The most effective way to improve your vitamin D levels is a combination of sun exposure, vitamin D-rich foods, and supplementation, with the right mix depending on where you live, your body weight, and the time of year. Most people with low levels will need supplements to make a meaningful difference, especially during winter months. A blood level of at least 20 ng/mL is considered sufficient, though many clinicians aim for 30 ng/mL or higher.
Sun Exposure: Your Body’s Primary Source
Your skin produces vitamin D when exposed to UVB rays from sunlight. For most people with lighter skin, 10 to 15 minutes of midday sun on the arms and legs several times a week is enough to trigger meaningful production. People with darker skin need longer exposure because melanin slows the process. Sunscreen, clothing, and glass all block UVB rays.
Geography matters more than most people realize. If you live north of the 37th parallel (roughly the latitude of Los Angeles), the sun sits too low in the sky from November through March for your skin to produce much vitamin D at all. That line cuts across the southern United States, meaning the majority of the country essentially gets zero vitamin D from sunlight for nearly five months. Anyone living in the northern half of the U.S., Canada, the UK, or northern Europe is relying almost entirely on food and supplements during winter.
Best Food Sources of Vitamin D
Very few foods naturally contain vitamin D, and the ones that do vary enormously in how much they provide. Fatty fish is the clear winner. A 3-ounce serving of cooked sockeye salmon delivers about 570 IU, and farmed rainbow trout comes in even higher at 645 IU. A single tablespoon of cod liver oil provides a massive 1,360 IU.
Beyond fish, the numbers drop quickly:
- UV-exposed white mushrooms (½ cup): 366 IU
- Fortified milk (1 cup): 120 IU
- Fortified plant milks (1 cup): 100 to 144 IU
- Fortified cereal (1 serving): 80 IU
- Sardines (2 canned): 46 IU
- One large egg (scrambled): 44 IU
The vitamin D in eggs is entirely in the yolk, so egg whites contribute nothing. Regular mushrooms that haven’t been exposed to UV light are nearly useless too, providing just 4 IU per half cup. Look for packaging that specifically says “UV-treated” or “high in vitamin D.”
In American diets, fortified foods like milk and cereal supply the bulk of dietary vitamin D. But even drinking three cups of fortified milk a day only gets you to 360 IU, which is well below the 600 to 800 IU daily recommendation for most adults. For most people, food alone won’t fix a deficiency.
Supplements: What Actually Works
If you’re deficient (blood levels below 20 ng/mL), a typical clinical approach starts with higher-dose supplementation for 6 to 8 weeks to build levels up, followed by a lower maintenance dose. Maintenance for most adults lands around 1,000 to 2,000 IU daily. Vitamin D3 (cholecalciferol) is generally preferred over D2 (ergocalciferol) because it raises blood levels more effectively per dose.
One simple change can make your supplement significantly more effective: take it with a meal that contains fat. A study published in the Journal of the Academy of Nutrition and Dietetics found that taking vitamin D with a fat-containing meal increased absorption by 32% compared to taking it on an empty stomach or with a fat-free meal. You don’t need a large amount of fat. A few nuts, some avocado, or eggs with breakfast will do.
Why Body Weight Changes the Equation
If you carry extra weight, reaching sufficient vitamin D levels takes more effort. Vitamin D is fat-soluble, meaning it gets sequestered in fat tissue rather than circulating freely in your blood. Research published in Endocrine Practice found that people with obesity need roughly 40% more vitamin D than non-obese individuals to reach the same blood concentration. Someone who maintains adequate levels on 2,000 IU daily at a normal weight might need closer to 3,000 IU at a BMI over 30. Blood levels in people with obesity also take longer to stabilize, sometimes requiring an extra one to three months before retesting gives an accurate picture.
Magnesium: The Overlooked Co-Factor
Your body can’t actually use vitamin D without magnesium. The enzymes that convert vitamin D into its active form all require magnesium as a co-factor, and the protein that transports vitamin D through your bloodstream does too. If you’re supplementing vitamin D but your magnesium is low, you may not see the improvement you’d expect in your blood work.
Many adults don’t get enough magnesium from their diet. Good sources include pumpkin seeds, spinach, almonds, black beans, and dark chocolate. If you’re addressing a vitamin D deficiency, it’s worth making sure your magnesium intake is adequate at the same time. Some clinicians recommend taking both together.
How Long Before Levels Improve
Vitamin D levels don’t change overnight. After starting supplementation, the standard recommendation is to retest after 6 to 8 weeks of a loading dose to see whether levels have reached the target range. Once you’ve shifted to a maintenance dose, a follow-up test 2 to 3 months later confirms you’re holding steady. People with obesity or absorption issues (from conditions like celiac disease, Crohn’s, or gastric bypass surgery) may need monitoring every 3 to 6 months until levels stabilize.
If you’ve never had your vitamin D tested, a simple blood draw measuring 25-hydroxyvitamin D is the standard test. Levels below 20 ng/mL are considered deficient. Between 20 and 30 ng/mL is sometimes classified as insufficient. Most guidelines consider 30 ng/mL and above to be sufficient for bone health and general function.
Safety and Upper Limits
Unlike sun exposure (your body self-regulates and stops producing excess vitamin D), supplements carry a real risk of overdoing it. The tolerable upper intake level for adults is 4,000 IU per day from supplements and food combined. Going well beyond that over weeks or months can cause calcium to build up in your blood, leading to nausea, kidney stones, and in severe cases, kidney damage. This doesn’t happen from food or sunshine. It’s exclusively a supplement risk, and it generally takes sustained intake well above the upper limit to cause problems. Sticking to a reasonable daily dose and checking your blood levels periodically keeps you in a safe range.