Vitamin D 101: What It Is and What the Science Says

A comprehensive guide to one of the most studied — and most misunderstood — nutrients in medicine

Few nutrients have generated as much scientific attention, public debate, and market enthusiasm as vitamin D. Walk into any pharmacy or health food store and you'll find it prominently displayed alongside headlines promising everything from stronger bones to protection against cancer. It is, by many measures, the most used dietary supplement in the United States. Yet despite decades of research and billions of dollars spent studying it, scientists still argue vigorously about what vitamin D can and cannot do — and whether the pill form does the same thing as the sunlight that earned it the “sunshine vitamin” nickname.

This article aims to answer key questions most people have: What is vitamin D and how do you get it? 

What Exactly Is Vitamin D?

The first thing to understand about vitamin D is that it isn't really a vitamin at all — at least not in the conventional sense. Vitamins are organic compounds that the body requires but cannot manufacture in sufficient quantities, making dietary intake essential. Vitamin D breaks this rule. The human body can synthesize it in abundance, entirely on its own, given adequate exposure to the right kind of sunlight.

More accurately, vitamin D behaves as a prohormone — a precursor that the body converts into a hormone with wide-ranging effects. The active form of vitamin D, called 1,25-dihydroxyvitamin D (or calcitriol), functions like a steroid hormone, binding to vitamin D receptors found in nearly every tissue in the body. These receptors regulate the expression of hundreds of genes involved in cell growth, immune function, inflammation, and metabolism.

The term "vitamin D" is actually an umbrella for several related fat-soluble compounds:

• Vitamin D2 (ergocalciferol): Found in some fungi and plants; the form used in many prescription supplements and some fortified foods.

• Vitamin D3 (cholecalciferol): The form produced in human skin upon UV exposure; also found naturally in fatty fish and egg yolks, and used in most over-the-counter supplements.

These two forms are not equivalent. Research consistently shows that vitamin D3 raises and maintains serum levels more effectively than D2, with one meta-analysis finding D3 produced a mean difference of 15.23 nmol/L (6.1 ng/mL) higher serum levels compared to equivalent D2 doses. For most people, when choosing a supplement, D3 is the preferable form.

How Does the Body Make Vitamin D from Sunlight?

The skin's ability to produce vitamin D is one of the more elegant examples of biochemistry in the human body — and understanding the mechanism helps explain why not just any light source will do.

The photosynthesis of vitamin D

The raw material for vitamin D synthesis is 7-dehydrocholesterol (7-DHC), a cholesterol precursor found in the cell membranes of keratinocytes and fibroblasts in the epidermis — the outer layer of your skin.

When ultraviolet B (UVB) light strikes the skin, photons are absorbed by 7-DHC, causing a specific bond between carbon atoms to break. This produces pre-vitamin D3, an unstable intermediate. Body heat then converts pre-vitamin D3 into vitamin D3 within the phospholipid bilayer of the skin cell membrane.

Vitamin D3 is then released slowly into the bloodstream over the course of a few days, where it binds to vitamin D binding protein (VDBP) for transport to the liver. In the liver, it is converted to 25-hydroxyvitamin D [25(OH)D] — the form measured in blood tests. Finally, the kidneys convert 25(OH)D into the active hormone calcitriol [1,25(OH)₂D].

Which Wavelengths of Light Produce Vitamin D?

Not all wavelengths of light are capable of driving vitamin D synthesis. This critical concept — called the action spectrum — determines exactly which wavelengths are effective, and it has major practical implications.

The action spectrum for vitamin D production is narrow and falls entirely within the UVB portion of the electromagnetic spectrum, roughly 280–315 nm, with peak efficiency at approximately 295–297 nm. Wavelengths above 315 nm have essentially no capacity to initiate vitamin D synthesis.

To put this in practical terms:

UVB (280–315 nm): The only wavelength range capable of stimulating vitamin D production in the skin.

• UVA (315–400 nm): Does NOT produce vitamin D. UVA penetrates deeper into the skin and does trigger other important biological responses, but vitamin D synthesis is not one of them.

• Visible light (400–700 nm): Does not produce vitamin D.

• Red light / near-infrared (700 nm+): Does not produce vitamin D.

• Blue light (~460 nm): Does not produce vitamin D.

This distinction matters enormously when evaluating common claims about light therapy devices.

Do Tanning Beds Produce Vitamin D?

Most conventional tanning beds are designed to emit primarily UVA radiation, which is more effective at producing a cosmetic tan with a lower immediate risk of sunburn. Because they emit little to no UVB, most tanning beds are largely ineffective at stimulating significant vitamin D production. Some tanning beds — those specifically designed to include a meaningful UVB component — can produce vitamin D, but the majority sold commercially do not.

