Vitamin D: Does It Actually Work?

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin your body makes when sunlight hits your skin. It’s also found in fatty fish, egg yolks, and fortified foods. But most people don’t get enough from food and sun alone.

About 42% of American adults are deficient. That number jumps higher if you have dark skin, live far from the equator, or spend most of your time indoors.

You’ll see vitamin D marketed for everything from bone health to cancer prevention to muscle growth. Some of those claims hold up. Most depend heavily on whether you’re deficient or not.

We analyzed 4 major meta-analyses and systematic reviews covering 162 individual studies. Here’s what the data actually shows.

The Evidence, Claim by Claim

The sections below break down each claim with real numbers from published meta-analyses. Every effect size, confidence interval, and study count comes from peer-reviewed research.

Bones and Fractures: Not What You’d Expect

This is the claim everyone assumes is settled. It isn’t.

Bolland et al. reviewed 81 randomized controlled trials with over 53,000 people. Vitamin D supplementation alone didn’t significantly reduce fractures (RR = 0.97). It didn’t reduce falls either.

There’s a catch. When you combine vitamin D with calcium, elderly people do see a small reduction in hip fractures. And high-dose bolus vitamin D (one mega-dose per year) actually increased falls and fractures in one major trial.

The takeaway: vitamin D alone won’t protect your bones if your levels are already normal. If you’re deficient and elderly, it might help, especially with calcium.

Immune Function: The Strongest Claim

This is where vitamin D shines. Jolliffe and Martineau ran an individual participant data meta-analysis. That’s the gold standard. They pooled data from 43 trials and nearly 49,000 people.

The results: vitamin D reduced acute respiratory infections by about 8% overall (OR = 0.92). But the real story is in the subgroups.

People with very low vitamin D (under 25 nmol/L) saw a 37% reduction in infections. Daily dosing worked. Monthly or yearly bolus dosing didn’t work at all.

If you’re deficient, daily vitamin D is one of the most evidence-backed ways to support your immune system.

Depression: Small But Real

Cheng et al. pooled 25 studies with about 7,500 people. They found vitamin D modestly reduced depression scores (SMD = -0.28). That’s a small effect.

It got bigger in two groups: people with clinical depression and people who were vitamin D deficient. The prediction interval crosses zero, meaning not every study agrees.

Don’t count on vitamin D to fix depression. But if you’re deficient and struggling with low mood, correcting it might help as part of a larger treatment plan.

Muscle Strength: Probably Not

Tomlinson et al. looked at 13 studies with about 2,200 people. The pooled effect wasn’t significant (SMD = 0.17, p = 0.09). The confidence interval crossed zero.

The only positive signals came from severely deficient elderly people. If your vitamin D levels are normal, supplementing won’t make you stronger.

This claim doesn’t hold up for the general population.

D3 vs D2: Which Form Is Better?

Go with D3 (cholecalciferol). Research shows D3 raises blood levels about 70% more than D2 (ergocalciferol).

D3 is what your skin makes from sunlight. It’s the natural human form. D2 comes from plants and irradiated fungi.

Most supplements now use D3. Some vegan options use D3 sourced from lichen. D2 is fine if it’s your only option, but you’ll need a higher dose to get the same blood level increase.

How Much Do You Need?

The answer depends on your starting level. Get a 25(OH)D blood test first.

• Deficient (under 20 ng/mL): Your doctor may recommend 4000-5000 IU daily for 8-12 weeks.
• Insufficient (20-30 ng/mL): 2000-4000 IU daily to get into the optimal range.
• Sufficient (30-50 ng/mL): 1000-2000 IU daily to maintain levels.
• Above 50 ng/mL: You probably don’t need to supplement.

Take it with a meal that contains fat. Vitamin D is fat-soluble. Taking it on an empty stomach reduces absorption.

Obese individuals often need 2-3 times the standard dose because vitamin D gets stored in fat tissue.

Who Should NOT Take Vitamin D

Don’t take vitamin D if you have hypercalcemia (high blood calcium). Vitamin D increases calcium absorption, which makes this worse.

Avoid it if you have severe kidney disease. Your kidneys can’t convert vitamin D to its active form properly. Your doctor will prescribe the active form (calcitriol) instead if needed.

Be cautious with sarcoidosis and other granulomatous diseases. These conditions make your body extra sensitive to vitamin D, raising the risk of high calcium levels.

If you take digoxin, talk to your doctor before supplementing. Vitamin D-induced calcium changes can make digoxin toxic.

The Bottom Line

Vitamin D is one of the most studied supplements in the world. The evidence is clear but nuanced.

For immune support, it works, especially if you’re deficient. Daily dosing of D3 at 1000-4000 IU reduces respiratory infections. The lower your starting levels, the bigger the benefit.

For bones, it’s not the miracle the marketing suggests. Vitamin D alone doesn’t prevent fractures in most people. Combined with calcium, it might help the elderly.

For depression and muscle strength, the evidence is weaker. Small benefits exist for deficient people, but don’t expect much if your levels are already normal.

The real question isn’t “should I take vitamin D?” It’s “am I deficient?” Get tested. If you are, supplement with D3 daily. If you aren’t, you probably don’t need it.