Why Coloradans Are Still Vitamin D Deficient (Even With 300 Days of Sunshine)
Why Coloradans Are Still Vitamin D Deficient (Even With 300 Days of Sunshine)
Here's a sentence I hear in our Centennial clinic, just south of Denver, almost every week: "I live in Colorado, I'm outside all the time, my vitamin D is fine."
Then we run the lab. And it's not.
Vitamin D deficiency is one of the most common — and most underestimated — nutrient issues we see in our patients. It cuts across age, sex, fitness level, and lifestyle. Marathoners with 25-hydroxy vitamin D levels under 20 ng/mL. Outdoor enthusiasts who hike every weekend. Patients who genuinely believe they're getting "plenty of sun." The deficiency is widespread, and the 300-days-of-sunshine story is doing real harm by making people think they don't need to check.
This post walks through what we actually see in clinical practice, why the Colorado sunshine myth doesn't protect you, what the lab numbers really mean, and what to do about it. No supplement hype, no fear mongering — just what a clinician would tell you if you asked.
The 300-Days-of-Sunshine Myth
Colorado does have a lot of sunny days. That part isn't a myth. What's misleading is the assumption that sunny days = adequate vitamin D synthesis. There are three reasons this breaks down, and they all stack on top of each other.
1. Latitude works against us for half the year
Centennial sits at about 39.6° north. That latitude matters because the sun has to be at a high enough angle (above roughly 50°) for the UVB wavelengths that trigger vitamin D synthesis to actually reach the ground. From roughly November through March, the sun in Colorado simply doesn't get high enough in the sky to produce meaningful vitamin D — regardless of how clear and bright the day looks.
You can stand outside on a brilliant January morning at 9,000 feet and produce essentially zero vitamin D. Your skin will still burn from UVA. Your sunglasses will still fog up. But the UVB needed for D synthesis is being filtered out by atmospheric angle.
2. High altitude is a paradox
Thinner air at higher elevations does let more UVB through. In theory, this should help. In practice, it doesn't, because the same conditions that make Colorado sunny also make Colorado cold for much of the year. We're covered up — long sleeves, pants, hats — during the months when UVB exposure would matter most. Skiing in March looks like sun exposure, but the skin that needs the UVB is fully covered by gear.
3. Modern indoor lifestyle
The third problem is the one nobody wants to talk about: even when conditions are good, most of us aren't outside very much. We commute, we work indoors, we drive home. The peak UVB hours (roughly 10am to 2pm in summer) are exactly when most people are at desks. The 30 minutes of summer evening sun in the backyard feels like a lot, but it's largely UVA — too low an angle for efficient vitamin D synthesis.
Stack these three factors and you get a population that looks like it should have abundant vitamin D — and doesn't.
What "Vitamin D Deficient" Actually Means
The 25-hydroxy vitamin D test (sometimes written as 25(OH)D) is the standard measure. Here's the working framework most clinicians use:
A few important nuances. There is genuine disagreement between major bodies on what counts as "sufficient." The Endocrine Society uses 30 ng/mL as the lower bound of adequacy; the Institute of Medicine has historically used 20 ng/mL. We tend to aim for the 40–50 ng/mL range for patients who are also addressing hormone, immune, or mood concerns, because the evidence on extra-skeletal benefits (beyond bone health) tends to point toward those higher levels.
One bad number doesn't define you. Vitamin D shifts seasonally, fluctuates with recent travel and sun exposure, and is influenced by what you ate that week. We look at the trend over time and the clinical picture together — not just a single value in isolation.
Why Vitamin D Matters Beyond Bones
The traditional teaching is that vitamin D matters because it regulates calcium absorption and bone health. That's still true. But the conversation has expanded a lot over the past two decades, and the evidence for vitamin D's role in other systems is now substantial:
- Immune function. Vitamin D receptors are present on most immune cells. Low levels are associated with more frequent respiratory infections and slower recovery from acute illness.
- Hormone metabolism. Vitamin D acts as a hormone itself and interacts with estrogen, progesterone, and testosterone metabolism. In our hormone therapy patients, optimizing vitamin D often improves how well a hormone protocol works.
- Mood regulation. The link between vitamin D and depression — particularly seasonal patterns — is consistent enough in research that it's now part of any thorough mood evaluation.
- Inflammation. Vitamin D modulates inflammatory cytokines. Patients with chronic inflammation often have low D, and correcting it can be part of a larger inflammation strategy.
- Muscle function. Vitamin D deficiency is associated with muscle weakness, falls in older adults, and impaired recovery from training.
Vitamin D will not cure cancer, reverse autoimmune disease, or be a single-cause explanation for whatever you're experiencing. The hype around it has gotten ahead of the evidence in some corners of the wellness world. But it's a real piece of a real picture, and ignoring it because the hype is overstated is its own mistake.
The Symptoms Most People Miss
Here's the inconvenient truth: many people with low vitamin D have no specific symptoms at all. That's part of why it goes undetected. When symptoms do appear, they tend to be vague enough to be attributed to something else — work stress, age, "not sleeping well lately."
Things we hear most often from patients who turn out to be deficient:
- Persistent low-grade fatigue that doesn't respond to better sleep
- Catching every cold that goes around the office
- Vague muscle aches, especially in the back and thighs
- Low mood or flat affect, particularly in winter
- Hair shedding more than usual
- Slower recovery from workouts
- "I just don't feel like myself, but I can't pin it down"
None of these is specific to vitamin D deficiency. All of them can be other things. But when they cluster — especially in a Coloradan who assumes the sun has them covered — it's worth checking the actual number.