Does Red Light Therapy Produce Vitamin D?

No. Red light therapy devices typically emit wavelengths in the 630-850 nm range, which is far outside the UVB action spectrum for vitamin D. Red light does have documented biological effects — photobiomodulation, anti-inflammatory responses, mitochondrial stimulation — but vitamin D synthesis is not among them. Claims that red light therapy raises vitamin D levels are not supported by the photobiological evidence.

What About "Vitamin D Lamps"?

Devices marketed as "vitamin D lamps" vary widely. The only devices capable of producing vitamin D in the skin are those that emit UVB radiation in the 280-315 nm range at sufficient intensity. Fluorescent bulbs emitting primarily visible or UVA light, regardless of any marketing claims, will not produce meaningful amounts of vitamin D.

Can You Get Too Much Vitamin D From Sunlight

One of the most remarkable features of sun-derived vitamin D is the body's ability to self-regulate production to prevent toxicity. When pre-vitamin D3 accumulates in the skin, continued UV exposure converts the excess into biologically inert photoproducts — most notably lumisterol and tachysterol — rather than allowing unlimited vitamin D3 to enter circulation. This self-regulating photochemical feedback occurs within seconds of sun exposure and operates entirely independently of skin pigmentation.

The practical implication: it is not possible to become vitamin D toxic from sun exposure alone. This is categorically different from oral supplementation, where no such natural ceiling exists.

The Four Sources of Vitamin D

1. Sunlight: The Original Source of Vitamin D

Sunlight is the original and, for most of human evolutionary history, the primary source of vitamin D. Estimates suggest that in populations with regular outdoor activity, sun exposure historically provided 80–90% of circulating vitamin D.

Advantages:

• The body self-regulates production, preventing toxicity.

• Sun-derived vitamin D3 remains in circulation significantly longer — roughly 2–3 times as long - as the same molecule taken orally, because skin production releases it slowly rather than in a single bolus.

• UVB exposure also initiates production of other photoproducts in the skin (including lumisterol) with potentially unique biological properties.

• Keratinocytes — the cells making up over 90% of the outer skin layer - cannot effectively use supplemental vitamin D; they require locally synthesized vitamin D for their own protective and repair functions.

• Sun exposure triggers numerous health-promoting responses beyond vitamin D synthesis.

Disadvantages and limitations:

• UVB availability is highly dependent on latitude, season, time of day, and cloud cover. At latitudes above approximately 33°N (or south of 33°S), UVB sufficient for vitamin D synthesis may be unavailable for four to six months of the year.

• UVB does not penetrate glass, so sitting near a sunny window does not produce vitamin D.

• Sunscreen, when applied liberally, significantly reduces (though does not eliminate) UVB transmission.

• Skin tone, age, and body surface area exposed all affect the rate of production. Darker-skinned individuals have more melanin, which competes with 7-DHC for available UVB photons, requiring longer exposure times to produce equivalent amounts of vitamin D.

• Excessive UV exposure increases the risk of skin cancer and photoaging.

How do you know if your sun exposure is actually working?

This is a genuinely difficult question, and one that deserves more attention than it typically receives. The skin cannot report its vitamin D production in real time. Importantly, vitamin D photosynthesis occurs at sub-erythemogenic doses — meaning you can produce meaningful amounts of vitamin D without getting pink or burned. In fact, once the skin begins to redden, excess UV is degrading vitamin D rather than producing more. The practical guidance most researchers offer: Brief, regular exposures — well short of burning — are more effective at sustaining vitamin D production than infrequent, prolonged sessions.

The most reliable way to know if your sun exposure is translating into adequate vitamin D is simply to measure your blood level.

“When you make vitamin D in your skin, it lasts two to three times longer in your body compared to taking it as an oral supplement, because skin production releases it slowly rather than in a single bolus. We did a study in healthy adults and showed that if you expose your whole body in a bathing suit to one minimal erythemal dose — a slight pinkness to your skin 24 hours later, not a sunburn — it’s equivalent to ingesting 15,000 to 20,000 units of vitamin D.”

— Dr. Michael F. Holick, Ph.D., M.D., a leading endocrinologist and professor at Boston University, Modern Healthspan, 2025

2. Vitamin D supplements: what to know about dosing and form

Supplements are the most controllable and, for many people living in northern latitudes or working indoors, the most practical route to maintaining adequate vitamin D.

What's available:

• Over-the-counter supplements are available in doses ranging from 400 IU to 10,000 IU, most commonly as vitamin D3 (cholecalciferol), which is superior to D2 for raising and sustaining serum levels.

• Prescription vitamin D is typically D2 (ergocalciferol) in doses of 50,000 IU, often used weekly for clinical deficiency correction.

Advantages:

• Not dependent on weather or season 

• Can be adjusted based on blood test results.

