On a blood test, the marker that shows whether you have enough vitamin D is 25(OH)D (25-hydroxyvitamin D). Under Japan’s assessment criteria, 30 ng/mL or higher counts as “sufficient,” below 20 ng/mL as “deficient,” and anything in between as “insufficient.” Many people in Japan are reported to fall short of 30 ng/mL. If you take a supplement and your level still won’t rise, check a few things before increasing the dose: the season and timing of the test, your build, any inflammation, and the status of other nutrients.
What 25(OH)D reflects
Vitamin D is made in the skin when it’s exposed to ultraviolet light (UVB) from the sun, and it also comes in from food, such as fish, eggs, and mushrooms, and from supplements. All of it is converted to 25(OH)D in the liver, and 25(OH)D stays in the blood for several weeks. So 25(OH)D reflects how much you’ve had from sunlight, food, and supplements combined over the past few weeks to months.
Some of the 25(OH)D is converted in the kidneys, as needed, into the active form, 1,25(OH)2D. The active form binds to the vitamin D receptor (VDR) inside cells and regulates the activity of specific genes in the nucleus. This works in much the same way as steroid hormones.
There’s also a test called “1,25(OH)2 vitamin D,” but because the body tightly regulates the active form, it can come out normal or even high when you’re short on vitamin D, so it isn’t suited to checking whether you have enough. When you look at your results, make sure the test name is 25(OH)D. In Japan, results are usually given in ng/mL; nmol/L, common in European sources and research papers, gives numbers about 2.5 times larger (20 ng/mL = 50 nmol/L).
Assessment criteria, and how common low levels are in Japan
Japan’s assessment criteria
25(OH)D has no nationwide reference range in Japan (it isn’t part of Japan’s shared reference intervals, JCCLS), so the categories in the 2017 assessment criteria from a Ministry of Health, Labour and Welfare research group, the Japanese Society for Bone and Mineral Research, and the Japan Endocrine Society are widely used.
| 25(OH)D | Assessment | Precision nutrition optimal range |
|---|---|---|
| 30 ng/mL or higher | Sufficient | 60–100 ng/mL |
| 20 ng/mL to below 30 ng/mL | Insufficient | — |
| Below 20 ng/mL | Deficient | — |
The right-hand column shows the precision nutrition optimal range I use in my counseling. This is a precision nutrition guide, not a medical reference range. It’s also separate from the categories in the assessment criteria.
The categories in the criteria were set based on how 25(OH)D relates to bone and calcium status. A 2011 report from the U.S. Institute of Medicine (IOM) concluded that 20 ng/mL meets the needs for bone health in the great majority of people, and the 2024 guideline from the U.S.-based Endocrine Society said it’s unclear what level is best for preventing disease.
Low levels are common, and they shift a lot with the seasons
In a study of healthy adults in Hokkaido and Kumamoto (in Japan’s north and south), tested at the end of summer and the end of winter, 47.7% were below 20 ng/mL in summer and 82.2% in winter. In a study of about 5,500 people who had checkups in Tokyo, 98% were below 30 ng/mL. In the blood data I’ve seen over the years, too, many people fall short of 30 ng/mL.
Levels are highest from late summer to early fall and lowest around February to March. In the Hokkaido and Kumamoto study, the average fell from 21.1 ng/mL in summer to 14.6 ng/mL in winter. In a study that measured young Japanese women month by month, about 9 in 10 were below 20 ng/mL in spring, just as in winter.
What a low level can mean
The most common reasons for a low level are spending little time in the sun (working indoors, using sunscreen or a parasol, winter, living in northern regions) and a diet with little fish. The skin’s ability to make vitamin D declines with age, and levels rise less easily in people with more body fat (a higher BMI). Illnesses that reduce absorption in the gut, liver or kidney disease, and some medications (such as antiepileptic drugs and steroids) can also be involved.
The clearest link with low vitamin D is in bone: the assessment criteria point to links with fractures, falls, rickets, osteomalacia (softening of the bones), and more. Links with immunity, mood, and blood glucose have also been studied, but outside bone, the results are mixed, and no clear conclusion has been reached. The Endocrine Society’s 2024 guideline, too, suggests taking vitamin D without testing, to help prevent disease, only for certain groups, such as children, people 75 and older, and people who are pregnant. A low value alone doesn’t mean you have any particular illness.
