How 2 people can stand in the same sun & make completely different amounts of vitamin D ....beyond skin type!
Same sun - 2 different amounts of vitamin D?
Many people assume that if 2 people are standing side by side in the same location - that they would make similar amounts of vitamin D, but it's just not that simple.
This week I got a few questions about how we calculate vitamin D in MyCircadianApp, and it sparked some useful questions I thought might be worthy of writing about - including the nuance beyond skin type.
So today's article will talk about a few factors that impact vitamin D levels, and why looking at just one marker might be short sighted.
Keep reading for the article....
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In spring 2024 when my friend Dr. Sara Pugh was visiting, we spent nearly identical time outside, but my vitamin D3 levels (active vs. storage was a 1:1 ratio) were close to 60 by the end of May, but hers stayed at 25 (same active to storage ratio). Why?
Sunlight isn’t a one-size-fits-all input, and much vitamin D you make depends on a mix of biology, environment, timing, and even other factors like hydration, genetic mutations and deuterium (covered at the end of this article).
This is why MyCircadianApp asks for immediate factors like skin type, weight, and location, because these details determine how your body translates light into vitamin D, and how much time you can spend in direct UV to avoid the risk of overexposure.
I also want to add that vitamin D isn’t just “one hormone.” The body produces dozens of metabolites from vitamin D, and researchers are still discovering their roles in immunity, metabolism, circadian signaling, and even mitochondrial health.
That means we don’t yet have the full picture of what optimal vitamin D status really means - or what optimal levels from sunlight really should look like. What we do know is that sunlight creates a symphony of metabolites that supplementation alone may not replicate.
Click here to read my blog post on why you can't overdose on vitamin D from sunlight, but you can from supplements.
Let's go over some of those immediate factors that impact storage D3 levels. ⬇️⬇️
We will start with the most obvious factor, and that is skin type/shade.
Melanin acts like a natural sunscreen, and the higher your melanin content, the more UVB is filtered before it can trigger vitamin D synthesis. This means individuals with darker skin often require longer exposure to make the same amount of vitamin D as those with lighter skin.
Respecting and understanding your skin type is essential for balancing protection and production, and this is another reason I started questioning people who say that fitzpatrick skin type 1 people should aim to spend endless hours per day outside in direct sunlight.
One thing Sara and I reflected on in 2024 was the fact that she was quite a bit tanner than me, which is one potential cause of the difference in our bloodwork (I still think it was deeper than this, but will talk more about that later).
This is one factor that shocks people: Elevated BMI & Leptin Resistance both create scenarios where it is more difficult for the body to make vitamin D.
I have a video and a more in depth article coming on this next week - so stay tuned if this is a point of interest.
Body composition changes the equation, and studies like Wortsman et al. (2000) show that individual with an elevated BMI have a 57% lower rise in vitamin D after the same UVB exposure compared to lean individuals.
Why? Vitamin D is fat-soluble - it gets sequestered in adipose tissue, lowering the amount circulating in blood. Others, like Drincic et al. (2012), highlight volumetric dilution - more body mass spreads vitamin D thinner across the system.
This research done on adolescents showed that once body fat climbed past about 30–40%, leptin shot up while vitamin D levels fell.
This means two people, side by side, can walk away with very different levels, especially if one is leptin resistant & the other is leptin sensitive,
(and no - I've seen her labs, and Dr. Sara is very leptin sensitive - so this is not a factor in why our levels were different)
Where you live plays a massive role in vitamin D production.
At higher latitudes or during winter, the sun’s angle makes UVB rays too weak to trigger vitamin D synthesis - even if you’re outside for hours.
Research shows that both latitude and season directly influence how much vitamin D you can make (Webb et al., 1988;Engelsen, 2010).
In other words, someone living in Florida at noon in July won’t get the same outcome as someone in Canada in January, and someone lives on the 33rd latitude year round (me) - vs. someone who lives on the 54th latitude (Dr. Sara) would potentially have higher D3 levels by default as long as they lived a circadian aligned life.
This is another scenario where I question "your vitamin D levels should be above 60 at a minimum" - as I don't believe that would have been the case ancestrally for our far northern latitude friends.
The strength of the sun at the time you’re outside matters.
The UV Index measures how intense UV radiation is in your location at any given hour, and research confirms that higher ambient UV levels directly predict serum vitamin D concentrations.
In practical terms, 10 minutes of sun at a UV Index of 7 is not the same as 10 minutes at a UV Index of 3. Timing and intensity shape the outcome (always use the Vitamin D timer in MyCircadianApp in stronger UV).
Now that being said: I think it is noteworthy - in my clinical experience - that going out in a UVI of 10 or 11 isn't always "better", and my fitzpatrick 1's & 2's do better with the earlier morning or later afternoon more gentle UVB.
But here’s the bigger picture: sunlight isn’t the only determinant of vitamin D status, and underlying health factors also matter.
Chronic infections and systemic inflammation can accelerate the breakdown of vitamin D. Certain genetic mutations affect how vitamin D is activated or transported. Dehydration and poor cellular water structure make it harder for the body to utilize vitamin D efficiently. And high deuterium levels - often from processed food and poor light environments - can impair mitochondrial function, reducing how effectively your cells respond to vitamin D signaling.
While there is no app that can take these things into account when estimating vitamin D levels, these factors are important to take into consideration if you are looking at your bloodwork & searching for answers.
In summary: Sunlight exposure is crucial to health, but there are many factors that impact how much vitamin D your body makes, uses and stores at a particular time. Going beyond "surface level" information & standard answers about vitamin D is highly encouraged.
I plan on writing more on this topic - so stay tuned!
If you enjoy this type of deep dive information, and you are an advanced learner or practitioner:
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P.S. You don’t have to be a practitioner to dive into this.
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