That means that from about September to June, you can’t rely on the sun to give you the vitamin D you need, she says. Plus, if you wear sunscreen in the summer, as you should, you can kiss your vitamin D production goodbye.

Sunny days aside, you can also get vitamin D from food, but the options are limited. Fatty fish like salmon, swordfish, tuna, mackerel and sardines are all sources of vitamin D, as are egg yolks. Some cereals, orange juice, milk, cheese and yogurt are fortified with vitamin D, too, meaning it’s added in during production to make it easier for people to sneak in vitamin D.

How much vitamin D do you need?

Unless you are really vigilant about tracking vitamin D in the foods you eat, it can be hard to know if you’re getting enough. Symptoms of vitamin D deficiency can be vague, says Lucille Marchand, M.D., a family medicine doctor at UW Medical Center.

Many people who have lower-than-normal vitamin D levels don’t realize it. Your doctor may suggest a blood test to measure your vitamin D level if you have fatigue, muscle aches or bone aches that can’t be attributed to something else, she says.

“If they correct the deficiency, chances are, they’re going to help that person feel better,” says Marchand.

How much do you need? The Institute of Medicine (IOM) recommended dietary allowance is 600 international units up to age 70, and 800 IU for people who are older. Both Marchand and Tick agree these recommendations are conservative.