I take around 2,000 IU of Vitamin D a day throughout the winter, and since October 1, 2008, I have averaged 1433 IU of Vitamin D per day. Next year, I will probably up that amount to around 3000 IU/day. It may seem obvious that supplementing Vitamin D is important during the winter months, when the sun is not strong enough in the northern U.S. to cause our skins to manufacture the vitamin, but I often wonder if I should supplement during the months when my skin can make the vitamin. I have thought about this because I want to make sure my levels are optimal, and I also don’t want to waste money taking a pill when the sun is providing plenty for free. By the way, according to Drs. Roizen and Oz, if you live north of North Carolina, the sun is not strong enough from October 1-April 15 to consistently cause our skin to produce Vitamin D.
I found the chart above that shows that Vitamin D levels tend to reach higher levels in the middle of summer, and start to decline by the end of the summer. However, the Vitamin D Council recommends a blood level of at least 50 ng/mL of calcidiol, which is not even reached, on average, in the chart above, so it is possible that most of us aren’t out in the sun enough to even make a basic amount of Vitamin D, even in the summer. I seem to recall that only lifeguards and a few others who were in the sun daily had truly normal levels without supplementing! Research I came across a few months ago (that I cannot seem to find again!!) suggests that by the late summer most of us are actually Vitamin D deficient, probably because tanned skin no longer produces a lot of Vitamin D. I believe this study suggested Vitamin D levels peak around May and June, and decline until spring the next year. I am guessing that I am one who has to watch my Vitamin D levels during the summer. I tend to get out in the sun a lot in April and May, so by the mid summer, I have a pretty strong tan (I am naturally a little darker-skinned, and tan easily). Also, research suggests that dark skinned people require more sunlight to make Vitamin D, which, if you live near the equator works out fine, but if you are dark-skinned and live far from the equator (like in the northern U.S. or Canada) you will have trouble making adequate Vitamin D from sun exposure.
I was out in the noonday sun for about 25 minutes a few days ago, and I also ran outside the last few days during peak hours, and given that it is past April 15th, I theoretically made plenty of Vitamin D. I am going to hold off on Vitamin D supplements for a few weeks if I consistently get this kind of sunlight, and may not start supplementing again until July or August, and even then, I will probably stick with 400 IU/day until October begins. Outside of getting regular blood tests, I don’t know how to consistently regulate my Vitamin D levels in the spring and summer months, but this post shows I am trying to make an educated guess about it. I have emailed the Vitamin D Council with this basic concern, but have yet to get a response. If I get one, I will share it.
Chart from When Your Body Gets the Blues by Brown and Robinson