Who's at Highest Risk in Colorado
Some Coloradans are at significantly higher risk than the average resident:
- Darker skin tones. Higher melanin content means less efficient UVB-to-vitamin-D conversion. The same sun exposure produces dramatically less vitamin D in darker skin than in lighter skin.
- Indoor workers. If you're in an office, lab, or warehouse during peak UVB hours most days, you're not getting meaningful sun exposure even in summer.
- Diligent sunscreen users. SPF 15+ blocks the vast majority of UVB. We absolutely don't recommend abandoning sunscreen — skin cancer risk in Colorado is real — but it's worth knowing that consistent use means you're producing very little vitamin D from sun.
- Older adults. Skin synthesis of vitamin D declines significantly with age. A 70-year-old produces roughly a quarter of the vitamin D from the same sun exposure as a 25-year-old.
- Higher body fat percentage. Vitamin D is fat-soluble. More adipose tissue sequesters more vitamin D, leaving less circulating in the bloodstream for the body to use.
- Certain medications. Long-term corticosteroids, some anticonvulsants, and a few other drug classes can lower vitamin D status meaningfully.
- Gut conditions affecting absorption. Celiac disease, Crohn's, ulcerative colitis, gastric bypass, and other malabsorption conditions all impair vitamin D uptake.
Testing — Why It Actually Matters
The only reliable way to know your vitamin D status is to test it. There's no symptom checklist that will tell you, no rule of thumb based on how much you're outside, no supplement dose that's universally correct.
The right test is a 25-hydroxy vitamin D level — sometimes called 25(OH)D or vitamin D, 25-hydroxy. It's a standard, inexpensive blood test that almost every lab runs. It's part of our comprehensive hormone and metabolic panels at Defiance because we use it to inform treatment decisions, not just check a box.
For most adults, an annual test is enough if levels are stable in the optimal range. If you're newly starting supplementation, recovering from a deficiency, or have one of the higher-risk profiles above, retesting at 8–12 weeks lets us confirm the dose is actually working before you settle into a maintenance plan.
Supplementation Done Right
If your level is low, supplementation works. The mechanics matter, though:
D3 over D2
Vitamin D3 (cholecalciferol) raises serum levels more efficiently than D2 (ergocalciferol). If you're going to take a supplement, take D3. It's the form your skin produces from sunlight and the form your body uses most easily.
Dose ranges, not specific prescriptions
Recommended adult daily intakes range from 600 IU (the IOM number for general adults under 70) to 1,500–2,000 IU (Endocrine Society guidance for those at risk of deficiency). Patients actively treating a deficiency are often dosed higher under clinical supervision — sometimes substantially higher — but those decisions belong in a clinical conversation, not a blog post. Vitamin D is fat-soluble. It builds up. Toxicity is rare but real, and it's worth not freelancing on high doses without testing.
Cofactors matter
Vitamin D doesn't work in isolation. Magnesium is required for vitamin D activation in the liver and kidney; low magnesium can mute your response to supplementation. Vitamin K2 helps direct calcium into bone rather than soft tissue, which is particularly relevant when you're optimizing D status. We test and consider both in patients who aren't responding to standard supplementation.
The honest summary: Test before you treat. Use D3, not D2. Don't go big on dose without supervision. And recheck in 8–12 weeks if you're correcting a deficiency, so you actually know whether the plan is working.
When Injections Make Sense
For most patients, oral D3 works fine. Take a capsule, get it absorbed, levels come up. But there are situations where injectable vitamin D is the better tool — and we offer it in our Centennial office for exactly those cases.
Common scenarios where we use injectable vitamin D:
- Significant deficiency where the patient wants to correct it quickly rather than wait 8–12 weeks for daily oral supplementation to catch up
- Absorption issues — patients with celiac, IBD, gastric bypass, or other gut conditions that limit oral vitamin D uptake
- Adherence concerns — patients who know they won't reliably take a daily supplement
- Combination with our broader care — patients already coming in for hormone therapy, weight loss, or other regular visits who'd rather handle it in-office than add another pill to their routine
Our pricing is straightforward: $30 per single injection, or a bundle of 4 for $100. Most patients addressing a real deficiency benefit from a series, which is why the bundle exists. We administer them during regular office visits at our Centennial location — no separate appointment required if you're already coming in for something else.
This isn't an alternative to testing. It's an alternative to a daily pill once we know your number and what dose you actually need.
The Defiance Take
Colorado's reputation for sunshine has done some of our patients a disservice. It makes it easy to assume your vitamin D is fine because you went hiking last weekend. The reality we see in lab after lab is that the assumption is often wrong, and the fix is usually straightforward — once you actually know your number.
If you've never had a vitamin D test, or it's been more than a year since your last one, that's a reasonable conversation to have with your provider — ours or anyone's. It's a five-minute lab order, and the answer changes what comes next.
Want to know your number?
Vitamin D is part of every comprehensive lab panel we run. Schedule a consultation to discuss whether testing is right for you.
Book a ConsultationThis blog post is for educational purposes only and does not constitute medical advice. Vitamin D requirements vary by individual, and all supplementation and treatment decisions should be made in consultation with a licensed medical provider. Individual results vary.