Why You Can Be Vitamin D Deficient Despite Plenty of Sun

The Lab Result That Surprises Everyone

A familiar scene plays out in primary care clinics from Phoenix to Brisbane to Mumbai. The patient lives in a sunny climate. They walk the dog at lunchtime, garden on weekends, sit on a patio in the afternoon. Their 25-hydroxyvitamin D blood test comes back at 22 ng/mL, well below the 30 ng/mL threshold that most clinical guidelines call sufficient. The first reaction is disbelief. How can someone who sees this much sun be deficient?

The answer is that “sun exposure” and “vitamin D synthesis” are not the same thing. The body manufactures vitamin D3 (cholecalciferol) when ultraviolet B radiation, a narrow slice of the UV spectrum between 290 and 315 nanometers, hits a cholesterol precursor in the skin. A long list of variables stand between the sun in the sky and a usable serum level, and any one of them can quietly hold a result back. This article walks through the eight reasons most commonly responsible, and what to do about each.

A row of labeled blood sample tubes ready for analysis, the only way to know what your actual 25-hydroxyvitamin D level is. — Photo by Maksim Goncharenok on Pexels.

1. Sunscreen and Clothing Block the UVB You Need

Sunscreen does its job by blocking UVB. A correctly applied SPF 30 product reduces UVB transmission to the skin by roughly 97 percent. Studies have shown that real-world sunscreen use, which usually involves under-application and missed areas, blunts vitamin D synthesis less dramatically than the lab numbers suggest, but it still reduces it. Long sleeves, hats, and pants do the same. If you walk outside fully covered in clothing or freshly slathered in SPF 50, your sun time is functionally indoor time as far as vitamin D is concerned.

The fix is not to abandon sun protection. Skin cancer is the leading cancer diagnosis in many sunny regions, and burns are not a sustainable strategy for any nutrient. The fix is to leave a small window of unprotected exposure (arms and legs for 10 to 20 minutes outside peak burn risk) before applying sunscreen for the rest of the day, and to recognize that supplementation is often the better lever once daily life involves a lot of clothing.

2. Glass Filters Out UVB Almost Entirely

Standard window glass blocks more than 95 percent of UVB while letting most UVA through. That is why a sunny office, a long drive, or an enclosed porch generates no meaningful vitamin D, even when the light feels warm and intense on the skin. Drivers, indoor workers near south-facing windows, and people who garden inside a sunroom are all in the “sunny but not synthesizing” category. The takeaway is simple. If the sunlight reached you through a pane of glass, it did not contribute to your vitamin D level.

3. Latitude and Season Shut Down the UVB Window

UVB intensity falls steeply with the angle of the sun. Above roughly 35 degrees latitude (a line that runs through Memphis, Tokyo, and the Mediterranean coast), there are months in winter when the sun never climbs high enough for UVB to reach the ground at synthesis-relevant intensity. Boston, London, Berlin, and Toronto all sit in a wide “vitamin D winter” from roughly October through March, when even hours of midday outdoor time generate little to no D3.

A useful rule of thumb is the shadow test. If your shadow at midday is shorter than your height, UVB is reaching you. If your shadow is longer than your height, the angle is too low and synthesis effectively stops. People in sunny climates closer to the equator are mostly exempt from this seasonal cliff, but they can still hit the same wall through any of the other factors on this list.

4. Skin Synthesis Declines With Age

The 7-dehydrocholesterol concentration in the skin (the precursor that UVB converts to pre-vitamin D3) drops significantly with age. A 70-year-old produces roughly a quarter of the vitamin D from the same dose of UVB as a 20-year-old. This is one of the most under-discussed contributors to deficiency in older adults who otherwise live active outdoor lives. Combined with reduced kidney activation of vitamin D in later decades, it is why supplementation guidance for adults over 70 is generally higher than for younger adults, and why “I get plenty of sun” carries less weight at 75 than at 35.

5. Melanin Slows the Reaction

Melanin is a natural sunscreen. People with darker skin tones produce vitamin D from sunlight more slowly than those with lighter skin, sometimes requiring three to six times the UVB exposure to make a comparable amount. This is an evolutionary feature, not a defect. It protected ancestors who lived in equatorial UV environments. In modern life at higher latitudes, in indoor jobs, or in cultures where modest dress is the norm, the same trait is the single largest demographic driver of vitamin D deficiency.

For populations affected by this combination, expert guidance increasingly leans toward routine supplementation rather than relying on sun exposure to close the gap.

6. Higher Body Weight Sequesters Vitamin D in Fat Tissue

Vitamin D is fat-soluble. In people with higher body fat percentages, a meaningful share of newly synthesized or supplemented vitamin D is stored in adipose tissue rather than circulating in the blood, which is what the 25-hydroxyvitamin D test measures. The result is that two people getting the same sun exposure or taking the same supplement dose can end up with very different serum levels, with the higher-weight person testing lower.

The Endocrine Society’s 2024 clinical guideline notes this explicitly and recommends that people with obesity may need higher daily intakes than the general population to reach the same target serum level. Weight loss can release stored vitamin D back into circulation, which is one of several mechanisms thought to explain post-bariatric improvements in serum D over time.

7. Magnesium Deficiency Stalls Vitamin D Activation

The vitamin D from sun and supplements is biologically inert until the liver converts it to 25-hydroxyvitamin D, and the kidneys then convert that to the active 1,25-dihydroxyvitamin D. Both of those enzymatic steps require magnesium as a cofactor. People who are magnesium deficient (a state estimated to affect close to half of adults in some surveys) cannot fully activate the vitamin D they have, regardless of where it came from.

This is why some patients with low serum D fail to respond meaningfully to high-dose supplementation. Adding a modest magnesium intake from leafy greens, nuts, seeds, beans, and dark chocolate, or a supplemental dose at a clinician’s recommendation, often allows the existing vitamin D to do its job.

8. Gut Absorption Issues Block Dietary and Supplemental D

The vitamin D you eat or swallow as a softgel has to be absorbed through the small intestine alongside dietary fat. Conditions that impair fat absorption (celiac disease, Crohn’s disease, inflammatory bowel disease, chronic pancreatitis, cystic fibrosis, and a history of gastric bypass surgery) can substantially reduce how much of an oral dose actually reaches the bloodstream. Fat-malabsorbing patients sometimes need two to five times the standard supplement dose to reach an adequate serum level, and a clinician may prescribe a high-potency form.

Less dramatically, taking a vitamin D capsule with a fat-free meal also reduces absorption. Pairing the supplement with the largest meal of the day, or with a meal that contains some fat (avocado, olive oil, eggs, nuts), measurably improves uptake.

What to Actually Do If Your Levels Are Low

The right next step depends on which of the above factors apply to you. A blood test before and after any change is the only way to know what is working for your specific physiology. Optimal serum levels remain debated among experts. The Endocrine Society and most North American guidelines target 30 ng/mL (75 nmol/L) as a sufficiency threshold, while some researchers argue for 40 to 60 ng/mL. The Institute of Medicine considers 20 ng/mL adequate for bone health in the general population.

A reasonable, conservative path looks like this. Get tested. Identify which of the eight factors are working against you. Prioritize the fixes that are within your control (a small unprotected exposure window where appropriate, magnesium-rich food, taking supplements with fat). Talk to your physician about whether supplementation is warranted, and at what dose, before you exceed standard daily intake recommendations. Retest in three to four months. Adjust from there.

The mistake to avoid is assuming that a sunny climate alone is enough. In a population where indoor jobs, cars, sunscreen, glass, age, melanin, body weight, and gut health all push synthesis and absorption downward, plenty of sun is no longer plenty.