Vitamin D

Less a vitamin than a hormone your skin makes from sunlight. It governs how much calcium you absorb, which is why a long shortage shows up in bone. At Nova Scotia’s latitude the sun is too low from roughly October to March for skin to make any at all.

Lets the gut absorb calcium and phosphorus, which is why it governs bone strength. Also acts on muscle and immune cells. Made in the skin from UVB sunlight rather than obtained mainly from food.

• Oily fish — salmon, mackerel, sardines
• Egg yolk
• Fortified milk and plant milks
• Cod liver oil
• UV-exposed mushrooms

Fat-soluble. Taking it with the largest meal of the day improves absorption.

600 IU (15 µg) · 800 IU (20 µg) over 70 for most adults. The upper limit, past which harm becomes likely, is 4,000 IU (100 µg).

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

A sustained shortage has a name here: Rickets (children) · Osteomalacia (adults). That is worth pausing on — nutrients whose deficiency earned a name are the ones where the consequences were severe enough, historically, to be described as a disease.

Most at risk: Anyone at Canadian latitudes between October and March — the sun is too low to make any. Also darker skin, covered skin, older adults, and people who are housebound. Health Canada recommends a supplement for adults over 50.

Latitude and season, indoor work, covering the skin, darker skin (melanin slows synthesis), older age, obesity (it is sequestered in fat), and malabsorption. Sunscreen matters less than time of year at this latitude.

Soft, deforming bones in children; bone pain and muscle weakness in adults. Milder shortage is common and often silent.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

Serum 25-hydroxyvitamin D is the standard measure. Reference ranges differ between laboratories and between countries, so a number is best read alongside whoever ordered it rather than against a figure found online.

Few foods carry much. Oily fish — salmon, mackerel, sardines, herring — are the main dietary source; egg yolks and fortified milks and cereals contribute smaller amounts. Diet alone rarely covers a winter this far north.

D3 (cholecalciferol) raises blood levels more reliably than D2. It is fat-soluble and does accumulate, so more is not better; the upper limit exists for a reason. Public health guidance in Canada suggests a supplement through the winter months for most adults.

Only from supplements, never from sun. Excess raises blood calcium, causing nausea, confusion, kidney stones and kidney damage. Case reports of harm generally involve doses well above the upper limit taken for months.

Interactions: Some anticonvulsants, corticosteroids and weight-loss medicines reduce levels.