• No skin cancer risk.

• Widely available and inexpensive.

Disadvantages and challenges:

Dosing uncertainty. Unlike sun exposure, where the body self-regulates production, the amount of vitamin D3 a given individual absorbs from a supplement varies considerably. Absorption depends on whether the supplement is taken with a meal containing fat (vitamin D is fat-soluble), the health of the digestive tract, liver and kidney function, and individual genetics including variations in the vitamin D binding protein and vitamin D receptors.

Quality control. Independent testing of dietary supplements has repeatedly found that actual vitamin D content can deviate substantially from label claims — sometimes delivering significantly less, occasionally more. Unlike pharmaceutical drugs, supplements are not subject to the same pre-market efficacy and consistency requirements in the United States. Choosing products that carry third-party verification (such as USP, NSF, or ConsumerLab certification) provides greater assurance of label accuracy.

Toxicity risk. Unlike natural vitamin stimulation from sun exposure, supplement overdosing is possible. Vitamin D toxicity — hypervitaminosis D — typically manifests as hypercalcemia (elevated blood calcium), with symptoms including nausea, vomiting, weakness, frequent urination, kidney stones, and in severe cases, cardiac arrhythmia and kidney damage. Most documented toxicity cases have involved either manufacturing errors (where supplements contained far more D3 than labeled) or prolonged self-administration of very high doses. The Tolerable Upper Intake Level set by the National Academies is 4,000 IU/day for adults, though many clinicians consider intakes up to 10,000 IU/day safe for healthy adults when monitored with blood tests. True toxicity generally requires sustained daily intakes well above these amounts — most reported cases have involved daily intakes of 50,000 IU or more over extended periods.

Monitoring. Periodic serum 25(OH)D testing is the only way to know whether a given supplement dose is achieving the desired effect. The same daily dose can produce very different blood levels in different individuals, making blood testing essential rather than optional.

3. Food sources of vitamin D: what's realistic

Very few foods naturally contain meaningful amounts of vitamin D, and the quantities involved make dietary intake an unreliable primary source for most people.

Foods with naturally occurring vitamin D:

• Fatty fish (wild salmon: 600-1,000 IU per serving; farmed salmon: approximately 100-250 IU; sardines: ~300 IU; mackerel: ~250 IU)

• Cod liver oil (~1,360 IU per tablespoon - though this also delivers substantial vitamin A)

• Beef liver (~40-50 IU per 3 oz serving)

• Eggs (~40-44 IU per yolk)

• Some mushrooms, particularly those exposed to UV light during growth (variable, up to several hundred IU)

Fortified foods: In the United States, vitamin D is added to cow's milk (typically 100 IU per 8 oz serving), some plant-based milks, orange juice, cereals, and yogurts — though levels vary by product and brand.

Can you get enough vitamin D from food alone?

For most people: no. The Recommended Dietary Allowance (RDA) for vitamin D is 600 IU/day for adults under 70 and 800 IU/day for those over 70 — figures that assume minimal sun exposure. Achieving even these relatively conservative targets through diet alone would require eating fatty fish multiple times per week, every week. Researchers hathe 6ve noted that it is effectively impossible to reach the higher serum levels — 40–60 ng/mL — that some scientists consider optimal, through food intake alone, unless significant amounts of fortified foods are included.

Vegans face a particular challenge, as the most vitamin D-rich whole foods are all animal products. Without supplementation and/or regular sun exposure, vitamin D deficiency in vegan populations is common.

4. UVB light therapy: what to know about this new solution 

UVB light therapy is another effective way to raise vitamin D levels, particularly for people who have limited sun exposure, live at higher latitudes, spend most of their time indoors, or have difficulty maintaining adequate vitamin D through diet alone. Unlike oral supplements, UVB light stimulates the skin's natural production of vitamin D using the same biological pathway activated by natural sunlight.

Not all light therapy devices are designed to stimulate vitamin D production. To support vitamin D synthesis, a device must emit UVB light, the specific portion of the ultraviolet spectrum that triggers vitamin D production in the skin. 

The takeaway

Vitamin D occupies a unique place in medicine: it's simultaneously one of the most important nutrients for human health and one of the most misunderstood. While it's tempting to look for simple answers — more sunshine, more supplements, or a single "optimal" blood level — the science is more nuanced. 

Your vitamin D status depends on where you live, how much time you spend outdoors, your skin tone, your diet, your health, and even your genetics. For most people, the goal isn't to maximize vitamin D at all costs, but to maintain healthy levels through a combination of sensible sun exposure, diet, and supplementation when appropriate. As research continues to evolve, one thing remains clear: Understanding where vitamin D comes from and what it actually does is far more valuable than chasing the latest headline or health trend.