Most of us assume we’re getting enough vitamin D. We spend time outside. We enjoy the sun when we can. We’ve been told our whole lives that sunshine is the source — and that feels like enough.
But for the vast majority of people, it isn’t. And the quiet, symptomless nature of vitamin D deficiency means that millions of people are running low on one of the body’s most essential nutrients without any idea it’s happening.
Understanding how vitamin D actually works — and why sun exposure alone is rarely sufficient — is one of the most important and overlooked pieces of foundational health I discuss with every client I work with.
How Your Body Actually Makes Vitamin D
Vitamin D is not passively absorbed through sunlight. Your skin produces it in response to a specific type of solar radiation called UVB rays. UVB is only strong enough to trigger meaningful vitamin D production during the middle hours of the day — roughly between 10 AM and 3 PM — when the sun is at a sufficient angle in the sky.
Morning walks and evening outdoor time, while valuable for many aspects of health, do not produce vitamin D. The sun’s angle outside of peak midday hours simply doesn’t deliver the UVB intensity required.
Research suggests that light-skinned adults need approximately 10 to 30 minutes of midday sun exposure on the arms and legs — without sunscreen — several times per week to produce meaningful vitamin D. Those with darker skin tones may need 30 to 60 minutes due to the natural UVB-filtering effect of melanin.
The Sunscreen Factor
SPF 30 sunscreen reduces vitamin D synthesis by up to 95%. This is not a reason to abandon sun protection — but it is a reason to be thoughtful about timing.
Unless you have a specific skin sensitivity or medical reason to apply sunscreen immediately upon going outside, consider allowing 15 to 20 minutes of unprotected midday sun exposure on your arms and legs before applying sunscreen. Don’t burn — that is never the goal — but give your body the opportunity to do what it was designed to do first.
For most people with busy schedules and indoor-focused workdays, getting outside consistently during peak midday hours with meaningful skin exposed simply isn’t realistic. This is one of the primary reasons supplementation is not optional for most adults — it is essential.
How Latitude and Season Affect Production
Where you live and what time of year it is dramatically affects your body’s ability to produce vitamin D from sunlight. Research has established that anyone living above approximately 35° to 37° North latitude experiences what scientists call a “vitamin D winter” — a period during which the sun’s angle is too low to trigger any meaningful vitamin D production in the skin, regardless of time spent outdoors.
For those in New England and the broader Northeast, this window runs from approximately November through February. In more northern regions it can extend even longer. But reduced UVB intensity affects vitamin D production across much of the country during winter months — meaning this is not exclusively a northern issue. Combined with indoor lifestyles, clothing coverage, and sunscreen use, most Americans in most regions are producing far less vitamin D from sun exposure than they realize.
Food Sources of Vitamin D
The best natural dietary sources of vitamin D include fatty fish such as wild-caught salmon, tuna, and trout, along with beef liver, egg yolks, and mushrooms that have been exposed to UV light. Fortified foods like milk and some orange juices contain added vitamin D as well.
While these foods are worth including in a well-rounded diet, the amounts of vitamin D they provide are modest. Relying on food alone to maintain optimal vitamin D levels is extremely difficult, and dietary sources cannot compensate for months of limited sun exposure. Supplementation remains the most reliable and practical solution for most people.
What Vitamin D Deficiency Actually Affects
Vitamin D deficiency is often silent — which is precisely what makes it so consequential. There may be no dramatic symptoms, only a gradual sense of running below your best. Fatigue that doesn’t resolve with rest. A mood that dips in the darker months. Increased susceptibility to illness. A vague sense of diminished vitality.
Over time, insufficient vitamin D affects a wide range of critical body systems. It is essential for calcium absorption and bone density. It supports immune function and the body’s ability to fight infection. It plays a meaningful role in cardiovascular health, with low levels associated with increased risk of heart disease. It supports mood regulation and cognitive function. It contributes to healthy muscle performance and reduces fall risk with aging.
And for metabolic health specifically: vitamin D plays a meaningful role in insulin sensitivity. Low vitamin D levels are associated with increased insulin resistance, which over time contributes to blood sugar dysregulation and elevated risk of prediabetes and type 2 diabetes.
Choosing the Right Supplement — and Why It Matters
Not all vitamin D supplements deliver equal results, and there are two things I want every person reading this to know before they choose a product.
First, the form matters. You want vitamin D3 specifically — not D2. D3 is the form your body naturally produces from sunlight and is significantly more effective at raising and maintaining vitamin D levels in the blood. Many inexpensive supplements use D2, which is far less effective and not the form your body is designed to use.
Second — and this is something most people are never told — vitamin D3 requires vitamin K2 for proper absorption and utilization. K2 acts as the essential companion that directs vitamin D to where it needs to go in the body, supporting both its effectiveness and its safety. Without K2, vitamin D3 simply cannot do its job as well. These two nutrients truly belong together, and any quality vitamin D supplement should include both.
Which is exactly why I love the Nanoemulsified D3K2 by Quicksilver Scientific. Not only does it pair these two essential nutrients together in one thoughtfully designed formula, but it delivers them via liposomal technology — a fat-based carrier that bypasses much of the digestive process and delivers the nutrients directly into the cells for dramatically superior absorption compared to standard capsule forms. It is one of the most bioavailable and well-formulated options on the market, and it is the one I personally use and recommend to every client I work with.
It is available through my Fullscript dispensary: Fullscript. Once you’re in the dispensary, you’ll find it under my Foundational Immunity Favorites collection — I’ve made it easy to locate!
I also recommend asking your healthcare provider to test your vitamin D levels — a simple blood test removes all guesswork and allows you to supplement with precision. And if you’d like to talk through what your results mean, I’m always happy to have that conversation. Book a call with me here.
The Bottom Line
Vitamin D is not a minor supplement. It is foundational to immune health, cardiovascular function, mood, metabolic health, and healthy aging. And for most people — regardless of where they live or how much time they spend outside — deficiency is far more common than adequacy.
Getting serious about vitamin D means understanding how your body actually produces it, making the most of the sun exposure opportunities you do have, and supplementing consistently with the right product — vitamin D3 and K2 together, in a form your body can actually use.
Your body needs this nutrient to function at its best. Make sure it’s getting it.
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