What a high level can mean
High values are rare in people who don’t take supplements; when a value comes out high, it may be that you’re getting a large amount from supplements. Too much vitamin D can raise calcium in the blood, which can lead to nausea, thirst, and strain on the kidneys.
The IOM report says higher levels aren’t consistently linked to benefits, and that values above 50 ng/mL call for caution about possible harm to the body. The Endocrine Society’s guideline also doesn’t recommend that healthy adults under 75 take more than the U.S. and Canadian Dietary Reference Intakes (15 μg = 600 IU a day for adults, and 20 μg = 800 IU for those 71 and older) to prevent disease. As the table above shows, the precision nutrition optimal range I use in my counseling is higher than the 50 ng/mL the IOM cautions about. So when raising levels with supplements, I proceed while also checking calcium and kidney function values. If your level is above 100 ng/mL, or you’ve been told your blood calcium is high, please talk with a doctor.
How and when to test
In Japan, 25(OH)D isn’t one of the standard checkup markers, and testing covered by national health insurance is mainly for situations involving bone disease. Otherwise, it’s measured as an add-on to a comprehensive health screening (what Japan calls a ningen dock) or as a test you pay for yourself.
The Endocrine Society’s guideline doesn’t recommend routinely testing healthy people who have no clear reason to be tested. When I look at this marker, it isn’t to find illness. It’s to read your current status alongside the season and your daily life, to decide whether to revisit your sun exposure, your diet, and how you take supplements, and to check that you aren’t taking too much.
If you test only once a year, the end of winter (around February to March), when levels are at their lowest, is the time when it’s easiest to check whether you have enough. Testing once in summer and once in winter shows how much your own levels shift with the seasons. If you start a supplement, test before you start and again after about 3 months. Values can differ depending on the measurement method, so if possible, compare results from the same lab.
If you take it but your level won’t rise
First, look at how long you’ve been taking it, and the season. It’s thought to take at least 2 to 3 months for levels to settle, and longer depending on the dose and your build. If you start in the fall and test in winter, the seasonal drop can cancel out the rise, so it may look as if nothing has changed. Levels can also read low when there’s inflammation, for example from an injury, surgery, or an infection.
Next, look at your build and how you take it. In people with more body fat, the same dose tends to raise levels less. In a randomized trial that compared taking it on an empty stomach, with a high-fat meal, and with a low-fat meal, there was no difference in how much 25(OH)D had risen after 3 months. Before worrying about whether you take it with a meal, check first whether you’re taking it consistently. If your gut has been feeling off for a while, or you’re taking medication, please check with a doctor about possible effects on absorption and metabolism.
One more thing to look at is magnesium. The enzymes that convert vitamin D are thought to need magnesium (based mainly on test-tube and animal studies). In a randomized trial in people, taking magnesium raised 25(OH)D in those who started at around 30 ng/mL, but actually lowered it in those who started higher (up to about 50 ng/mL). It isn’t as simple as “take magnesium and the level goes up.” Blood magnesium doesn’t reflect the amount in your body very well, so look back on whether you’re eating foods such as seaweed, beans, nuts, and less-refined grains.
If none of these apply and your level still won’t rise, please don’t keep increasing the dose on your own; talk with a doctor.
If your level rises but you don’t feel any different
Vitamin D’s clearly established roles are in bone and calcium, and it isn’t the kind of thing you can feel right after taking it. Feeling off, such as tiring easily, has many possible causes, and vitamin D alone can’t explain it.
Variants of the VDR gene that many people carry (such as FokI and TaqI) have also been studied as a source of individual differences on the receiving side. But reviews of the research are split on whether the rise in 25(OH)D from supplements differs by variant, with some finding a difference and some not, and how the variants change the receptor’s activity isn’t yet well understood. I don’t recommend the idea of “my gene variant makes it work less well, so I’ll take more.” I prioritize revisiting zinc status (serum zinc, with ALP as a clue), inflammation and allergies (CRP and eosinophils), and sleep and diet together.
How I read the numbers
The optimal range for 25(OH)D
Under the assessment criteria, 30 ng/mL or higher counts as “sufficient,” but from a precision nutrition perspective, I look at whether a “sufficient” value also falls within the optimal range in the table above.
