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Vitamin D: Why You're Probably Deficient & How to Fix It

Roughly 40% of adults are deficient and most don't know it. Here's what vitamin D actually does, how to test your levels, and how to get enough.

Vitamin D: Why You're Probably Deficient & How to Fix It

What Vitamin D Actually Is

Vitamin D is technically a hormone, not a vitamin. When UVB rays from sunlight hit your skin, your body synthesizes vitamin D3 (cholecalciferol), which the liver and kidneys then convert to its active form, calcitriol. That active form acts on receptors found in nearly every tissue in your body.

You can also get vitamin D from a small number of foods and from supplements. Unlike most vitamins, the vast majority of your supply historically came from the sun, not food — which is why indoor modern life has created widespread deficiency.

What It Does

  • Bone health: regulates calcium and phosphate absorption, preventing rickets in children and osteoporosis in adults
  • Immune function: supports both innate and adaptive immunity; deficiency is linked to higher rates of respiratory infection
  • Mood and mental health: low vitamin D is associated with increased risk of depression, particularly seasonal
  • Muscle function: affects strength and balance, especially in older adults
  • Testosterone: deficiency is correlated with lower testosterone in men
  • Inflammation and autoimmunity: emerging research links low vitamin D to higher autoimmune disease risk

Why Deficiency Is So Common

A few factors combined have created a modern epidemic of low vitamin D:

  1. Indoor lifestyles: most people spend the majority of daylight hours indoors behind glass (which blocks UVB).
  2. Latitude: anyone living above about 35° latitude (most of the US, Europe, and all of Canada) cannot produce meaningful vitamin D from sunlight for 4-6 months of the year.
  3. Sunscreen use: SPF 15+ blocks roughly 99% of vitamin D synthesis — important for skin cancer prevention, but a trade-off.
  4. Darker skin: higher melanin reduces vitamin D synthesis; people with darker skin need significantly more sun exposure or supplementation.
  5. Age: skin produces less vitamin D with age, so older adults are particularly at risk.
  6. Obesity: vitamin D is fat-soluble and gets sequestered in fat tissue, so higher body fat often means lower circulating levels.

How Much You Need

Official recommendations vary widely. The US RDA is 600-800 IU per day, but many vitamin D researchers argue this is far too low based on research into optimal blood levels. Most integrative and functional medicine guidelines target 2,000-5,000 IU per day for adults, with the exact dose individualized by weight and blood testing.

Target Blood Levels

General targets (values in ng/mL):

  • Below 20 ng/mL: deficient — needs correction
  • 20-30 ng/mL: insufficient — still suboptimal
  • 30-60 ng/mL: adequate to optimal — this is where most research suggests targeting
  • Above 100 ng/mL: excessive — reduce supplementation

The only way to truly know your vitamin D status is a blood test measuring 25-hydroxyvitamin D (25(OH)D).

How to Get Enough

Sunlight

The ideal source when available. Expose arms, legs, or torso (not just face and hands) to midday sun for 10-30 minutes several times per week, before your skin starts reddening. People with darker skin may need 3-6 times longer. This is not possible in winter above mid-latitudes.

Food Sources

Food provides modest amounts. The best sources:

  • Fatty fish: salmon (~570 IU per 3 oz), sardines, mackerel, trout
  • Cod liver oil: 1,300 IU per teaspoon — a traditional solution for good reason
  • Egg yolks: ~40 IU each
  • Beef liver: small but meaningful amounts
  • Fortified foods: milk, some plant milks, some cereals, fortified mushrooms

Supplements

For most people in most places, a supplement is the practical answer. Always choose D3 (cholecalciferol), not D2 — D3 raises blood levels more effectively and is the form your skin makes. Doses of 1,000-5,000 IU per day are typical. Take with a fat-containing meal for better absorption. Many experts now recommend taking D3 together with vitamin K2 (MK-7) to direct calcium into bones rather than arteries.

Toxicity and Safety

Vitamin D toxicity is rare but real. It usually requires sustained intake of 40,000+ IU per day for months, not occasional high doses. Symptoms include elevated blood calcium, nausea, kidney stones, and kidney damage. Sticking to 1,000-5,000 IU daily and testing blood levels every 6-12 months keeps you safely in the optimal range.

Testing

A 25(OH)D blood test is inexpensive, widely available, and the only way to personalize your vitamin D intake. If you're starting supplementation, test before you start and 3 months after to confirm you've reached the target range. Re-test annually after that, ideally in late winter when levels are typically at their lowest.

The Bottom Line

Vitamin D is one of the most common and most easily corrected nutritional deficiencies in the modern world. If you live above 35° latitude, work indoors, wear sunscreen regularly, or have darker skin, you're statistically very likely to be deficient — and the fix is cheap, safe, and supported by decades of research.

Get your levels tested, target 30-60 ng/mL, and build a plan with sunlight, fatty fish, and (most practically) a D3 supplement. Your bones, immune system, mood, and testosterone will all thank you.

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