Often called the “sunshine vitamin,” vitamin D is produced by the body after the skin . This fat-soluble nutrient is also obtainable from certain foods like fatty fish, mushrooms, egg yolks, and fortified dairy products, though ultraviolet B (UVB) light exposure is a major contributor to vitamin D stores.

With fewer daylight hours and less intense sunlight during winter in many regions, maintaining adequate vitamin D levels can be difficult. A from Slovenia found that 63% of adults not taking vitamin D supplements had insufficient levels in winter, compared to only 6% in summer.

“When you’re completely covered up and get minimal sun exposure during northern hemisphere winters, your body absorbs far less vitamin D than it would in warmer weather,” explains Kelsey Higgins, a nutrition support dietitian at Vanderbilt University Medical Center.

Sufficient vitamin D is crucial for general health and wellness. In addition to enhancing calcium and phosphorus absorption—essential for developing and preserving bone strength—vitamin D contributes to immune function, reduces inflammation, and supports muscle coordination, according to Carla Bouwmeester, a clinical professor in pharmacy and health systems sciences at Northeastern University.

In children, vitamin D is essential for preventing rickets, a bone-softening disease. And vitamin D to have mood-regulation benefits in adults.

How much vitamin D is enough?

“Certain individuals can fulfill some or all of their vitamin D requirements through sun exposure,” notes Dr. Elizabeth Ko, associate clinical professor at UCLA’s David Geffen School of Medicine and medical director of the UCLA Health Integrative Medicine Collaborative. “However, factors like season, time of day, day length, cloud cover, smog, skin melanin levels, and sunscreen application all influence UV radiation exposure and vitamin D production.”

Although recommendations for adequate vitamin D intake remain constant throughout the year, your body’s actual vitamin D levels may fluctuate seasonally, Ko explains.

Regarding dietary sources, recommended vitamin D intake depends on age: 600 I.U. daily for children and adults up to age 70, and 800 I.U. daily for adults ages 71 and .

The challenge is that “few foods are rich sources of vitamin D,” states Joan Salge Blake, clinical nutrition professor at Boston University and host of the Spot On! nutrition podcast. The vitamin occurs naturally in fatty fish (like trout, salmon, sardines, and tuna), beef liver, egg yolks, certain mushrooms, and fortified products including cow’s milk, some plant milks, and breakfast cereals—but most other foods lack it.

Some individuals face higher risk of vitamin D deficiency year-round. This includes older adults (since the skin’s vitamin D synthesis capacity declines with age) and those with medical conditions like celiac disease, Crohn’s disease, ulcerative colitis, and other fat-absorption disorders. Gastric bypass surgery patients can also develop vitamin D deficiency, Bouwmeester notes.

Those with darker skin tones are also susceptible to low vitamin D, as melanin pigment decreases their capacity to generate vitamin D from sunlight, according to Bouwmeester.

Additionally, some medications—such as corticosteroids (including prednisone) and certain weight-loss medications (like Orlistat)—can impair vitamin D absorption.

To supplement or not to supplement?

“During winter, we usually lack adequate sun exposure to produce vitamin D,” says Jacqueline Vernarelli, public health nutritionist and associate professor at Sacred Heart University. “Consequently, we must ensure we’re obtaining more from dietary sources or supplements during these months.”

If you don’t regularly eat enough vitamin D-rich foods, you might consider supplementation. The optimal approach is to “have your physician check your vitamin D status,” Salge Blake recommends. A basic blood test can assess your vitamin D levels. Generally, blood levels of 20 ng/mL for most people.

“If levels are low, you can talk with your doctor about supplementation,” Salge Blake says. Should you choose to take supplements, recheck your vitamin D status after several months “to confirm it has reached the appropriate range,” she advises.

Supplements contain either vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol). Though both forms can elevate blood vitamin D levels, D3 raises concentrations “more effectively than D2 and sustains those higher levels for extended durations,” Ko explains.

For those using supplements, “because vitamin D is fat-soluble, taking it with a fat-containing meal enhances absorption,” Vernarelli notes.

However, remember that “with supplementation, exceeding requirements isn’t necessarily beneficial,” Higgins cautions. The tolerable upper intake level for vitamin D from all sources—food, drinks, and supplements—is 4,000 IU.

The wisest approach is maintaining intake between the recommended daily allowance and the upper limit, Bouwmeester advises. “Prolonged excessive vitamin D intake can elevate blood calcium, resulting in high urinary calcium and potential kidney stone formation,” Bouwmeester warns. Excessive blood vitamin D may also trigger heart rhythm disturbances, Salge Blake adds.

You don’t want to exchange one health risk for another—making it crucial to identify and maintain the optimal vitamin D intake level throughout the year.