Health

Vitamin D Basics: What It Is, Where It's Found, and How to Read the Label

By Steven Motes · 10 min read

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Vitamin D might be the most talked-about nutrient in the supplement aisle — and the least understood. People buy it because a friend mentioned it, because a headline scared them, or because the bottle looked convincing. Very few of them could tell you what the "D" actually does, where it comes from, or what the numbers on the label mean. It's also one of five nutrients that dietitian DJ Blatner, RDN, named in a 2026 "starter kit" list (multivitamin, vitamin D, magnesium, omega-3s, probiotic) featured in a NOW press release — at a moment when supplement use has hit record levels, with 76% of shoppers taking something daily per a September 2025 NOW-commissioned survey.

This post fixes that. Consider it vitamin D explained for beginners: no hype, no scare tactics, no medical advice — just the plain facts about what it is, where your body gets it, how to read it on a label, and what to ask your doctor or pharmacist. I'm not a doctor or a dietitian. This is educational information. Talk to a qualified professional about your own situation before taking anything.

What Vitamin D Actually Is

Vitamin D is a fat-soluble vitamin, which means your body stores it in fatty tissue rather than flushing out the excess in urine the way it does with water-soluble vitamins like vitamin C. That storage ability is a double-edged sword: it means your body can keep a reserve, but it also means more isn't automatically better — fat-soluble vitamins can build up over time.

Your body gets vitamin D from three sources, and most people use a mix of all three:

1. Sunlight on skin. When UVB rays hit your skin, your body produces vitamin D on its own. How much depends on where you live, the season, the time of day, your skin tone, and how much skin is exposed — which is why this source varies wildly from person to person.
2. Food. Relatively few foods naturally contain meaningful amounts of vitamin D. The notable ones are fatty fish (like salmon), fish liver oils, egg yolks, and UV-exposed mushrooms. Many dairy products, plant milks, and cereals are fortified with it — meaning the manufacturer added it. Check the label: fortified foods list it.
3. Supplements. Pills, gummies, drops, and softgels containing vitamin D2 or D3 (more on the difference below).

That's the whole picture. Sun, food, supplements — in some combination.

D2 vs. D3: What's the Difference?

Walk down the supplement aisle and you'll see two versions: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Here's what those names actually mean:

• Vitamin D3 is the form your skin makes from sunlight, and it's also the form found in animal-sourced foods. In supplements, D3 is typically derived from lanolin (from sheep's wool) or fish oil. There are also vegan D3 options sourced from lichen.
• Vitamin D2 comes from plant and fungal sources — it's the form found in UV-exposed mushrooms, and in supplements it's typically derived from yeast.

Both are forms of vitamin D. If the source matters to you — for example, if you're vegan and want to avoid lanolin-derived D3 — the label or product description should tell you where the D came from. If it doesn't say, that's a transparency gap worth noting (see my guide, How to Read a Supplement Label Like a Pro (Before You Spend a Dime), for the full checklist).

The Label Math: mcg, IU, and %DV

Vitamin D labels are confusing because the vitamin is measured in two units, and products use them inconsistently. Here's the decoder ring, straight from FDA labeling rules:

• The Daily Value (DV) for vitamin D is 20 mcg (800 IU) for adults and children age 4 and older. (Source: FDA Daily Value tables, verified September 2026.)
• 1 IU of vitamin D = 0.025 mcg. So a product labeled "1,000 IU" contains 25 mcg — and 25 mcg ÷ 20 mcg = 125% DV. The FDA's own labeling guidance uses exactly this example.
• The 5/20 rule (from FDA guidance): 5% DV or less per serving is LOW in the nutrient; 20% DV or more is HIGH. A supplement at 125% DV is, by definition, a high amount relative to the daily reference value.

Why two units? The FDA requires vitamin D to be declared in mcg on Supplement Facts labels; manufacturers may also list IU in parentheses voluntarily. Older products and many articles still talk only in IU, which is why you'll see both. When comparing two products, convert to the same unit first — mcg is the one the FDA requires, so use that.

A practical example: Product A says "25 mcg (1,000 IU), 125% DV." Product B says "50 mcg (2,000 IU), 250% DV." Same vitamin, different amounts — and now you can see that Product B gives you double the reference daily amount per serving. Whether that's appropriate for you is a question for your doctor or pharmacist, not a label.

