The Vitamin Your Body Makes From Sunlight — and Why So Many of Us Are Running Low

Vitamin D deficiency has been linked to everything from depression to dizziness — but the research is more nuanced than the headlines suggest.

Here's a strange paradox: vitamin D is the one nutrient your body can manufacture for free, simply by standing in sunlight. And yet, vitamin D deficiency is one of the most common nutritional shortfalls on the planet. A 2020 Cochrane review put it bluntly — vitamin D deficiency is "common worldwide," contributing to nutritional rickets and bone-softening osteomalacia, with a major impact on the health and development of infants, children, and adolescents¹.

So if sunshine is free and abundant, why are so many people short on it? And does topping up your levels actually do anything beyond protecting your bones? That second question turns out to be where the science gets genuinely interesting — and where the honest answer is sometimes "it depends."

What vitamin D actually is (and isn't)

Despite the name, vitamin D behaves less like a vitamin and more like a hormone. Your skin makes it when ultraviolet light hits it, your liver and kidneys convert it into its active form, and that active form then travels around the body docking onto receptors found in a surprising number of tissues — not just bone, but immune cells, the thyroid, and beyond.

Researchers measure your status by checking blood levels of 25-hydroxyvitamin D, usually written as 25(OH)D. This is the storage form, and it's the number your doctor is looking at when they tell you you're "low." Almost every study in this field uses it as the yardstick, which makes it easy to compare findings across very different health conditions.

The interesting thing is how widely vitamin D's reach seems to extend. Once you start measuring 25(OH)D in people with various conditions, patterns emerge — though, as we'll see, a pattern is not the same thing as proof.

Who tends to run low

One of the clearer associations in the research involves body weight. A 2015 systematic review and meta-analysis pulled together 23 observational studies and found that vitamin D deficiency was 35% more prevalent in people with obesity compared to those at a normal weight (prevalence ratio 1.35), and 24% higher than in the overweight group².

Why would this be? The leading explanation is that vitamin D is fat-soluble — it gets sequestered in body fat, effectively diluting the amount circulating in the blood. The more adipose tissue you have, the more vitamin D gets "locked away" and the lower your measurable blood levels. Whether that means people carrying more weight simply need more vitamin D, or whether low vitamin D plays some role in weight regulation, the meta-analysis couldn't say — observational data shows you who's correlated, not why.

Breastfed infants are another group at genuine risk. The 2020 Cochrane review noted that vitamin D levels are naturally low in breast milk, which means exclusively breastfed babies are vulnerable to insufficiency or deficiency unless supplemented¹. This is precisely why pediatricians routinely recommend vitamin D drops for breastfed infants.

The mood connection

The link between vitamin D and depression has been debated for years, and a 2013 systematic review and meta-analysis tried to settle it by analyzing studies covering a combined 31,424 participants³.

The findings were intriguing. People with depression had significantly lower vitamin D levels than those without (a standardized mean difference of 0.60). In the cross-sectional studies, being in the lowest vitamin D category came with a 31% higher odds of depression compared to the highest category (odds ratio 1.31). And cohort studies — which follow people forward in time — showed an increased hazard of developing depression among those with the lowest vitamin D levels.

Here's the crucial caveat: most of this evidence is observational. When you find that low vitamin D and depression travel together, you can't tell which came first. Does low vitamin D nudge mood downward? Or do depressed people spend less time outdoors in the sun, eat more poorly, and end up with lower levels as a result of their depression? The chicken-and-egg problem looms large here, and this meta-analysis couldn't fully resolve it.

Blood sugar and type 2 diabetes

This is one area where we actually have a lot of trial data. A 2024 systematic review and meta-analysis gathered 39 randomized controlled trials — the gold standard for cause-and-effect — involving 2,982 people with type 2 diabetes⁴.

The results showed measurable improvements in glycemic control with vitamin D supplementation. Fasting blood glucose dropped by about 0.49 mmol/L, HbA1c (a marker of average blood sugar over months) fell by 0.30%, and markers of insulin resistance improved as well (HOMA-IR down 0.39, fasting insulin down 1.31 µIU/mL).

These are modest but real effects. A 0.30% reduction in HbA1c won't replace diabetes medication, but it's the kind of nudge that adds up alongside other lifestyle changes. Importantly, the researchers found that the benefits depended heavily on the dosage and duration of supplementation — short, low-dose courses didn't deliver the same results as more sustained, adequate dosing. This is a recurring theme in vitamin D research: dose matters, and so does giving it enough time to work.

The immune and skin angle

Vitamin D appears to play a role in calming the immune system, and two conditions in the research illustrate this.

Hashimoto's thyroiditis, an autoimmune condition where the immune system attacks the thyroid, shows a consistent pattern. A 2024 review found that vitamin D levels are significantly lower in people with Hashimoto's compared to healthy individuals. More compellingly, after supplementation with cholecalciferol (vitamin D3) in patients who had both Hashimoto's and vitamin D deficiency, thyroid autoantibody titers — a measure of the autoimmune attack — decreased significantly⁵. The proposed mechanism is that vitamin D helps promote immune tolerance, essentially teaching the immune system to stand down rather than attack the body's own tissue.

Psoriasis, a chronic inflammatory skin condition, tells a similar story. A 2023 systematic review and meta-analysis examined 25(OH)D levels in 1,876 psoriasis patients versus 7,532 controls and confirmed that people with psoriasis have significantly lower vitamin D levels⁶. The review also looked at whether oral vitamin D could improve the Psoriasis Area and Severity Index (PASI), the standard scoring system for how bad the disease is. Hypovitaminosis D, the authors concluded, appears to be a genuine risk factor for the disease.

