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Vitamin D3

Also known as Cholecalciferol / the sunshine vitamin
AEVIDENCE

The one most people actually are short of.

The honest take
Vitamin D is the rare supplement where the case for a real deficiency is strong and the case for taking more than you need is weak. The VITAL trial gave 2,000 IU a day to nearly 26,000 people for five years and found no reduction in cancer or cardiovascular events — a null result on the two claims the category was built on. What survives is what was always true: correcting a genuine shortfall matters for bone and muscle, and in a Canadian winter a shortfall is likely. Take enough. There is no prize for more, and the upper limit is real.
What it is
A fat-soluble vitamin your skin makes from sunlight, and the one nutrient a northern winter reliably denies you.
How it works
Skin makes cholecalciferol from UVB. The liver converts it to the storage form measured in a blood test, and the kidney activates it into a hormone that tells the gut to absorb calcium. Above about 37° latitude, winter sun is too weak for that first step for months at a time — which is why this is the deficiency that actually shows up in bloodwork here.
What it does
Required to absorb calcium and maintain bone, and involved in normal muscle and immune function.
Who it's for
Almost everyone at this latitude between October and April. Anyone who works indoors, covers up, or has darker skin needs more sun for the same result, so the case is stronger still.
How to use it
Daily with a meal containing fat. If you take more than one product with D3 in it, add the numbers before you add a third.
What to expect
Nothing you can feel, unless you were genuinely deficient — in which case fatigue and aching muscles may lift over weeks. A blood test is the only honest way to know where you started.
The research
Grade A for bone health and for correcting deficiency. VITAL (NEJM, 2018) found no reduction in invasive cancer or cardiovascular events at 2,000 IU/day over five years, with a suggestive reduction in cancer deaths on secondary analysis. The tolerable upper limit is 100 mcg (4,000 IU) per day.

Read the labelCount every source — INTAKE and NERVE together are 70 mcg. Do not exceed 100 mcg/day without a blood test. Speak to a doctor first with kidney disease, sarcoidosis or high blood calcium.

Evidence grades describe how well an ingredient is studied — A well-supported, C emerging — not a promise of results. Educational, not medical advice. Doses are typical ranges; follow the label on the product.