I also read it alongside the season. The same 32 ng/mL means something different when it’s measured in September than when it’s measured in February. Levels drop in winter compared with summer, so if you’re only just above 30 at the end of summer, you may fall into the “insufficient” category by winter.
Using supplements to find your balance
For adults whose 25(OH)D falls short of the optimal range, what I suggest in my counseling is starting at 5,000 IU (125 μg) a day. This is more than the tolerable upper intake level in the Dietary Reference Intakes for Japanese from Japan’s Ministry of Health, Labour and Welfare (100 μg = 4,000 IU a day for adults). So it isn’t something to start or continue on your own: it assumes that after about 3 months, you’ll have your 25(OH)D, calcium, and kidney markers measured and revisit the dose. If you’re pregnant or breastfeeding, under a doctor’s care, or taking medication, and especially if you’ve been prescribed an active vitamin D medication, please check with your regular doctor before you start.
Reading it alongside other markers
The optimal ranges for the markers below are also the precision nutrition optimal ranges I use in my counseling, just like the one for 25(OH)D. These are precision nutrition guides, not medical reference ranges. Falling outside them doesn’t by itself mean you have an illness.
When 25(OH)D is low and ALP is above the reference range, it may mean bone is being rebuilt more actively, so I recommend having a doctor check it. In Japan, the way ALP is measured began changing in April 2020, facility by facility. The current adult range in Japan’s shared reference intervals (JCCLS) is 38–113 U/L, and the old method gave numbers about three times as high. For adults, the optimal range for ALP is a single point, 63 U/L, not a range. It’s meant for values from the current method (the IFCC method), so I don’t apply it to results measured the old way.
When ALP is on the low side, on the other hand, I keep zinc status in mind. ALP is an enzyme that contains zinc, but the Japanese Society of Clinical Nutrition’s “Clinical Practice Guideline for Zinc Deficiency 2024” removed low ALP from its criteria, noting that ALP doesn’t drop in many people who are low in zinc. So I use ALP only as one cue to notice, and I check zinc status with serum zinc. The optimal range for serum zinc is 90–120 μg/dL. The VDR, too, has a zinc-containing structure (a zinc finger) in the part that binds to DNA, so I look at vitamin D and zinc together, but it hasn’t been confirmed whether low zinc weakens how the VDR works in people.
When eosinophils or CRP are on the high side, I don’t judge from the vitamin D number alone; I also look at whether inflammation or allergies could be in the background (see the five types of white blood cells). For eosinophils, the optimal range is 1–2% for women and 0–2% for men; for CRP, it’s 0–0.05 mg/dL. Inflammation lowers 25(OH)D itself. In a study after knee surgery, it fell by about 40% within 2 days as CRP rose, and 3 months later it was still about 20% below its level before surgery. If 25(OH)D was measured while CRP was high, I have it retested once the inflammation has settled. If your CRP is clearly high, or you’re feeling off, please talk with a doctor.
For people taking supplements, I also check for changes in calcium and kidney markers (creatinine and eGFR).
What you can revisit in everyday life
The UV light that makes vitamin D is strongest around midday, when the sun is high, and weaker in the morning and evening, in winter, and in northern regions; it barely passes through window glass. One estimate for a clear December day in Tsukuba, Japan, found that, with your face and the backs of your hands exposed, making 5.5 μg (the adequate intake at the time) takes about 22 minutes at noon but about 1 hour 45 minutes at 9 a.m. There’s no need to stay out until you get sunburned; adjust the amount to your skin and the season.
In food, vitamin D is plentiful in fish such as salmon, sardines, Pacific saury, and dried baby sardines (shirasu), as well as in egg yolks, wood ear mushrooms, and dried shiitake mushrooms. The Dietary Reference Intakes for Japanese (2025 edition) from Japan’s Ministry of Health, Labour and Welfare set the adequate intake for adults at 9.0 μg a day and the tolerable upper intake level at 100 μg (4,000 IU) a day. The upper level applies to the total from food and supplements combined.
If you’re concerned
Please talk with a doctor if your 25(OH)D is below 20 ng/mL or above 100 ng/mL, if bone pain or muscle weakness won’t go away, or if you’ve been told your bone density is low or your calcium is abnormal. If you’re pregnant or breastfeeding, under a doctor’s care, or taking medication, and especially if you’ve been prescribed an active vitamin D medication, check with your regular doctor first, before starting a supplement. Please don’t stop any prescribed medication on your own.