Vitamin D in Fall and Winter: When Testing Makes Sense

Every October the same question reappears: the days are shorter, the sun is lower, and a friend mentions they just had their vitamin D checked. Should you? The honest answer is less satisfying than a wellness influencer would like. Major U.S. bodies do not recommend testing everyone, yet testing is clearly sensible for some people. This guide explains why winter changes the picture, what the numbers on a lab report mean, who has a reason to ask, and what food and sunlight can and cannot do.

ENERGYbits® • Nutrient Basics

Why vitamin D testing comes up every fall

Vitamin D is unusual among nutrients because most people get it from two very different places: a little from food and, in the right season, a good deal from sunlight striking the skin. Fall is when the second source starts to fade for people living in the northern half of the United States, Canada and Europe. Add more time indoors, long sleeves and, for many, a desk job, and it is easy to see why people wonder about their levels.

The trouble is that “I might be low in winter” and “I should pay for a blood test” are not the same statement. Plenty of people are fine, and the guidelines are cautious about testing healthy people with no symptoms. This article separates three questions that often get blurred together: does winter really reduce vitamin D production, what is a low result, and who benefits from knowing their number.

If you are interested in the wider nutrient picture, our posts on micronutrient gaps on a low-calorie diet and immune support nutrients and food sources cover related ground. This piece focuses on testing and the fall-to-winter transition.

The vitamin D winter: what latitude and season do

Your skin makes vitamin D3 when ultraviolet B (UVB) light, roughly 290 to 320 nanometers in wavelength, reaches it. The NIH Office of Dietary Supplements notes that season, time of day, cloud cover, smog, skin melanin and sunscreen all affect how much your skin makes, and that UVB does not pass through window glass [1][2]. So sitting by a sunny office window in January does not count.

The classic latitude study

The best-known demonstration is a 1988 study by Webb, Kline and Holick in the Journal of Clinical Endocrinology and Metabolism. The researchers exposed skin samples and a vitamin D precursor to sunlight on cloudless days. In Boston (42.2°N), no previtamin D3 formed from November through February. In Edmonton (52°N), the dead period stretched from October through March. At 34°N and 18°N, sunlight still produced previtamin D3 in midwinter [5]. The authors concluded that higher latitudes have a longer “vitamin D winter,” when dietary vitamin D may be advisable [5].

What this means for fall

Fall is the transition. In the northern U.S. and Canada, the UVB window can close earlier than most people expect: in the 1988 data, as early as October at the latitude of Edmonton. For those living in the southern tier of the country, the window stays open longer. That is general geography, not a personal prediction, and it is one reason a single seasonal rule of thumb (“everyone should test in November”) does not hold up.

What your 25(OH)D number means

The standard blood test measures 25-hydroxyvitamin D, written 25(OH)D, which reflects vitamin D from both food and skin. Results come in nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL). To convert, multiply nmol/L by 0.4 to get ng/mL [2]. The NIH Office of Dietary Supplements summarizes the commonly used cutoffs below [1][2].

Two points are worth keeping in mind. First, these cutoffs come from a consensus process, and experts still debate where “optimal” sits; some labs and clinicians use higher targets than the NIH figures, which is a major reason your report might flag a value as “low” that the NIH calls adequate. Second, the NIH notes that most U.S. adults have adequate levels, though nearly one in four have levels that are too low or inadequate [2]. A single reading is also a snapshot: it moves with season and recent intake.

What the Dietary Reference Intakes say

The recommended dietary allowance (RDA) is 15 micrograms (600 IU) a day for ages 1 to 70, including pregnancy and lactation, and 20 micrograms (800 IU) for adults over 70. The tolerable upper intake level (UL) for ages 9 and up is 100 micrograms (4,000 IU) a day [1]. Those are the official reference points. Any amount above the RDA, and certainly above the UL, is a decision for you and your clinician, informed by a test where appropriate.

What the guidelines say about testing

USPSTF: insufficient evidence for screening

In April 2021 the U.S. Preventive Services Task Force issued an “I statement,” meaning the current evidence is insufficient to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic adults. The statement covers community-dwelling, nonpregnant adults and excludes people who are hospitalized or living in institutions [3]. Note what an I statement is not: it is not a recommendation against testing, and it is not a statement that deficiency is unimportant. It says the research cannot yet show that screening everyone leads to better outcomes.

Endocrine Society: no routine testing without an indication

The 2024 Endocrine Society guideline (Demay and colleagues, JCEM) states that the panel “suggests against routine 25(OH)D testing in the absence of established indications.” That applies to the general population as well as to adults with obesity or dark complexion, and to pregnancy [4]. The same guideline suggests empiric vitamin D for children and adolescents aged 1 to 18, adults 75 and older, people who are pregnant, and adults with high-risk prediabetes, and suggests against going beyond the Dietary Reference Intakes in generally healthy adults under 75 [4]. These are guideline suggestions with varying certainty, not individual medical advice.

Who might reasonably consider testing

If routine testing is not recommended, who is the exception? The NIH describes groups at higher risk of inadequate vitamin D. Those groups are a reasonable place to start a conversation with your clinician, who can decide whether a test is warranted [1][2].

