Signs of Deficiency and How to Get More

I’m writing this from a coffee shop in Portland, Oregon, where it is, as I type, (shockingly!) raining. If you’ve watched Portlandia or know anything about my—against all weather odds—favorite place in the country, this is not unusual. What is unusual is that I have spent the better part of the last decade convincing myself that the four weeks of actual sunshine we get each summer are probably enough to keep my vitamin D levels in a reasonable place. Reader, they are not.

According to Maya Feller, MS, RD, CDN, founder of Brooklyn-based Maya Feller Nutrition and author of Eating from Our Roots, vitamin D deficiency is estimated to affect up to 75% of adults—and that number holds even for people who would describe themselves as health-conscious, outdoorsy, doing everything right. The signs that you’re running low are easy to write off as something else entirely. Fatigue. Low mood. A general sense of running on empty that you attribute to your schedule, your sleep, your stress—anything but a nutrient your body is supposed to make from sunlight.

We asked Feller to walk us through what’s actually going on—and what we can do about it.

Why Is Vitamin D Deficiency So Common?

The short answer is that the system we rely on to produce vitamin D is more fragile than most people realize. “Vitamin D is synthesized subcutaneously via the skin,” Feller explains. “UVB rays from the sun trigger a reaction where the skin makes vitamin D—but depending on the amount of melanin in a person’s skin, this process can take anywhere from 5 to 30 minutes when skin is exposed to the sun.” Add in the fact that most of us wear sunscreen, spend the majority of our daylight hours indoors, and live far enough from the equator that UVB rays are weakened for months at a time—and the math starts working against you.

Food isn’t a reliable backup. Vitamin D occurs naturally in a fairly short list of foods—egg yolks, salmon, liver, certain mushrooms—and fortified options like milk and cereal rarely contain enough to move the needle on their own. “These foods tend to be underconsumed,” Feller notes, “and in turn don’t have a meaningful impact on vitamin D levels.”

Sorry to write it, but some people are also starting at a disadvantage. Feller flags several groups at higher risk: people with liver disease (since the liver is involved in metabolizing vitamin D), those with GI or malabsorptive disorders, people with darker skin living in northern regions or far from the equator, and anyone who spends extended periods indoors. If you fall into more than one of these categories—or if you live somewhere that gets, say, four weeks of reliable sunshine a year—it’s worth paying attention.

How Do You Know If You’re Low?

The frustrating thing about vitamin D deficiency is that its symptoms are exactly the kind you’d explain away. Feller points to increased susceptibility to illness as one of the more telling signs—adequate vitamin D is needed to support immune function, so if you’re catching everything that goes around, that’s worth noting. Low mood and fatigue are also common indicators, and ones that are particularly easy to attribute to everything except a nutrient deficiency. People with low vitamin D levels are at higher risk of depression, and over time, chronically low levels can affect bone health in ways that compound for years before becoming obvious.

What Vitamin D Actually Does (Beyond Bone Health)

Bone health is the benefit most people associate with vitamin D—and it’s real—but it’s also the least surprising thing on the list. The more interesting case is what’s happening everywhere else.

Mood is the one worth leading with. “Low vitamin D levels are associated with increased symptoms of depression and anxiety,” Feller says. “It’s thought that the deficiency can make the symptoms worse.” This is an area where the research is suggestive rather than definitive—vitamin D isn’t a treatment for depression—but the association is consistent enough that Feller considers safe supplementation worth discussing for people managing anxiety or depression.

Beyond mood, adequate vitamin D supports immune function, helps regulate calcium absorption, and helps reduce inflammation. The research on its broader disease-prevention potential—cardiovascular health, certain cancers, cognitive function—is still developing, and Feller is cautious about where the evidence is solid versus where it gets overstated. Vitamin D is necessary, its deficiency has real downstream effects, and getting enough is a reasonable baseline priority.

How to Actually Get More Vitamin D

The three sources are sun, food, and supplements—and they’re not interchangeable. “The processing of vitamin D is reliant upon a number of factors, including the individual’s baseline health and ability to absorb vitamin D from both food and supplements,” Feller explains. What works for one person may not move the needle for another, which is part of why deficiency is so persistent even among people who think they’re covering their bases.

Sun

The most efficient source, when it’s available. According to Harvard Health, general guidance is:

• Aim for around 10-15 minutes of direct sun exposure on arms and legs, ideally around midday when UVB rays are most intense
• Skip the SPF for that window—sunscreen blocks UVB rays and significantly reduces vitamin D synthesis

Food

For people who eat animal products, Feller recommends eggs, salmon, liver, and fish liver oils as a starting point. “People can safely enjoy one egg per day, seafood twice weekly, and two ounces of liver weekly as functional therapy,” she says. For plant-based eaters, options are more limited:

• UV-exposed mushrooms are the standout natural source
• Fortified plant milks and cereals offer supplementary options, though rarely enough to correct a deficiency on their own

Supplements

This is where most people end up—and where the most common mistake happens. Feller’s guidance:

• Don’t supplement without knowing your baseline levels first
• Follow up with lab work to confirm the plan is working
• Look for D3 (cholecalciferol) over D2—it’s the form the body produces naturally and absorbs more efficiently
• Third-party quality seals (USP, NSF, or Informed Sport) are worth looking for on the label

A Note on Testing and Dosage

If the symptoms sound familiar, testing is straightforward. “Lab tests are mostly accurate and quite reliable,” Feller says. Ask your doctor to include a vitamin D test in your next annual bloodwork—it’s a simple add, covered by most insurance, and knowing your baseline changes everything about how you approach supplementation.

According to the NIH Office of Dietary Supplements, the recommended daily intake is 600 IU for most adults up to age 70, and 800 IU for those over 70—with higher amounts sometimes recommended for those managing a deficiency. But because vitamin D is fat-soluble and accumulates in the body, more isn’t always better. A healthcare provider can help you find the right number for your specific levels and circumstances.

The Takeaway

Vitamin D is not a wellness trend. It’s not a supplement with a moment, or a nutrient that’s been rebranded for a new audience. It’s foundational—the kind of thing that works in the background, supporting the systems that make everything else function better. Getting enough of it is one of the smaller adjustments you can make with one of the longer payoffs. Even if you live somewhere it’s currently, as I type, raining.

This post was last updated on June 30, 2026, to include new insights.