Health

Vitamin D: Testing, Sun, and When Food Isn’t Enough

by Admin

Vitamin D testing has become one of the most requested blood tests, and vitamin D supplements sit in millions of kitchen cupboards. Yet the major guidelines are more cautious than wellness marketing suggests: most healthy adults do not need routine testing, the sun is an unreliable (and risky) source, and only a few foods contain much vitamin D at all.

This guide walks through what vitamin D does, what current guidance says about testing, how sun and food fit in, and the situations where food alone may genuinely fall short. It is educational information, not a diagnosis or a personal supplement plan.

What vitamin D actually does

Vitamin D helps your body absorb calcium, which is one of the main building blocks of bone. The NIH Office of Dietary Supplements (ODS) notes that vitamin D also plays roles in muscle function, nerve signaling, and the immune system (NIH ODS: Vitamin D fact sheet for consumers).

Long-term shortages of vitamin D and calcium can make bones fragile, contributing to osteoporosis in adults; in children, severe deficiency causes rickets. A shortage may also lead to weak, painful muscles (NIH ODS). That bone-and-muscle link is one reason vitamin D comes up alongside strength training for beginners over 40, since muscles help maintain balance and protect bones from falls.

What vitamin D has not convincingly shown is a broad cure-all effect. The American Academy of Dermatology summarizes that evidence for benefits beyond bone health has been inconsistent and insufficient to set nutritional requirements (AAD: Vitamin D statistics and position). The ODS also notes that vitamin D supplements or vitamin D–rich foods do not help with weight loss (NIH ODS).

How much vitamin D do adults need?

The ODS lists these average daily recommended amounts, based on National Academy of Medicine reference values (NIH ODS):

  • Adults 19–70 years: 15 mcg (600 IU)
  • Adults 71 years and older: 20 mcg (800 IU)
  • Pregnant and breastfeeding teens and women: 15 mcg (600 IU)
  • Infants birth to 12 months: 10 mcg (400 IU)

These recommendations were set assuming minimal sun exposure (the AAD notes the RDA was developed on that basis), so they describe intake from food, beverages, and supplements combined (AAD).

There is also an upper limit. For adults, including during pregnancy and breastfeeding, the ODS lists 100 mcg (4,000 IU) per day from all sources combined, unless a doctor prescribes more for a period of time to treat a deficiency (NIH ODS). More is not automatically better.

Vitamin D testing: who needs it?

What the test measures

A vitamin D blood test measures 25-hydroxyvitamin D, the main circulating form, reported in nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL). One nmol/L equals 0.4 ng/mL, so 50 nmol/L is the same as 20 ng/mL (NIH ODS).

According to the ODS, levels of 50 nmol/L (20 ng/mL) or above are adequate for most people for bone and overall health, while levels below 30 nmol/L (12 ng/mL) are too low (NIH ODS). Very high levels, above 375 nmol/L (150 ng/mL), can cause harm.

Labs and clinicians do not all use the same cutoffs, which is one reason a single number on a private lab report can be confusing. Ask the clinician who ordered it how they are interpreting it.

What the guidelines say about routine testing

Two major bodies have looked closely at testing people who feel well:

  • The US Preventive Services Task Force concluded that current evidence is insufficient to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic, community-dwelling, nonpregnant adults (USPSTF: Vitamin D deficiency screening).
  • The Endocrine Society’s 2024 guideline suggests against routine 25(OH)D testing in healthy adults, including adults under 50, adults 50–74, adults 75 and older, during pregnancy, and in generally healthy adults with dark complexion or obesity. The reason given is that blood levels which deliver specific health benefits have not been established in clinical trials (Endocrine Society: Vitamin D for the prevention of disease).

Importantly, these recommendations apply to people without an established reason for testing. They do not mean testing is never useful.

When a clinician may order a test

MedlinePlus explains that a provider may order a vitamin D test if you have symptoms or risk factors for low vitamin D, such as bone weakness or problems absorbing nutrients, or to check whether treatment for a diagnosed deficiency is working (MedlinePlus: Vitamin D test). The Endocrine Society also refers to established indications such as low blood calcium (Endocrine Society).

If you are wondering whether you should be tested, that is a good question to bring to a doctor, along with any symptoms, medications, and health conditions.

Can you get enough vitamin D from the sun?

Your skin makes vitamin D when bare skin is exposed to sunlight, and most people get at least some this way. But the amount varies a lot. Clouds, smog, older age, and darker skin all reduce how much vitamin D the skin makes, sunscreen limits production, and your skin does not make vitamin D from sunlight coming through a window (NIH ODS). The USPSTF adds that winter season, high latitude, and sun avoidance are associated with lower levels (USPSTF).

The bigger issue is safety. UV radiation is a known cause of skin cancer. The American Academy of Dermatology states there is no safe level of UV exposure that allows maximum vitamin D production without raising skin cancer risk, and it does not recommend getting vitamin D from sun exposure or indoor tanning (AAD). MedlinePlus likewise notes that supplements or vitamin D–rich foods are usually a safer way to raise levels than more sun (MedlinePlus).