Note the Endocrine Society's position even for some of these groups: its guideline suggests against routine testing in adults with obesity or dark complexion in the absence of other indications [4]. Higher risk of inadequacy and a recommendation to test are different things. That is exactly why this decision belongs in a conversation with a clinician who knows your history.

Medicines that matter

The NIH consumer fact sheet lists several interactions worth mentioning to your provider: orlistat can reduce vitamin D absorption, steroids such as prednisone can lower vitamin D blood levels, statins may work less well with high-dose vitamin D supplements, and thiazide diuretics combined with vitamin D supplements could raise blood calcium too high [2]. If you take any of these, mention it before adding or changing a supplement.

Signs that call for a doctor, not a home kit

Persistent bone pain, muscle weakness, frequent fractures or unexplained fatigue can have many causes, and vitamin D is only one. Those symptoms deserve a proper evaluation instead of an assumption that sun or supplements will fix them. A clinician can order the right tests, including calcium and other markers, and interpret them together.

If you do get tested: how to make the result useful

Suppose you and your clinician decide a test is worth doing. A few practical habits make the result easier to interpret.

• Tell your clinician what you already take. Multivitamins, cod liver oil, fortified products and standalone vitamin D all contribute to the number.
• Note the season and recent travel. A level drawn after a sunny holiday reads differently from one drawn in February.
• Ask what range the lab uses. Compare it with the NIH cutoffs above so a “low” flag does not cause needless alarm, and ask your clinician which target they use and why [1][2].
• Ask what the plan is before you test. If no result would change your approach, the test may not add much. If it would, it is worth doing.
• Consider retesting only if there is a reason. There is no universal schedule; follow your clinician’s advice.

Direct-to-consumer kits are widely marketed. The guidelines above are about routine testing in general; if you do use a kit, share the result with your clinician instead of acting on it alone, especially before taking large amounts of any supplement.

Food, sunlight and supplements: the NIH picture

The NIH is clear that few foods naturally contain vitamin D, and that in the United States fortified foods supply much of what people get [2]. Fatty fish and fish liver oils are the best natural sources. The table below lists examples from the NIH Office of Dietary Supplements [1].

Practical ways to build a fall menu

• Fish twice a week is a common pattern: trout or salmon deliver a large share of a day’s value in one serving [1].
• Check labels on milk, plant milks, cereals and yogurts; fortification varies by product [2].
• UV-exposed mushrooms are one of the few plant-based sources. Look for the UV or “high in vitamin D” label [1].
• Eggs contribute small amounts, found in the yolk [1].
• Cod liver oil is potent, but it is also a source of vitamin A, so check labels and talk with your clinician before making it a habit.

Sunlight: useful, not a prescription

The NIH notes that some experts suggest roughly 5 to 30 minutes of midday sun on the face, arms, hands and legs several times a week, without sunscreen, as a way to make vitamin D. It also notes that sunscreen of SPF 8 or higher appears to block vitamin D-producing UV rays, while recommending sun protection for longer outdoor time [1][2]. In winter at northern latitudes, that approach may simply not produce much [5]. Skin protection and vitamin D are a real tradeoff, and your dermatologist and primary care clinician are the right people to help you strike it.

Supplements

Supplements are a legitimate tool, particularly for the groups the Endocrine Society names [4]. How much to take is a clinical decision. This article deliberately stays at the level of the official reference values: an RDA of 15 to 20 mcg (600 to 800 IU) depending on age, and a UL of 100 mcg (4,000 IU) for ages 9 and up [1]. If a clinician recommends a specific amount for you, follow that guidance, not a blog.

Testing, food and supplements at a glance

Where ENERGYbits fits (and does not)

Since you are reading this on the ENERGYbits blog, one clear point: ENERGYbits is not a vitamin D source. Spirulina and chlorella do not appear among the food sources the NIH Office of Dietary Supplements lists for vitamin D [1], and we are not suggesting they replace fish, fortified foods, sunlight, a supplement or your clinician’s advice on vitamin D.

What spirulina and chlorella are is whole foods with their own nutritional story: spirulina is rich in protein, phycocyanin, chlorophyll, vitamins and minerals, and chlorella offers chlorophyll with a cracked cell wall for digestibility. If you already eat well and want to add a whole-food algae, ENERGYbits spirulina is a single-ingredient tablet with no fillers or additives, grown in freshwater, gently dried at low temperatures and third-party tested for purity and contaminants. The VITALITYbits spirulina and chlorella blend combines both. A full ENERGYbits serving is 30 tablets of 250 mg each, 7.5 g in total. None of that changes the vitamin D conversation above.

As with any supplement, check with your healthcare provider first if you are pregnant or nursing, have phenylketonuria, an autoimmune condition, a bleeding disorder, or thyroid or iodine concerns (relevant to chlorella), or take blood thinners or immunosuppressants.

