How to choose a Vitamin D supplement

Choosing a vitamin D supplement comes down to six things: the form of D, whether it includes K2, the dose, the carrier it's dissolved in, the ingredient list, and whether it's third-party tested.

Six things. Most brands get the first two wrong and bury the rest.

Vitamin D looks like the simplest buy in the supplement aisle. A few dollars, a plastic bottle, done. That price is exactly the problem. It reflects how little the category takes a foundational nutrient seriously, and it's why so many people who supplement still come up short.

Roughly 70% of US adults sit below 30 ng/mL, the level often used as the cutoff for sufficiency, based on NHANES data analyzed in the British Journal of Nutrition. So the supplement you pick actually matters. Here's the checklist.

1. D3, NOT D2

There are two forms on the shelf: D2 (ergocalciferol) and D3 (cholecalciferol). D3 is the form your body makes from sunlight, and it raises blood levels of vitamin D more effectively than D2.

This one isn't close. A 2012 meta-analysis in The American Journal of Clinical Nutrition pooled randomized trials that directly compared the two and found D3 significantly better at raising serum 25(OH)D, the marker doctors actually measure. If the label says D2, put it back. D3X uses D3.

2. IT SHOULD INCLUDE K2

D3 helps your body absorb calcium.* K2 helps direct where that calcium goes.* They work as a pair, which is why the better supplements include both.

The mechanism is well mapped. K2 activates two proteins: osteocalcin, which binds calcium into the bone matrix,* and matrix Gla protein, which helps keep calcium out of soft tissue like arteries.* The form of K2 matters too. MK-7 has higher bioavailability and a much longer half-life than MK-4, which makes it the better fit for a once-a-day habit. D3X uses K2 as MK-7.

3. THE DOSE SHOULD BE SENSIBLE

Bigger is not better here. The NIH puts the recommended intake at 600 to 800 IU and the tolerable upper limit at 4,000 IU per day for adults. Plenty of brands sell 5,000 or even 10,000 IU, because a big number sells. For daily maintenance most people don't need a megadose, and vitamin D is fat-soluble, so it builds up in the body over time.

A sensible daily dose is the one you can take every day without a second thought. D3X is 2,000 IU per serving. If you think you're very low, get a blood test and talk to your doctor about correcting it.

4. THE CARRIER MATTERS

Vitamin D is fat-soluble, which means it needs fat to absorb well. A dry tablet gives it none. In one study in the Journal of Bone and Mineral Research, simply taking vitamin D with the largest, fattiest meal of the day raised blood levels by about 50% compared with taking it on an empty stomach.

So the carrier isn't a footnote. It's part of whether the thing works at all. The cheapest products use a dry filler or a processed seed oil. D3X dissolves its D3 and K2 in organic olive oil, so the fat that drives absorption is built into every dropper.

5. READ THE INGREDIENT LIST

Flip the bottle over. This is where the corners get cut.

• Proprietary blends. If the label lists a "D3 and K2 blend" as one combined number with no per-ingredient breakdown, you don't actually know what you're getting. Walk away.
• Fillers. Magnesium stearate, silicon dioxide, and similar additives are there to help machines run smoothly, not to help you.
• Dyes and sweeteners. A daily nutrient doesn't need a candy coating.

The shorter the list, the less there is to hide. D3X has three ingredients: D3, K2, and organic olive oil.

6. IT SHOULD BE THIRD-PARTY TESTED

A brand can print anything it wants on a label. Third-party testing is how you know what's inside matches it. Look for products made in a cGMP facility and verified by an outside lab, and prefer glass over plastic so nothing leaches in over the life of the bottle. D3X is made in the USA in a cGMP facility, third-party tested, and bottled in glass.

WHAT IT COMES DOWN TO

Run any vitamin D supplement through the six: D3 over D2, K2 as MK-7, a sensible dose, a fat-based carrier, a short and clean ingredient list, third-party tested. Most of the aisle fails at least two of them. The cheap ones fail most.

That's the case for taking this seriously. Vitamin D is foundational,* and roughly seven in ten people are short on it. The supplement you choose is the difference between fixing that and just owning a bottle.