Vitamin D Discovery: Why a Pharmacist’s 2-Year Routine Almost Failed
Vitamin D supplements fly off the shelves at my pharmacy every single day.
When I ask customers why they're taking it, I almost always get the same answer:
"I heard it's good for you."
Honestly? I used to be the same way. Back in pharmacy school, vitamin D was basically filed under "bone health" and left there. When I opened my own pharmacy, I didn't pay it much attention either. It wasn't until I started digging into research for a lecture I was preparing that things changed — and what I found genuinely surprised me.
So let me share what I learned, mixing in some real data, my own experience, and a few stories from people I know.
A pharmacist analyzing the latest 2024 Vitamin D clinical guidelines and deficiency data.
The short version — 3 things worth remembering:
1. Get outside for 10–15 minutes at lunch. Sunlight is more effective than any supplement.
2. Build a habit of outdoor exercise. Skipping exercise more than triples your deficiency risk.
3. If you're in a high-risk group, consider supplementing. This includes children, pregnant women, adults over 75, and those with prediabetes.
Below, I'll walk through why I landed on these conclusions — using research data and real-world examples.
How Low Are Vitamin D Levels, Really?
A major study came out in 2024. A research team led by Professor Eun-Hee Na at Chonnam National University Hospital analyzed vitamin D data from roughly 120,000 people who had health checkups across 13 South Korean cities between 2017 and 2022.
The average blood vitamin D level in that population: 21.6 ng/mL.
For context: a healthy level is 30 ng/mL or above. Below 20 ng/mL is considered deficient, and the 20–30 range is "insufficient." The average in this study was already in the insufficient zone.
Now, you might be thinking: "That's a Korean study — does it apply to me?" Fair question. But the picture in the U.S. looks remarkably similar. A large analysis of NHANES data covering over 71,000 Americans found that roughly 1 in 4 adults had deficient or insufficient vitamin D levels — and that figure climbs sharply among Black and Hispanic Americans, people with sedentary indoor lifestyles, and — as we'll cover in a moment — young adults in their 20s. This isn't a uniquely Korean problem. It's a modern, indoor-living problem that crosses borders.
When I saw those numbers, I thought about my own situation.
My Wife and I Took Vitamin D Every Day for Two Years
My wife and I have been taking 2,500 IU of vitamin D daily for two years. As a pharmacist, I picked that dose carefully — I figured it would be more than enough to keep us in a good range.
Then we both got blood tests.
Our levels came back in the low 30s. Barely above the lower end of normal.
Two full years of consistent supplementation and we were just scraping past the threshold. That was a wake-up call. Vitamin D is fat-soluble, which means it doesn't absorb quickly or spike easily. It builds up slowly — and if you're not getting sunlight on top of it, even supplements can only do so much.
(Source: Park J et al., Nutrients, 2024)
Young Adults Are the Most at Risk — 1 in 5 Is Severely Deficient
The same 120,000-person study broke down "severe deficiency" (under 10 ng/mL) by age group. Here's what stood out:
| Age Group | Severe Deficiency — Women | Severe Deficiency — Men |
|---|---|---|
| Ages 20–29 | 23.0% | 20.1% |
| Overall average | 8.9% | 6.1% |
Nearly 1 in 4 women in their 20s are severely deficient. That's almost three times the overall average.
In my pharmacy, I see this constantly. Customers in their 20s and 30s come in looking for energy supplements, saying things like "I'm exhausted all the time" or "I sleep plenty but I never feel rested." When I ask about their lifestyle, the pattern is almost always the same: working indoors all day, little to no exercise, eating on the go. That's a perfect recipe for vitamin D deficiency.
U.S. data backs this up. According to a NHANES-based study covering 2001–2018, Americans in their 20s showed some of the highest deficiency rates of any age group — higher than people in their 40s, 50s, or 60s. The same pattern, on two different continents. Young age is not a shield against vitamin D deficiency; if anything, the modern lifestyle of that demographic makes it a liability.
Why Are Younger People More Deficient?
A research team at Chosun University analyzed national health survey data focused specifically on adults in their 20s. The results were striking: 84.2% of people in their 20s were vitamin D deficient (below 20 ng/mL). Among women, it was 88.6%. Among men, 79.4%.
The study also identified who was most likely to be deficient:
Risk factors for vitamin D deficiency in young adults:
• Being female → ~1.8x higher risk
• Being unmarried → ~2.1x higher risk
• Perceiving one's own health as poor → ~1.9x higher risk
• Getting little to no moderate-intensity exercise → ~3.4x higher risk
That last one is the big one. Not exercising increases your deficiency risk by more than three times.
The logic is simple: vitamin D is produced in the skin when it's exposed to sunlight. No exercise usually means no reason to go outside, which means no sun exposure. Add sunscreen and indoor jobs into the mix, and you have almost no opportunity for your body to make its own vitamin D.
(Source: Ju MH et al., Journal of the Korea Academia-Industrial Cooperation Society, 2018)
Does Where You Live Make a Difference?
Yes — and it's not just about latitude. The 120,000-person study found that deficiency rates were highest in major cities like Seoul and Incheon, and lowest in areas like Jeju Island and Changwon.
The researchers suggested this is likely because cities concentrate desk workers and people doing overnight or rotating shifts — groups who rarely see daylight.
This matches what I see in my community. Older residents who spend hours working outdoors tend to have decent vitamin D levels. People who've recently moved from cities — even to rural areas — often still show low levels until they adjust their routines.