The practical takeaway: everyday outdoor time is good for you for many reasons, but “sunbathing for vitamin D” is not a strategy health experts endorse. Keep using sun protection.

Food sources of vitamin D

Few foods naturally contain much vitamin D, which is why fortification matters. MedlinePlus lists egg yolks, liver, and fatty fish as natural sources, with vitamin D added to many foods such as breakfast cereals, milk, and other dairy items (MedlinePlus). The ODS adds that some mushrooms are exposed to ultraviolet light to boost their vitamin D content, and that beef liver and cheese contain small amounts (NIH ODS).

Useful options to consider within a balanced diet:

  • Fatty fish such as salmon, trout, sardines, and mackerel
  • Eggs (the vitamin D is in the yolk)
  • Fortified milk, plant milks, yogurts, and breakfast cereals (check the label, since fortification varies by country and brand)
  • UV-exposed mushrooms

Building meals around a protein source, as in our guide to protein at breakfast, is an easy place to fit eggs, fortified dairy, or fish.

When food isn’t enough

Even with a good diet, some groups have a harder time getting enough vitamin D. The ODS lists people more likely to fall short (NIH ODS):

  • Breastfed infants
  • Older adults, whose skin makes less vitamin D
  • People who rarely expose their skin to sunshine, because they stay indoors or keep their body and head covered
  • People with darker skin
  • People with conditions that limit fat absorption, such as Crohn’s disease, celiac disease, or ulcerative colitis
  • People with obesity or who have had gastric bypass surgery

The Endocrine Society also suggests that certain groups may benefit from vitamin D beyond the usual recommended intake without testing first: children and adolescents, adults 75 and older, people who are pregnant, and adults with high-risk prediabetes (alongside lifestyle changes). For healthy adults under 75, it suggests against supplementing above the recommended daily amounts (Endocrine Society).

If you fit one of these groups, or you simply don’t eat fish, eggs, or fortified foods, it is worth asking a doctor, pharmacist, or dietitian whether a supplement makes sense for you and what amount is appropriate. As with other supplements, such as magnesium glycinate for sleep, product quality and your other medications matter.

Too much vitamin D is a real risk

Vitamin D toxicity is uncommon, but it happens, and it is almost always caused by taking too much from supplements. You cannot get too much vitamin D from sunshine, because the skin limits how much it makes (NIH ODS).

Very high blood levels can cause nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive urination and thirst, and kidney stones. Extremely high levels can lead to kidney failure, irregular heartbeat, and even death (NIH ODS).

Vitamin D supplements can also interact with some medications, including the weight-loss drug orlistat, statins, corticosteroids such as prednisone, and thiazide diuretics, so tell your clinician and pharmacist about any supplement you take (NIH ODS: Health professional fact sheet).

A sensible vitamin D checklist

  1. Look at your food first. Do you regularly eat fatty fish, eggs, or fortified foods?
  2. Be honest about sun. Outdoor time helps, but don’t seek extra UV for vitamin D, and keep using sun protection.
  3. Check whether you’re in a higher-risk group. Older age, limited sun, darker skin, absorption problems, obesity, pregnancy, or bariatric surgery are worth raising with a clinician.
  4. Don’t chase a number without context. Routine testing in healthy adults isn’t recommended by major guidelines; testing is most useful when there’s a specific reason.
  5. Respect the upper limit. Count all sources, and don’t take high-dose products unless a clinician has prescribed them.

Frequently asked questions

Should I get a vitamin D test if I feel tired?

Tiredness has many possible causes, from sleep and stress to anemia, thyroid problems, and other conditions. Rather than ordering a vitamin D test alone, it is better to see a doctor who can look at the whole picture. Our guide to burnout vs ordinary stress covers some non-nutritional causes of exhaustion.

Is 20 ng/mL enough?

The ODS describes 50 nmol/L (20 ng/mL) or above as adequate for most people for bone and overall health (NIH ODS). Some labs and clinicians use different targets, so ask how your result is being interpreted.

Does sunscreen cause vitamin D deficiency?

Sunscreen does reduce vitamin D production, but health experts still recommend it, along with protective clothing and shade, because of skin cancer risk (NIH ODS; AAD).

Sources referenced

Medical disclaimer: This article is for general educational purposes only and is not medical advice, diagnosis, or a personalized supplement or treatment plan. It cannot tell you whether you are deficient in vitamin D, whether you need testing, or what dose, if any, is right for you. Always consult a qualified healthcare professional before starting, stopping, or changing a supplement, especially if you are pregnant or breastfeeding, are an older adult, have kidney, liver, parathyroid, or digestive conditions, have had bariatric surgery, have a history of kidney stones or high calcium, or take prescription medications. If you have symptoms such as persistent nausea or vomiting, confusion, severe weakness, irregular heartbeat, or bone pain, seek medical care promptly. In an emergency, contact local emergency services immediately.