Your EXACT vitamin D status in YOUR city (important immune system info for winter)
And I'm sure you've heard about the plethora of studies linking higher vitamin D levels to lower risks of cancer, heart disease, diabetes, and many other diseases.
Obviously, paying attention to your vitamin D levels is extremely important to your health!
But is supplemental D as good as the sun? Almost, but not quite...
Sure, you can supplement with a good oil-based vitamin D3 throughout the winter. I personally do this in winter, although I don't supplement in the summer since I personally get plenty of daily sun with outdoor activities like hiking and biking, and if you supplement too much in the summer, your D levels could actually go too high (this is rare though).
However, many vitamin D researchers believe that the D obtained from mid-day sun produced in your skin (and then absorbed into your bloodstream) from exposure to UVB rays is the best utilized source of vitamin D in your body, and more effective than any oral vitamin D.
So even though you can supplement with D3 in the fall and winter, I prefer to try to get as much D as I can from the sun until the point in the year known as the "vitamin D winter", which is different based on what latitude you live.
As I mentioned in this detailed blog post about this topic, expert vitamin D researcher, Dr Joseph Mercola estimates that the sun needs to be higher in the sky than approx 30-40 degrees above the horizon in order for UVB rays to be strong enough to trigger vitamin D production in the body. He previously estimated this as 50 degrees above the horizon, but later modified that stance after some additional research into it.
I think 30 degrees in the sky is a bit low, and would not be high enough to trigger vitamin D production if there's any haziness in the sky, smog, or air pollution. On the other hand, as an example, if you're hiking in the mountains at 9000 feet elevation with clean air and clear skies, a sun angle of 30 degrees above the horizon might be just barely a high enough sun angle to produce vitamin D in your body.
I recently found a table on a Navy website that gives you exact calculations of the sun's height in the sky on any day of the year and at any location -- latitude being the key here.
For example, I've been on a road trip through California this past week, and we did some hiking near Los Angeles the other day. Because Los Angeles has a significant amount of air pollution, I was looking to see how long during the day the sun went above 40 degrees height in the sky, since 30 degrees height would not be sufficient for UVB rays to penetrate the LA smog. According to that Navy sun angle table, the sun angle was above 40 degrees from 11:10 am to approx 2 pm Pacific time on Oct 22 (with a peak height in the sky of 44.6 degrees at 12:30 pm), the day we did the hike. This means there was about a 3 hour window mid day to produce vitamin D if you were to get out in the sun on Oct 22.
Once we get to Nov 22nd, the peak sun height is only approx 35 degrees max height in the sky mid-day, and considering the air pollution in LA, this is probably not a high enough sun angle to produce vitamin D. This low sun angle will continue through January before the sun gets high enough in the sky again for vitamin D production in February in LA. So as you can see, even in sunny Los Angeles, you have approximately 3 months per year as your "vitamin D winter".
A couple more examples of your time of day to produce Vitamin D based on different cities on Oct 24:
New York City Sun Angle on Oct 24th:
Peak sun angle: 37.3 degrees height above horizon
Time above 35 degrees height = 11:40 am to 1:40pm approx.
If air pollution is heavier and you need a sun angle above 40 degrees, as you can see, you can't produce any vitamin D at all in the end of October in NYC. If it's an absolutely clear day with no clouds or smog, perhaps you have 1 or 2 hours MAX mid day to still produce at least some vitamin D in your skin.
For comparison, in the highest sun angle month of June, in New York City latitude, you have approximately 8 hours in the day (9am to 5pm) that the sun angle is high enough in the sky to produce vitamin D, with a peak sun height mid-day of approx 73 degrees above the horizon.
Coincidentally, those are the same hours each day that most people are locked inside offices though, which is why many people can be vitamin D deficient even in the summer months. Sadly, most people are also "sun-phobic" and lather up with sunscreens when they do actually get outside, so this also blocks any vitamin D production in the skin.
Seattle Sun Angle on Oct 24th:
Peak sun angle: 30.4 degrees height above horizon
Since the sun barely gets above 30 degrees height in the sky, as you can see we're already in the "vitamin D winter" in Seattle. Cities at a similar latitude to Seattle would also be in the "vitamin D winter" already.
Dallas Texas Sun Angle on Oct 24th:
Peak sun angle: 45.2 degrees height above horizon
Since Dallas weather might have some typical haze and/or smog, we'll assume that we need at least a 40 degree sun angle to produce vitamin D...
Time above 40 degrees height = 11:40 am to 2:35 pm approx.
As you can see, at the end of October in Dallas, Texas, you still have at least a 3-hour window mid-day on a sunny day that you could produce vitamin D if out in the sun without sunscreen on. It's possible that window is a little more than 3 hours if the minimum sun angle is a little lower than 40 degrees.
Your "vitamin D winter" at your latitude...
What you can see from these examples is that in many parts of the northern hemisphere, based on how far north you live, you could already be in your "vitamin D winter", whereas in more southern locations of the US or Europe, you could still have a few weeks of good mid-day sun remaining for vitamin D production to keep you healthy and with a strong immune system this winter. If your blood levels of vitamin D are in an optimum range of approx 50-80 ng/dl, it may take more than 30 days without sun this fall/winter for your levels of vitamin D to dwindle to lower levels that start to affect your immune system.
Note that Miami has a max sun angle of 40 degrees height in the sky in the end of December, which represents the lowest sun angle of the year in the northern hemisphere. So you can likely produce vitamin D all 12 months of the year in Miami, as well as Hawaii, which is the lowest latitude location in the US.
As a general guideline, for the northern half of the US, your vitamin D winter is approximately from mid-October through the end of February. In Canada, this would be a few weeks longer on both ends. The same guidelines would apply for Europe at similar examples of latitude that I gave in this article.
The big takeaway from this is that vitamin D supplementation might be necessary for you if you want to keep a strong immune system and hormone balance from approximately October through March if you live north of an imaginary line from Atlanta to Los Angeles (both are around 34 degrees North Latitude). If you live further south of this, your "vitamin D winter" will be shorter, and if you get as far south as Miami or Hawaii, you can actually produce vitamin D all 12 months of the year on a clear sunny day at mid-day.
Note that new research is showing that you NEED good levels of vitamin K2 if you're supplementing with vitamin D3. Vitamin K2 is essential for the proper utilization of vitamin D in your body. Vitamin K2 can be found in grass-fed dairy fat (butter and cream), organ meats, and is also supplied in Athletic Greens, which is my favorite drink that I use first thing every morning.
Speaking of vitamin K2 and it's relationship to preventing calcification in the body (in joints, arteries, etc), please make sure to also read my article about vitamin K2 in dairy fat and how this helps PREVENT heart disease. This is something not many people understand, considering that dairy fat has been falsely implicated in heart disease since the anti-fat propaganda became popular in the 80's.
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Certified Nutrition Specialist
Certified Personal Trainer