test for vitamin D, and your level is below 20 ng/mL, it is recommended that you increase your intake of vitamin D fortified foods and/or a vitamin D supplement of 1000 IU/day, in times of inadequate sunlight. In addition, you should make sure to get the recommended sun exposure as previously discussed, in order to increase your blood level of vitamin D.

For a more unorthodox approach to vitamin D supplementation, Dr. Michael Holick, a scientist and physician, actually prescribes 50,000 IU of vitamin D once a week for eight weeks (for those who are vitamin D deficient). He notes that this method “fills the vitamin D tank.” He also prescribes an alternative of 2,000-4,000 IU of vitamin D per day for 60 days, in order to correct a vitamin D deficiency.

If you are going to start a vitamin D supplement regimen however, it is imperative that you seek a physician’s advice. If supplementation of vitamin D is taken in too large a dose (evidence indicates ingestion of 10,000 IU/day for prolonged periods) toxicity can occur. It should be noted that sun exposure does not cause intoxication of vitamin D.

The bottom line is that, if you get moderate exposure to the sun in the spring, summer, and fall, you will saturate your body with vitamin D, and the resulting excess vitamin D that is stored in the body's fat can then be used during the winter months. And, to make sure you maintain adequate vitamin D levels during times of insufficient sun exposure, it would be wise to take a vitamin D supplement, or have enough vitamin D from fortified foods for insurance.



References:

Shils, ME, Shike, M, Ross, AC, Caballero, B, Cousins, RJ. (2006). Modern nutrition in health and disease, tenth edition. Philadelphia: Lippincot Williams & Wilkins.

Holick MF. Photobiology, metabolism, and clinical applications. In: DeGroot LJ, ed. Endocrinology. Philadelphia: WB Saunders, 1995:990-1013.

Eisman JA. 1,25-Dihydroxyvitamin D3 receptor and the role of 1,25-Dihydroxyvitamin D3 in human cancer cells in vitamin D. In: Kumar R, ed. Vitamin D. Basic and Clinical Concepts. Boston: Martinus Nijhoff, 1984.

Tangpricha V, Koutkia P, Rieke SM et al. Fortification of orange juice with vitamin D: a novel approach for enhancing vitamin D nutritional health. Am J Clin Nutr. 2003;77:1478-1483.

Holick, MF. High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc. 2006;81(3):353-373.

Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, DC: National Academy Press, 1997:S1-S13.