This is the Optimal Amount of Sun Exposure

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03/11/2014
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Dr. Dowd: We always think of this 15 minutes, how much is enough? Oh, fifteen minutes of casual sun exposure. What does that mean? Okay? Fifteen minutes of casual sun exposure to a Vitamindologist means 15 minutes with a UV index of six or seven, meaning mid-summer or...just say six or seven. That's mid-summer in northern half the United States. In Hawaii it's mid-winter. UV index of six or seven, 50 percent skin exposure is shorts and t-shirt or a bathing suit for 15 minutes, middle of the day. Because UVB light is only available in the middle of the day so early morning sun before 10:00 or 11:00 in the morning is all UVA. And then as the sun gets closer to the surface of the Earth at the middle of the day, there's less ozone to absorb UVB, that's when all your UVB is getting through and that's usually between sometime around 11--and it varies. This window closes with the winter and in the spring it opens back up. And here early in the day it's 10-11:00 in the morning, late in the day it's 4-5:00 in the evening and then as the season closes you close in at that solar noon which is somewhere afternoon and it's two hours, three hour--three two hours then it's gone. But it's the middle of the day, it's not -- I have patients who are in Florida and they say "Oh I don't need to take Vitamin D I'm going to my place in Florida we have a swimming pool, I'm out there the whole time I'm outside." First of all I don't believe they're outside the whole time because it's not comfortable outside the whole time. And yes they do come back with a tan, but remember a tan is a red flag. If you have dark skin that means you're at greater risk for Vitamin D deficiency not less risk. It may mean you've had more sun exposure, but when you really darken up it means you make lots of melanin and block the sun. So I say why don't we measure your Vitamin D before you go to Florida and we'll measure it when you come back and we'll see how much D you made outside in the sun. And usually they come back and their D has not changed or in many cases it's gone down. And they're shocked because they come back with this nice bronzed tan being at their pool and their D levels lower. I said "I told you so." The reason is--I said "Let me see what your day is like." They get up in the morning, they go outside, they work in the garden, they work in the yard. It's cool in the morning in Florida, the sun is out, it's nice, it's very comfortable. Around 11:00 it starts getting hot and sticky. They go inside. I'm going to do some bills, going to do my grocery shopping and then at 3:00 in the afternoon they go back outside. So the exact time of the day when they could be making all this D, they decide to do all their indoor work and they go back out when it's more comfortable. Not making any D when you do that. And so the key is making sure you're out in the sun with enough skin exposure, 50 percent, middle of the day, UV index has to be high enough. Then that 15 minutes may be enough. But otherwise at 15 minutes it's really pretty meaningless. Because most of us don't meet all those requirements. How many of us get 50 percent skin exposure, for 15 minutes with a UV index of six or seven? How many days a year does that actually happen?

Interviewer: Right. Not many.

Dr. Dowd: You can probably count them on your fingers and toes. It's just not that many.

Interviewer: And you're talking about per day, right? To get optimal amount.

Dr. Dowd: Well it may not need to be every day. Maybe three or four days a week is all you need to get that, but that's still -- we're lucky to get three or four days a month. And only in the season that it's available or it's warm enough to do that. I did a little calculation in the book and I counted something like 58 days out of the year was as many days as we could make Vitamin D. If you say I only do it on the weekends and only the weekends that have the right amount of sun exposure, which means limited times of the year, it came out to like 40 or 50 or something--50 plus or minus days in the Detroit area that you could make Vitamin D on a normal schedule. So you said okay this is how much Vitamin D I need on a daily basis, how much sun will I have to cram into those days. You'd have to be like baking in the sun all day and even then you couldn't store up enough to last because it decays quicker than that. The decay is faster than the length of your winter. And so it disappears before the winter is over. Which means it just keeps going down so you still run out of gas mid-winter.

Interviewer: So for those of us, you and I included that live in the northern half of the country. The winter, even if we go out there at noon on December 21st in our bathing suit on a sunny day, we're out of luck.

Dr. Dowd: You're not making anything. So in those mid-winter warm-ups where it's 60 degrees outside and you're dreaming about Spring and you think I'm going to put some shorts on, it's going to be great, you're not making any Vitamin D. Enjoy the sun, you might get burned even, because Ultraviolet A will cause your skin to burn or darken, but you won't make any Vitamin D.

Interviewer: How about in the summer when it's cloudy? There's times when we still can be outside on a slightly cloudy day and get a bit of a burn. On those days can we still get--

Dr. Dowd: You can still make Vitamin D if that UV index is above three. In mid-day sun, even on a cloudy day so you can still get sun but there's still lots of UVA coming through, there's probably less UVB coming through, but the UV index is a calculation that the weather the National Oceanic and Atmospheric Administration does, something like 80 percent of that calculation comes from UVB. Because historically we thought UVB was the bad stuff and UVA was the good stuff. And so they came up with a calculation to measure the sun's intensity based primarily on UVB so that they could use it to caution us against sun exposure. Thankfully they did that, because now we have a tool that we can use to help us know when we're making the most Vitamin D. Because if it was a formula based mainly on UVA, it wouldn't help us any. But since it's based primarily on UVB, we can use it as a tool, a surrogate tool to tell us how much Vitamin D making sunlight is available.
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Sun is essential for making vitamin D. But can too much sun be dangerous? How do you know how much sun is enough and when is the right time to get that amount? Dr. James Dowd explains what the optimal time of day and amount of sun should be for healthy vitamin D levels

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