Sunlight truly is the primary source of vitamin D. It's easy to forget that when you're inside all day.
What Actually Happens When You're Deficient?
Most people know "vitamin D = bone health," but the effects are broader than that.
Well-established effects:
Vitamin D helps your gut absorb calcium. Without enough of it, calcium absorption drops, bones weaken, and over time you're looking at rickets in children, osteomalacia in adults, and increased risk of osteoporosis.
Associations being studied:
Research continues to find links between low vitamin D and conditions like obesity, high blood pressure, elevated cholesterol, fatty liver disease, type 2 diabetes, cardiovascular disease, and certain cancers. These connections keep appearing in studies.
But here's an important distinction:
Saying "people with low vitamin D tend to have these conditions more often" is not the same as saying "low vitamin D causes these conditions." Most of this evidence is associational, not causal. The jury is still out.
This matters because it's the difference between sound health advice and hype. Vitamin D is not a magic pill that prevents cancer. Anyone telling you otherwise is overselling it.
Do You Actually Need to Get Your Vitamin D Tested?
This is where things get a bit counterintuitive.
In 2021, the U.S. Preventive Services Task Force (USPSTF) concluded:
"There is insufficient evidence to recommend screening for vitamin D deficiency in asymptomatic adults."
In other words: the evidence that testing healthy people and then supplementing them actually improves health outcomes isn't strong enough yet to make it a standard recommendation.
The 2024 Endocrine Society Clinical Practice Guidelines echoed this — routine vitamin D blood testing isn't recommended for generally healthy adults without specific clinical reasons (like hypocalcemia).
That said, this is about testing. The question of whether to supplement is a bit different.
(Source: Seo MH, Korean Journal of Medicine, 2024)
Who Should Actually Take Vitamin D Supplements?
The 2024 Endocrine Society guidelines recommend what they call empiric supplementation — meaning, supplementing based on known risk rather than waiting for a blood test — for the following groups:
| Group | Reason |
|---|---|
| Children and teens (ages 1–18) | Prevents rickets; may reduce respiratory infection risk |
| Adults over 75 | Associated with reduced mortality risk |
| Pregnant women | Prevents rickets in the baby; may reduce respiratory infection risk |
| People with prediabetes | May slow progression to type 2 diabetes |
For healthy adults under 75 who don't fall into these categories, the guidelines say high-dose supplementation isn't necessary.
One more practical note: for adults over 50 who are supplementing, the guidelines recommend taking it daily rather than in large weekly or monthly doses. Consistency beats cramming.
A Story From a Friend
A friend of mine had a routine health checkup and came back with a vitamin D level so low that his doctor was visibly alarmed. He called me right away — "Can you send me something?"
I recommended an appropriate supplement, and he took it consistently for three months. When he retested, his levels had come back into the normal range. He called me laughing: "It actually worked."
The lesson here isn't complicated. If you take vitamin D consistently, your levels will go up. The flip side — as my wife and I discovered — is that it takes longer than you'd expect, and it doesn't happen automatically. Your body doesn't top itself off.
My Honest Take, After Nine Years as a Pharmacist
1. Vitamin D deficiency is genuinely widespread. Among young adults, 84% being deficient is close to a public health crisis level — not a niche concern.
2. The main cause is indoor living and not exercising. Less sunlight = less vitamin D. It's that simple.
3. Universal testing for vitamin D isn't well-supported by current evidence. If you're worried, it doesn't hurt to check — but don't assume a test is necessary.
4. If you're a child, pregnant, over 75, or have prediabetes, supplementation is worth taking seriously regardless of your test results.
5. Vitamin D builds up slowly. Two years of daily supplements got my wife and me to just barely normal. Patience and consistency are non-negotiable.
6. The single most effective thing you can do? Step outside for 10–15 minutes at lunch. No supplement beats natural sunlight.
Selling supplements is good for my business. So it's a little against my own interests to say this — but supplements are a backup plan. Sunlight comes first. Outdoor activity comes second. If you've genuinely exhausted those options, then reach for the bottle.
And if you're in your 20s or 30s reading this thinking "I'm young, I'm probably fine" — the data says you might be the most at risk. Worth paying attention to.
References
• Park J et al. (2024). Vitamin D Status and Reference Intervals in a South Korean Population during 2017–2022. Nutrients, 16, 604.
• Seo MH (2024). Screening and Prevention of Vitamin D Deficiency. Korean Journal of Medicine, 99(6), 279–283.
• Ju MH et al. (2018). Analysis of Factors Related to Vitamin D Deficiency in Korean Adults in Their 20s. Journal of the Korea Academia-Industrial Cooperation Society, 19(5), 303–311.
• US Preventive Services Task Force (2021). Screening for Vitamin D Deficiency in Adults. JAMA, 325, 1436–1442.
• Demay MB et al. (2024). Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab, 109, 1907–1947.
• Dai et al. (2022). Prevalence, trend, and predictor analyses of vitamin D deficiency in the US population, 2001–2018. Frontiers in Nutrition, 9.
Disclaimer: This post is written for general informational purposes only and does not constitute medical advice. The content reflects the author's personal experience and interpretation of published research, and should not be used as a substitute for professional medical consultation, diagnosis, or treatment. Vitamin D needs vary by individual. If you have concerns about your health or nutrient levels, please consult a qualified healthcare provider before making any changes to your diet or supplement routine.
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