As we move into the unofficial start of summer with Memorial Day today, this is a particularly important topic. We are seeing increased outdoor activity with less amounts of sun protective clothing-especially as we open up at the end of the Covid pandemic. Sunlight and vitamin D production are intimately related. Most days I get questions about vitamin D: is it important, what it does and how much to use. This question is particularly important in the area we live since more than half of the year we have typically more cloudy skies and less sun exposure throughout the end of the fall, winter and early spring. In December, Omaha gets on average 134 hours of sunshine per month with an average UV index of 1/10. However in July, Omaha gets 333 hours of sunshine per month with an average UV index of 9/10. So in July, this means that not only do we get more hours of sunlight per day, the sun is much more direct with increasing UV radiation. The amount of sun exposure is not only important for vitamin D production in our body but also important for our skin health.
Vitamins are organic compounds that are needed for a variety of metabolic and cellular functions throughout the body. Vitamin D is a fat soluble vitamin. Other fat soluble vitamins are vitamin A, vitamin E, and vitamin K. The water soluble vitamins are B vitamins and vitamin C. All vitamins with the exception of vitamin D are acquired through diet. Vitamin D is the only one that humans can synthesize on our own. Very few foods with the exception of eggs, some mushroom and fish liver (cod liver oil is the classic example, yum!!) contain vitamin D. For example one large egg contains about 50 IUs of vitamin D. The recommended daily allowance or RDA runs anywhere from about 600 to 800 IUs daily to prevent vitamin D deficiency.
The well-known functions of vitamin D are to increase calcium absorption from the G.I. tract as well as bone metabolism. If we do not have enough vitamin D, we do not absorb calcium very well. There are likely other functions of vitamin D which are being actively researched.
Vitamin D is synthesized in the skin from a form of cholesterol. Approximately 10 to 15 minutes of exposure to the sun on the arms and face between 10 AM and 3 PM during spring, summer and fall is enough to produce sufficient vitamin D synthesis in individuals with light skin pigmentation. As skin pigmentation increases, the amount of sun exposure needed to produce the same amount of vitamin D increases. Individuals with very dark pigmentation may require 6-10 times (1-2 hours) of sunlight exposure to produce sufficient vitamin D.
Other factors that play a role in the amount of vitamin D produced: Factors that reduce vitamin production include small amounts of skin exposed, wearing sunscreen, cloud cover, lower altitudes, higher latitudes (particularly above 40°N (Omaha is about 41°N), morning/evening hours (lower sun angle), UV blocking clothing, obesity (more fat sequesters circulating vitamin D) and staying indoors. Sunscreen with an SPF of 30 reduces vitamin D synthesis by as much as 95% (but it also blocks the harmful UV rays that damage skin...more on that in my next post).
It is difficult to measure each and everyone of those factors at all times estimate whether an individual is producing enough vitamin D. That is where lab work comes in the play to verify whether or not someone is truly vitamin D deficient. Prior to the use of vitamin D supplementation either as a supplement or food additive, vitamin D deficient individuals could potentially develop rickets in children or osteomalacia in adults. These were bone formation and metabolism disorders from severe deficiency. Since there are now vitamin D fortified foods such as dairy products and formulas, these severe disorders are much less common. However osteoporosis, which is low bone density disorder, is partially attributable to low vitamin D.
There is some controversy about the optimal levels of 25(-OH) vitamin D (the main circulating form of vitamin D, and the best measurement of the adequacy of vitamin D amounts). Most lab reference ranges are between 30 and 80 ng/mL. The Institute of Medicine (IOM) supports levels above 20 ng/mL. Other experts including the National Osteoporosis Foundation, International Osteoporosis Foundation and the American Geriatric Society recommend a minimum level of 30 ng/mL, particularly in older adults to minimize risk of fractures.
Let‘s assume that you are living in Omaha, NE with minimal sun exposure during the winter and early spring, and you are diligent about putting on sunscreen throughout the year to help reduce your risk of skin cancer. More than likely you will be vitamin D deficient, particularly towards the end of winter into the spring. Optimal dosing of vitamin D supplements is difficult to define. There are many factors that play a role in the dosing of vitamin D supplementation. There are two main vitamin D supplements, vitamin D2 or ergocalciferol, and vitamin D3 or calciferol. Vitamin D3 is likely a more efficient route to getting increased blood levels of vitamin D. Different healthcare professionals have different approaches, some will use high-dose vitamin D2 weekly for 6-8 weeks then a smaller daily dose of vitamin D3 daily after that. Others like myself will typically use anywhere from 1000 to 5000 IUs of vitamin D3 on a daily basis rather than a larger doses of vitamin D2. in general, most people will not get into too much trouble with vitamin D excess if you keep your vitamin D3 supplement at 2000 IUs or less. If you are considering vitamin D supplementation, and if you are my patient, we can check your vitamin D levels and try to figure out what you were optimal dose would be.
In my next post we will discuss the issues with sun exposure and skin health, particularly since we are going into summer. Happy Memorial Day!!