Your vitamin D result depends on the month it was drawn

Why the calendar moves the number

Your skin makes vitamin D when bare skin is exposed to ultraviolet B (UVB) light. In winter, the sun sits lower in the sky, days are shorter and more skin is covered, so production falls. The U.S. Preventive Services Task Force lists little or no UVB exposure, for example because of winter season, high latitude or sun avoidance, as a risk factor for low vitamin D.

Other factors reduce production too. The NIH Office of Dietary Supplements notes that clouds, smog, older age, darker skin and sunscreen all lower the amount made, and that sunlight through a window does not produce vitamin D. Few foods contain much vitamin D naturally, so for many people diet does not fill the winter gap.

What the season does to your results

Because the body stores vitamin D and levels change gradually, the low point usually comes in late winter or early spring, weeks after the darkest days. The high point tends to follow late summer.

That means a result drawn in February and one drawn in August are not directly comparable. A drop between them may simply be the season, not a sign that something is wrong. If you are tracking vitamin D over time, test in the same season each time.

Reading the number

Labs may report in ng/mL or nmol/L; 1 nmol/L equals 0.4 ng/mL. Some labs flag 'optimal' ranges higher than these NIH thresholds; ask your clinician how they interpret your result.

• Below 12 ng/mL (30 nmol/L): associated with deficiency, which can weaken bones
• 12 to under 20 ng/mL (30 to under 50 nmol/L): generally considered inadequate for bone and overall health
• 20 ng/mL (50 nmol/L) or more: adequate for most people
• Above 50 ng/mL (125 nmol/L): linked to potential adverse effects

When a test is worth doing

Routine testing of healthy people is not recommended by major groups. The USPSTF found insufficient evidence to recommend screening adults without symptoms. The Endocrine Society's 2024 guideline suggests against routine vitamin D testing in healthy adults, including those with darker skin or obesity, and in adults 75 and older, unless there is a specific reason.

Specific reasons include conditions that affect bone, such as osteoporosis or low calcium, malabsorption disorders, gastric bypass surgery, certain medicines, or very limited sun exposure. If you are already being tested as part of a broader panel, interpret the result with the season in mind and avoid retesting every few weeks, which mostly adds seasonal noise.

An honest word on supplements

Vitamin D is essential for bone. The NIH fact sheet notes that sufficiency prevents rickets in children and osteomalacia in adults, and that evidence supports bone benefits, particularly with calcium. But large trials have not shown that supplements prevent cancer, heart disease, depression, type 2 diabetes or multiple sclerosis, or help with weight loss.

The recommended intake is 600 IU a day for adults through age 70 and 800 IU over 70. The Endocrine Society suggests against routinely taking more than these amounts for healthy adults under 75. The upper limit for adults is 4,000 IU daily unless your clinician advises more for a specific reason. Too much vitamin D, almost always from supplements, can raise calcium levels and cause kidney problems and heart rhythm disturbances.

Vitamin D also interacts with some medicines, including steroids, thiazide diuretics, statins and orlistat. Tell your clinician what you take.

Practical winter steps

• Eat vitamin D sources: fatty fish such as salmon and trout, eggs, and fortified milk, plant milks and cereals
• Get outdoors in daylight when you can; it also helps sleep and mood
• If you take a supplement, choose a daily dose within recommended limits unless your clinician advises otherwise
• If you are tested, note the month and compare like with like
• If you have risk factors for low bone density, ask about calcium and bone health as well