Winter Supplements: Vitamin D3 and K2, Vitamin C, Zinc and Magnesium, and When to Take Each

Four cover it: vitamin D3 with K2, because most people north of Atlanta are low from November to March and low D tracks with more respiratory infections; vitamin C, which modestly shortens colds; zinc, which shortens a cold if it starts in the first day and which many diets run short on; and magnesium, which most people are low in and which the body needs to use vitamin D at all. Sleep, sun when there is any, and real food do more than all four; the supplements fill the winter gap.

With the biggest meal of the day, usually dinner, because both are fat-soluble and absorb far better with dietary fat; a D3 dose taken with a fatty meal absorbs about a third better than one taken on an empty stomach. Once a day is fine; D is stored, so the timing within the day matters less than taking it with fat. K2 (MK-7) rides along in the same softgel and handles the calcium D helps you absorb.

For absorption, yes: 30 minutes before or 2 hours after a meal, and away from dairy, calcium and iron, which block it. If it makes you queasy, and it does for a lot of people, take it with a small amount of food that is not dairy. Zinc lozenges for a cold are different: those are sucked slowly every few hours in the first 24 hours of symptoms, and the point is contact with the throat, not the stomach.

Better not. They compete for the same absorption pathway, and a large zinc dose cuts magnesium uptake. Zinc in the morning on an empty stomach, magnesium in the evening with or after dinner, which also happens to be when magnesium helps sleep. Zinc and iron have the same problem, so keep those apart too. D3, K2 and C can go with anything.

Every step that turns vitamin D into its active form runs on magnesium-dependent enzymes. Someone who is low on magnesium, which is most people on a modern diet, gets less out of the D they take, and a big D dose on top of low magnesium can pull magnesium down further and cause cramps and poor sleep. 200 to 350 mg of a well-absorbed form, glycinate or malate, split through the day, is the fix.

Zinc: oysters by a mile, then beef, lamb, crab, pumpkin seeds, cashews, chickpeas and lentils, with meat zinc absorbing far better than plant zinc. Vitamin D: fatty fish (salmon, mackerel, sardines, herring), cod liver oil, egg yolks from pasture-raised hens, liver, and fortified milk. Almost nobody gets enough D from food in winter, which is why it is the one supplement on this list I consider non-negotiable from November to March.

The adult upper limit is 4,000 IU a day taken long term without testing, and toxicity is real: it shows up as high blood calcium, nausea, kidney stones and worse, usually from months of 10,000 IU or more. Most people do well on 1,000 to 4,000 IU a day in winter. The way to take more is a blood test showing you are low and a doctor's number. A 10,000 IU softgel is a twice-a-week dose, not a daily one, unless a test says otherwise.

Not for most people. Regular vitamin C did not reduce how many colds people caught in the big reviews, but it shortened them by about 8 percent in adults and 14 percent in children, and it did cut the number of colds in people under heavy physical stress, such as soldiers and marathoners. Starting it after a cold begins does little. So 500 mg twice a day through winter is a small, cheap benefit, and the upper limit is 2,000 mg a day, above which it just causes loose stools.