Supplements

Vitamin D

3.4

Vitamin D: Start with a blood test.

4 sections, about 6 minutes in total

600–800

The standard adult reference intake is 600 to 800 IU daily (NIH).

4,000

The daily upper limit without clinician oversight is 4,000 IU.

25(OH)D

25(OH)D is the blood test that measures your vitamin D status.

Section 1 of 4:

Why so many run low

Your skin makes it, but you spend your days indoors.

Your skin makes vitamin D from midday sun, and most modern life happens indoors. Few foods carry meaningful amounts, so you can eat well and still run low. Click through the four reasons.

Tap a step to open it. Each one carries the detail.

Why we say get tested

Low vitamin D is common, so you may be low without knowing. Vitamin D is fat-soluble, so large doses build up in the body. A 25(OH)D blood test shows your actual level. The test decides the dose.

Section 2 of 4:

What the evidence says

What vitamin D helps with.

The bone claim is well supported. The cancer and heart disease claim is not. The falls, immunity and mood claims sit in between, and the evidence is strongest in people who were deficient.

The claimIt keeps my bones strong.
What the evidence saysHolds up. Your body needs vitamin D to absorb calcium. Correcting a deficiency, together with calcium, is the main reason the RDA exists.
The claimIt prevents falls and fractures.
What the evidence saysPartly. There is reasonable support in older adults who are deficient. Trials in people with adequate levels show little effect. A blood test tells you which group you are in.
The claimIt stops me catching everything.
What the evidence saysNot settled. The respiratory infection trials are mixed. Some pooled analyses show small effects, mostly in people who were deficient. The largest trials show little.
The claimIt prevents cancer and heart disease.
What the evidence saysNot supported. The VITAL trial gave vitamin D to 25,871 adults for over five years. It found no reduction in cancer or cardiovascular events.
The claimIt fixes low mood.
What the evidence saysWeak. Low vitamin D and low mood appear together in observational studies. Trials of supplements improving mood are inconsistent. Winter affects mood in other ways too.

These summaries reflect the cited research and will change if the research changes. This is coaching context, not a prescription.

Section 3 of 4:

How much is enough

What each daily amount means.

The RDA is 600 IU a day for most adults and 800 IU after age 70. The adult upper limit is 4,000 IU. Above that is a medical decision. Tap a range to see where it sits.

Tap a range, in IU a day.

Each range shows what the research says about that amount.

Reference values are from NIH ODS, not a prescription. 25(OH)D is the blood test that measures your level. It is read against the NIH ranges. Take vitamin D with a meal that contains some fat. It absorbs poorly on an empty stomach.

Section 4 of 4:

Why testing comes first

Why a blood test comes first.

Vitamin D is fat-soluble. Extra builds up in the body instead of leaving it. A 25(OH)D blood test shows your level, so you do not have to guess.

Without a test

  • You may assume you are fine. Indoor life, higher latitude and darker skin make low levels more likely.
  • You may assume more is better and buy 10,000 IU bottles on a podcast's advice.
  • Excess can build up over time. Vitamin D toxicity is rare, and it almost always comes from large doses taken without a test.

With a test

  • One 25(OH)D test at a routine appointment. It is cheap, standard and done with your other blood work.
  • If you are low, your doctor sets the dose, the schedule and the recheck.
  • If you are fine, you skip years of unneeded pills. Maintenance is sunlight when you can get it, food, and at most an RDA-level amount agreed with your doctor.

Vitamin D supports bone alongside calcium and strength training. The Longevity & Bone Health lab in the Physique wing covers that side of the topic.

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