Conclusion

Fall is a sensible time to think about vitamin D, and a poor time to panic. Winter sun at northern latitudes really can fall short, and some people have genuine reasons to check their level. But the leading U.S. guidance does not support testing everyone, and the most useful steps are the unglamorous ones: eat the foods that carry vitamin D, understand your own risk factors, and ask your clinician whether a test would change anything for you.

If you decide to test, bring the result to someone who can read it in context. If you decide to skip, you are in line with current guidance for healthy adults. Either way, food, curiosity and a good clinician beat guesswork.

Frequently asked questions

When is the best time to test vitamin D? +

There is no universal best time. Levels tend to fall with less sun, so a reading in late winter may be lower than one in late summer. Because guidelines do not recommend routine testing for healthy adults, timing matters mainly if you and your clinician have a reason to test [3][4]. Ask your clinician which timing suits your situation.

Do I need a vitamin D test if I feel fine? +

Usually not. The USPSTF found insufficient evidence to assess screening in asymptomatic adults, and the Endocrine Society suggests against routine testing without an established indication [3][4]. If you have risk factors such as a fat-absorption condition, discuss them with your clinician.

What is a normal vitamin D level? +

According to NIH ODS, 50 nmol/L (20 ng/mL) or more is generally adequate, 30 to under 50 nmol/L is generally inadequate, and under 30 nmol/L (12 ng/mL) is associated with deficiency. Over 125 nmol/L (50 ng/mL) is linked to potential adverse effects [1][2]. Labs may flag different ranges.

How do I convert nmol/L to ng/mL? +

Multiply nmol/L by 0.4 to get ng/mL, so 50 nmol/L equals 20 ng/mL [2]. To go the other way, divide ng/mL by 0.4.

Can you get enough vitamin D from sun in winter? +

At higher latitudes often not. A 1988 study found no previtamin D3 formed in Boston from November through February and in Edmonton from October through March [5]. Windows block UVB, and clouds, smog, sunscreen and skin pigmentation also reduce production [1][2].

Does skin color affect vitamin D? +

Yes. The NIH lists people with darker skin among groups more likely to have inadequate vitamin D, because melanin reduces UVB-driven production [1][2]. The Endocrine Society nonetheless suggests against routine testing in adults with dark complexion absent other indications [4]. Discuss it with your clinician.

What foods are highest in vitamin D? +

Per the NIH, cod liver oil (34 mcg per tablespoon), cooked farmed rainbow trout (16.2 mcg per 3 oz) and cooked sockeye salmon (14.2 mcg per 3 oz) are highest. UV-exposed mushrooms, fortified milk and egg yolks provide smaller amounts [1].

Are vegans more likely to be low in vitamin D? +

The NIH says people with milk allergy, lactose intolerance, or ovo-vegetarian or vegan diets are more likely to have inadequate intake, since most natural sources are animal-based [1]. Fortified plant milks and UV-exposed mushrooms can help; check labels [1][2]. Ask your clinician whether testing makes sense.

What is the RDA for vitamin D? +

The RDA is 15 mcg (600 IU) for ages 1 to 70, including pregnancy and lactation, and 20 mcg (800 IU) for adults over 70 [1].

How much vitamin D is too much? +

The tolerable upper intake level is 100 mcg (4,000 IU) a day for ages 9 and up, with lower limits for younger children [1]. Blood levels over 125 nmol/L (50 ng/mL) are linked to potential adverse effects [1]. Talk with your clinician before using amounts above the RDA.

Do medications affect vitamin D? +

Some do. The NIH lists orlistat (reduced absorption), steroids such as prednisone (lower blood levels), statins (possibly less effective with high-dose vitamin D) and thiazide diuretics (risk of high calcium when combined with supplements) [2]. Tell your provider about everything you take.

Does spirulina or chlorella provide vitamin D? +

They are not recognized vitamin D sources and do not appear among the food sources the NIH lists [1]. ENERGYbits is not a vitamin D source. Spirulina and chlorella are whole foods valued for other nutrients, such as protein and chlorophyll, and they do not replace guidance on vitamin D.

Should I take a vitamin D supplement in winter? +

It depends on you. The Endocrine Society suggests empiric supplementation for children and adolescents, adults 75 and older, pregnancy and high-risk prediabetes, and suggests against going beyond the Dietary Reference Intakes in generally healthy adults under 75 [4]. Ask your clinician what fits your situation.

This article is for educational purposes only and is not medical advice. These statements have not been evaluated by the FDA. ENERGYbits® products are whole-food supplements and are not intended to diagnose, treat, cure, or prevent any disease. Talk with your doctor before starting any supplement or ordering a lab test, especially if you are pregnant, nursing, taking medication, or managing a medical condition.

References

1. National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
2. National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Consumers. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
3. US Preventive Services Task Force. Screening for Vitamin D Deficiency in Adults: Recommendation Statement. JAMA. April 13, 2021. doi:10.1001/jama.2021.3069. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/vitamin-d-deficiency-screening
4. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947. doi:10.1210/clinem/dgae290. https://academic.oup.com/jcem/article/109/8/1907/7685305
5. Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol Metab. 1988;67(2):373-378. https://pubmed.ncbi.nlm.nih.gov/2839537/