Food Sources Worth Knowing

If you'd rather get vitamin D from food than a bottle, here's what's actually on the menu. These are the foods the NIH's Office of Dietary Supplements lists as meaningful sources:

• Fatty fish — salmon, trout, and similar fish are among the richest natural food sources.
• Fish liver oils — concentrated, and also a source of vitamin A (which is why dosing matters — talk to a professional).
• Egg yolks — modest amounts; the vitamin D is in the yolk, not the white.
• UV-exposed mushrooms — the one meaningful plant source, and the source of most D2.
• Fortified foods — milk, many plant milks, some yogurts, and many breakfast cereals have vitamin D added. Fortification levels vary by product, so read the Nutrition Facts panel.

Notice what's not on the list: most fruits, vegetables, grains, and meats contain little to no vitamin D naturally. That's why fortified foods exist — and why the nutrient gets so much attention in the first place.

Sunlight: The Complicated Source

Your skin makes vitamin D when exposed to UVB sunlight. That sounds simple, but in practice it's the most variable source of all:

• Geography and season matter. UVB is weaker in winter and at higher latitudes. Someone in Texas in July and someone in Minnesota in January are working with very different sunlight.
• Skin tone matters. More melanin means less UVB gets through, so people with darker skin generally produce vitamin D more slowly from the same sun exposure.
• Sunscreen, clothing, and windows matter. Sunscreen blocks the UVB that triggers production. Glass blocks UVB too — sitting by a sunny window doesn't count.
• Time of day matters. Midday sun carries more UVB than morning or evening sun.

I'm not going to tell you how much sun to get — that's between you, your dermatologist, and your own risk tolerance, because UV exposure is also the primary driver of skin damage. The point is just to understand why sunlight is an unreliable sole source for many people, and why food and label-reading skills matter.

What to Look for on a Vitamin D Supplement Label

If you do shop for one, run it through the same label checklist from my How to Read a Supplement Label Like a Pro (Before You Spend a Dime) guide, with these vitamin-D-specific additions:

1. Check the form — D2 or D3? The label should say which. If the source matters to you (vegan, allergies), look for the source too.
2. Do the unit math — convert IU to mcg (multiply IU by 0.025), then check the %DV against the 20 mcg Daily Value.
3. Check the serving size — is that 125% DV per one softgel or per two? Serving-size math is where price comparisons go wrong.
4. Look for third-party seals — USP Verified, NSF, or Informed Choice/Sport. Voluntary, but it's the only independent check on what's in the bottle.
5. Read the warnings — fat-soluble means buildup is possible over time. A responsible label tells you who should check with a professional first (pregnant or nursing women, people on medications, children).
6. Watch the claims — "supports bone health" style structure/function language with the FDA disclaimer is normal. Anything claiming to treat, cure, or prevent a condition is a red flag — supplements legally cannot make disease claims.

Questions to Ask Your Doctor or Pharmacist

The label tells you what's in the bottle. It can't tell you what you need. These are the questions worth bringing to a professional:

• Should I be getting vitamin D from food, sun, supplements, or a combination?
• Does anything I take (medications, other supplements) interact with vitamin D?
• What does my total intake look like across food plus supplements?
• Is there a blood test that would tell me where I stand, and do I need one?
• Given that it's fat-soluble, what should I know about taking it long-term?

A pharmacist can answer most label questions on the spot — they're an underused resource, and consultations are usually free.

FAQ

Vitamin D isn't complicated once you strip away the marketing: sun, food, or a bottle — measured in mcg against a 20 mcg Daily Value, in D2 or D3 form, with the same label-reading rules as everything else in the supplement aisle. Learn the label, ask the pharmacist, and keep your money for the products that earn it.

This article is for educational purposes only and is not medical advice. Supplements can interact with medications and health conditions — talk to your doctor or pharmacist about your own situation before starting anything new.

Next step: run any bottle you're considering through the 60-second label checklist in How to Read a Supplement Label Like a Pro (Before You Spend a Dime) — it takes a minute and it works on every supplement, not just this one.

Related reading: Magnesium Basics: Food Sources, Supplement Forms, and What the Label Tells You and How to Store Supplements So They Last (Heat, Humidity, Expiration Dates) — the rest of this health vertical.