Vitamin D: Importance of the Sunshine Vitamin

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I find vitamin D fascinating. It is classified as a vitamin and present in small amounts in a few foods. But vitamin D is a hormone that is produced by our own skin with exposure to the sun. Living in Phoenix, you would think it would be no problem to make enough vitamin D. But you can imagine my surprise when I tested slightly low for vitamin D in my blood at the end of August! What’s more, I had been supplementing vitamin D daily as well. This discovery, along with an excellent lecture I attended at the AND’s Food and Nutrition Conference and Expo (FNCE) 2016, inspired this blog post on vitamin D. If I was low, maybe you are too!

Functions of Vitamin D

Most people know that vitamin D is important for bone health because of its role in calcium and phosphorus regulation. When a child is deficient in vitamin D, they develop a condition called rickets in which the bones are malformed. When an adult is deficient, they can develop osteomalacia, a condition in which the bones soften and are more prone to fracture.

But vitamin D has revealed itself to be important in many other critical body functions including the immune, cardiovascular, respiratory, and skeletal muscle systems. For example, in the presentation titled, “Vitamin D: Friend or Foe?” at FNCE 2016, research was presented that demonstrated a lower incidence of upper respiratory tract infections with higher vitamin D concentrations in blood. A nod to the importance of vitamin D during the winter months when it becomes difficult for our body to make it (read on to learn more).

Optimum Levels of Vitamin D

When your doctor orders a vitamin D lab, they are measuring the form of the vitamin named 25(OH)D, which is subsequently converted to the “active” form by your body when needed. It is preferable to measure 25(OH)D because it reflects vitamin D storage in the body, and not the hormonal concentration of the active form, which can fluctuate greatly.

Per the Institute of Medicine (IOM), a 25(OH)D level of <20 ng/ml is considered deficient. Under 30 ng/ml is considered insufficient, and sufficiency is reached at levels above 30 ng/ml. The ideal range is disputed. Many researchers argue the IOM’s standards are too low. An ideal level may lie somewhere between 40 and 100 ng/ml, as stated by the FNCE presenters. The Vitamin D Council recommends a level of 50 ng/ml as ideal. Levels that reach over 150 ng/ml are regarded as toxic and lead to hypercalcemia (too much calcium in the blood).

Sources of Vitamin D: Sun Exposure

The primary source of vitamin D is the exposure of your skin to UVB rays from the sun. In the time it takes your skin to turn slightly pink in the sun, your body can make 10,000 to 25,000 IUs of vitamin D. Many factors affect your body’s ability to produce vitamin D including skin pigmentation, age, weather conditions, and sun screen use. A sunscreen with a SPF of only 8 blocks your skin’s ability to produce vitamin D! Another interesting deterrent is glass; UVB rays cannot penetrate glass (and your body will therefore not produce vitamin D while you’re driving in the car, for example).

Exposing our skin to the sun is tricky business, especially here in Phoenix where I live and skin cancer is a valid concern. Per Dr. Michael Hollick, renowned vitamin D researcher, safe sun exposure involves exposing your arms, legs, and torso 2 times a week for 5 to 30 minutes close to solar noon (when the sun is at its highest elevation in the sky and the angle of the rays are correct for vitamin D synthesis). He has created a helpful app called “dminder” to help you track your window for vitamin D synthesis.

Perhaps most significant, the latitude at which you live is greatly important to vitamin D synthesis from the sun. If you live above a latitude of 35 degrees, you cannot make vitamin D in winter months (this includes much of the United States!). For example, Boston is located at 42 degrees North, which means that no vitamin D can be produced in the skin of Bostonians from November through February. As you can see, in some locations, for certain periods of the year, supplementation is the only option.

Sources of Vitamin D: Food and Supplements

Vitamin D availability in food is relatively limited when you consider that 10,000 to 25,000 IUs can be produced by your own skin in a relatively short amount of time. Compare that to the quantities available in the best food sources:

  • 1 tbsp cod liver oil = 1300 IU
  • 3.5 oz wild salmon = 980 IU
  • 1 oz sun-dried shitake mushrooms = 500 IU
  • 1 cup fortified milk = 100 IU

The current RDA for vitamin D is 600 IU daily for most adults. It is thought that this level may be too low to maintain levels of 25(OH)D in the blood above 30 ng/ml (the lowest level considered to be sufficient). The Endocrine Society recommends that individuals with limited sun exposure require at least 1500 to 2000 IU daily.

Supplement Options

When you visit the vitamin section of your local grocer, you will see vitamin D2 and vitamin D3 in various dosages. Vitamin D2 is a vegan form of the vitamin, if that is a concern for you. If it is not, opt for vitamin D3 as it is more effective at larger doses and less likely to be toxic than D2, as instructed by Dr. Enette Larson-Meyer at FNCE. Keep in mind also that multi-vitamins usually contain 400-1000 IU, so factor that into your dosage. Finally, as noted above, cod liver oil is a good source of vitamin D (it used to be given to children with rickets in the old days) but it may be too high in vitamin A. Better to find an individual D3 supplement at the appropriate level for you.

It is recommended that you discuss your vitamin D level with your doctor, and get your 25(OH)D tested before supplementation.

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