The dizziness nobody expects

One of the more surprising findings in this field concerns benign paroxysmal positional vertigo, or BPPV — the most common cause of that sudden, spinning dizziness when you tilt your head or roll over in bed. A 2024 systematic review and meta-analysis focused on populations in the Northern Hemisphere (where sunlight, and therefore vitamin D, is more seasonal) examined the relationship between serum vitamin D and both the incidence and recurrence of BPPV⁷.

The biological logic is actually quite elegant. BPPV happens when tiny calcium carbonate crystals in your inner ear become dislodged. Vitamin D is central to calcium metabolism, so it's plausible that low vitamin D could affect the stability of those crystals — and the data linked deficiency to both developing BPPV and having it come back. If you've ever been told your vertigo "just happens," this is a reminder that nutritional status may be quietly involved.

Pregnancy, miscarriage, and the next generation

Vitamin D demands rise during pregnancy. A 2020 overview of randomized trials and meta-analyses explained that maternal 25(OH)D levels progressively decline through pregnancy as the developing fetus draws on the mother's stores⁸.

In women with low vitamin D status, supplementation may improve fetal growth and reduce the risks of several complications: small-for-gestational-age babies, preeclampsia, preterm birth, and gestational diabetes. The review went further, noting that children born to mothers with sufficient vitamin D had fewer dental enamel defects and lower rates of attention-deficit/hyperactivity disorder and autism. These are associations drawn from the available evidence rather than definitive cause-and-effect, but they underscore why vitamin D status in pregnancy gets so much attention.

On the question of miscarriage, the picture is less clear-cut. A 2022 systematic review and meta-analysis specifically examined whether vitamin D status is associated with miscarriage and recurrent miscarriage, analyzing 10 studies⁹. The researchers set out to determine both whether low vitamin D predicts miscarriage risk and whether vitamin D treatment reduces it — a reminder that even when a nutrient seems important, the evidence for any single outcome needs to be weighed on its own.

The big trial that brought everyone down to earth

For all the associations, the question that matters most is: if you give healthy people extra vitamin D, do they get sick less often? The Finnish Vitamin D Trial set out to answer exactly that with a rigorous design¹⁰.

This was a five-year, randomized, placebo-controlled trial in 2,495 older adults — men 60 and over, postmenopausal women 65 and over — none of whom had prior cardiovascular disease or cancer. They were split into three groups: placebo, 1,600 IU per day, or 3,200 IU per day of vitamin D3. The primary endpoints were major cardiovascular events and invasive cancer.

The result was sobering for vitamin D enthusiasts: even at these substantial doses over five years, supplementation didn't significantly move the needle on cardiovascular disease or cancer in this generally healthy, well-nourished population.

This is one of the most important lessons in the entire vitamin D story. Supplementation seems most valuable for correcting a genuine deficiency — and far less impressive when given to people who already have adequate levels. Pouring more vitamin D into an already-full tank doesn't appear to buy extra protection. The benefit lives in fixing a shortfall, not in megadosing for insurance.

Putting it all together

When you step back and look at all ten studies, a coherent picture emerges. Vitamin D deficiency is genuinely common, especially among people with obesity² and exclusively breastfed infants¹. Low levels travel alongside a striking range of conditions — depression³, Hashimoto's thyroiditis⁵, psoriasis⁶, and BPPV vertigo⁷.

But association is not causation, and the strongest, most carefully controlled evidence tells a measured story. Vitamin D supplementation meaningfully helps blood sugar control in type 2 diabetes⁴, reduces thyroid autoantibodies in deficient Hashimoto's patients⁵, and supports better outcomes in pregnancy for women who are low⁸. Yet in healthy, already-replete older adults, even years of high-dose supplements didn't prevent heart disease or cancer¹⁰.

The takeaway is elegantly simple: this is about correcting deficiency, not chasing super-physiological doses.

Practical takeaway

If you're concerned about vitamin D, the smartest first step isn't to start swallowing pills blindly — it's to find out where you actually stand. A simple blood test for 25(OH)D tells you whether you're deficient, insufficient, or just fine. The research consistently shows that the people who benefit most from supplementation are those who start out low.

For specific groups, the evidence points to concrete actions. Breastfed infants should be supplemented, since breast milk alone doesn't provide enough¹. For pregnant women, the reviewed evidence supports a baseline of 600 IU per day of vitamin D3, with higher doses in the range of 1,000–4,000 IU per day potentially useful for achieving better status in those who start out low⁸. For people with type 2 diabetes, the diabetes meta-analysis emphasized that adequate dose and sufficient duration are what separate the trials that worked from those that didn't⁴.

A few honest cautions. Vitamin D is fat-soluble, which means it accumulates in the body and can build up to toxic levels if you dramatically over-supplement — this is not a "more is always better" nutrient. The Finnish trial is a strong reminder that if your levels are already adequate, extra pills are unlikely to help¹⁰. And many of the associations described here — depression, psoriasis, vertigo — come from observational research that can't prove vitamin D caused the difference.

Talk to your doctor before starting supplementation, especially if you are pregnant, have an autoimmune or thyroid condition, take medications, or have kidney issues. Ask for a 25(OH)D test, find out your actual number, and let that guide the decision rather than the latest headline. Sunlight, sensible supplementation when you're genuinely low, and a realistic understanding of what vitamin D can and can't do — that's the evidence-based middle path.

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