You get vitamin D three ways: sunlight, food, and supplements. Sunlight is the most efficient source for most people, but where you live, your skin tone, and the time of year all determine whether you’re actually producing enough. Most adults need 600 IU (15 mcg) per day, rising to 800 IU (20 mcg) after age 70.
Sunlight: Your Body’s Built-In Source
Your skin manufactures vitamin D when ultraviolet B (UVB) rays hit a cholesterol compound sitting in your outer skin layers. That compound converts into a precursor form of vitamin D, which then transforms into vitamin D3 over the following hours. Your liver and kidneys finish the job, converting D3 into the active hormone your body actually uses.
The practical recommendation: expose your bare arms and legs to midday sun (between 10 a.m. and 3 p.m.) for 5 to 30 minutes, twice a week. That window matters because UVB rays are strongest when the sun is highest in the sky. Early morning and late afternoon sun won’t trigger much vitamin D production no matter how long you stay outside.
Skin tone significantly changes how much time you need. People with darker skin can require up to ten times longer than fair-skinned individuals to produce the same amount of vitamin D. If you have very dark skin, you may need closer to that 30-minute mark, while someone with light skin might need only 5 to 10 minutes.
Why Winter and Location Matter
If you live above roughly 42°N latitude (think Boston, Chicago, or Portland, Oregon), your skin produces essentially zero vitamin D from sunlight during winter months. The sun sits too low in the sky, and the atmosphere filters out nearly all UVB radiation before it reaches you. Even in slightly lower latitudes, winter vitamin D production drops dramatically, with useful UV exposure limited to a narrow window around solar noon.
Summer UV index values can exceed 12 in some regions, but winter values often fall below 2. At that intensity, you could stand outside all day and barely produce any vitamin D. Cloud cover, air pollution, window glass, and sunscreen (SPF 30 or higher) all block UVB as well. If any of these factors apply to you for large portions of the year, food and supplements become your primary sources.
Foods That Contain Vitamin D
Very few foods naturally contain meaningful amounts of vitamin D, which is part of why deficiency is so common. The richest natural sources are fatty fish: salmon, mackerel, sardines, and trout. A 3-ounce serving of cooked salmon delivers roughly 400 to 600 IU, which alone can cover most of your daily need.
Beyond fish, the options thin out quickly:
- Egg yolks provide about 40 IU each, so you’d need a lot of eggs to make a real dent.
- Beef liver contains modest amounts but isn’t a food most people eat regularly.
- UV-treated mushrooms are one of the few plant sources. Some brands expose mushrooms to UV light during processing, boosting their vitamin D content significantly. Check the label.
- Fortified foods are where most people actually get dietary vitamin D. Milk, orange juice, cereal, and some yogurts are commonly fortified with 100 to 150 IU per serving.
Vitamin D is fat-soluble, meaning your body absorbs it much better when you eat it alongside some dietary fat. Taking a supplement or eating fortified cereal with a completely fat-free meal reduces how much you actually absorb. Even a small amount of fat, like the milk on your cereal or butter on your toast, helps.
Choosing the Right Supplement
If sunlight and food aren’t covering your needs (and for many people, especially in winter, they aren’t), a supplement is the most reliable fix. You’ll see two forms on store shelves: D2 and D3.
Pick D3. It’s the more potent form in humans, raises blood levels more effectively, and stays active in your body longer. D2 has a shorter shelf life, binds less effectively to the proteins that transport vitamin D through your bloodstream, and follows a slightly different metabolic pathway that makes it less efficient overall. D2 is plant-derived, which once made it the default for vegetarians, but many D3 supplements are now made from lichen and are fully vegan.
For most adults, a daily supplement of 600 to 1,000 IU is sufficient. People over 70 should aim for at least 800 IU. Some individuals with diagnosed deficiency may need higher doses temporarily, but that’s best guided by a blood test rather than guesswork.
How Much You Actually Need
The recommended daily amounts set by the National Academies of Sciences, Engineering, and Medicine are:
- Infants (0 to 12 months): 400 IU (10 mcg)
- Children and adults (1 to 70 years): 600 IU (15 mcg)
- Adults over 70: 800 IU (20 mcg)
These numbers represent the amount sufficient for the vast majority of healthy people. Some researchers argue they’re too conservative, but they remain the official benchmark. The tolerable upper limit for adults is 4,000 IU per day. Going well beyond that over time can cause calcium to build up in your blood, leading to nausea, kidney problems, and other serious issues. Toxicity doesn’t happen from sunlight (your skin self-regulates production) or food, only from oversupplementing.
Testing Your Levels
A simple blood test measures your vitamin D status. The test looks at a form called 25-hydroxyvitamin D, which reflects your total intake from sun, food, and supplements combined. Most labs define levels below 20 ng/mL as deficient, 20 to 29 ng/mL as insufficient, and 30 ng/mL or above as adequate. Levels above 50 ng/mL offer no additional benefit and may carry risk.
Testing is especially worth considering if you spend most of your time indoors, live in a northern climate, have dark skin, are over 65, or have a condition that affects fat absorption (like celiac disease or Crohn’s). These factors all raise the likelihood that your levels are lower than you’d expect.
Getting the Most From Any Source
Magnesium plays a critical role in vitamin D metabolism. Your body needs it to complete the conversion steps that happen in the liver and kidneys. If you’re low on magnesium (and many people are), supplementing with vitamin D alone may not fully correct a deficiency. Foods rich in magnesium include nuts, seeds, leafy greens, and whole grains.
Consistency matters more than occasional large doses. Taking a daily supplement with a meal that includes some fat, getting regular midday sun when the season allows, and eating fatty fish once or twice a week creates a reliable baseline. Any single strategy can fall short on its own, but combining two or three makes deficiency unlikely.