Vitamin D vs D3: what's different, and how to tell if your vitamin D supplement is working

Vitamin D is the umbrella name for two forms, D2 (ergocalciferol) and D3 (cholecalciferol), and D3 is the one your skin makes and most Canadian supplements contain. To tell whether your supplement is working, test your 25-hydroxyvitamin D level three to four months after you start and compare it with where you began.

The blood test doesn't take sides: your liver turns both forms into 25-hydroxyvitamin D, and the test measures the total. Numia Health, a physician-ordered, at-home blood testing service in Greater Vancouver, includes it on its Basic Nutrients panel.

What's the difference between vitamin D2 and D3?

BC Guidelines recommend D3, calling it "three times more effective" than D2 at raising blood levels. The 2012 meta-analysis behind that found D3's edge was clearest with large, infrequent doses; with daily supplements, "the effect was lost" (Tripkovic et al., 2012).

How do you know if your vitamin D supplement is working?

A blood test is the clearest way to know. Osteoporosis Canada advises checking "no sooner than three to four months" after starting a standard dose. On average, 1,000 IU (25 µg) of D3 a day raises the level by 15–25 nmol/L (Hanley et al., 2010).

Here's the catch: that's an average. A review found the rise from a given dose "differs between individuals," shaped most clearly by your starting level, body fat, and the dose and type of vitamin D (Mazahery & von Hurst, 2015). A before-and-after pair of Numia results shows your own response.

Season matters too. In a Canadian survey, supplement users' levels didn't differ by season, while more than a third of non-users were below 50 nmol/L in winter (Whiting et al., 2011). Retest in the same season each year, and Numia keeps every result in one app.

How much vitamin D is too much?

Health Canada's upper limit for adults is 4,000 IU (100 µg) a day from food and supplements combined. Count milk, too: since January 2026, Canadian cow's milk carries 200 IU per 250 mL cup. If too much is suspected, your doctor can check calcium and parathyroid hormone.

Where does Numia fit?

Retesting is how a single number becomes a trend. Numia's Basic Nutrients panel measures vitamin D with B12 and magnesium, folate, zinc and iron markers, and needs no fasting. Instead of the usual appointment, lab visit, wait and follow-up, a licensed Canadian physician approves your requisition online in about 3 minutes, and a BC-licensed clinician does a 10-minute morning draw at your home. Results arrive in plain English in the Numia app in 7 days, beside your earlier ones, and the Numia Companion, an AI that has read them, answers questions. Out-of-range results are physician-reviewed with next steps, an optional wellness consultation is available, and Numia is an HSA-eligible service.

Frequently asked questions

What's the difference between vitamin D and vitamin D3?

"Vitamin D" covers two forms, D2 and D3. D3 is the form your skin makes and the one in most Canadian supplements. The test on Numia's Basic Nutrients panel counts both.

When should I retest after starting vitamin D?

Three to four months after starting a standard dose. On average, 1,000 IU of D3 a day adds 15–25 nmol/L, but your response may differ; a Numia retest shows yours.

Is 4,000 IU of vitamin D a day safe?

4,000 IU is Health Canada's upper limit for adults, from food and supplements combined. Going above it is a decision for your physician.