Specialty Testing Guide › TruHealth Micronutrient Testing
Yes, people in Pennsylvania make little or no vitamin D from sunlight for several months each winter, because the sun sits too low for the skin to use it. Most adults still get through winter without a true deficiency, thanks to body stores built up in summer and vitamin D from fortified foods.
The bigger questions are who is actually at risk, whether you need a blood test, and how much supplement makes sense. On the last two, current guidelines are more cautious than many people expect.
Key takeaways
- From late fall to late winter, sunlight at Pennsylvania's latitude does little to produce vitamin D in your skin.
- Older adults, people with darker skin, people who stay indoors and people with absorption problems carry the highest risk.
- Routine vitamin D blood testing is not advised for healthy adults with no specific medical reason.
- Going above standard intake to prevent disease is mainly suggested for adults over 75, pregnancy and high-risk prediabetes.
Why Pennsylvania winters matter for vitamin D
Your skin makes vitamin D when ultraviolet B (UVB) light hits it. In winter the sun is lower in the sky, its light passes through more atmosphere, and much of the UVB never reaches the ground.
A classic study measured this directly. In Boston, sunlight on clear days from November through February produced no vitamin D precursor in skin samples.1 Boston sits close to the latitude of Pennsylvania's northern border, so most of the state has a similar "vitamin D winter", perhaps a little shorter in the south. Clouds, short days and heavy coats add to the effect.
Summer is different. Short periods of midday sun on bare arms and legs make meaningful amounts of vitamin D, and your body stores some of it for the darker months. That is one reason winter deficiency is less common than the lack of winter sun might suggest.
Who is most at risk
Low vitamin D is more likely if you:
- Are older, since aging skin makes less vitamin D from the same sun
- Have darker skin, which needs more sun exposure to make the same amount
- Spend most days indoors, work night shifts or cover most of your skin outdoors
- Have a condition that limits fat absorption, such as celiac disease, Crohn's disease or past weight-loss surgery
- Have obesity, which is linked with lower blood levels
- Take medicines that affect vitamin D, such as some seizure drugs or long-term steroids
If absorption is the issue, our guide to gut absorption and nutrient levels explains why supplements alone may not fix it.
What the guidelines say about testing
The standard test is 25-hydroxyvitamin D, a blood test that reflects vitamin D from both sun and diet. Many people assume everyone should have it checked every year. The major guideline groups disagree.
The US Preventive Services Task Force found the evidence insufficient to recommend for or against screening adults who have no symptoms.2 The Endocrine Society's 2024 guideline goes further and suggests against routine vitamin D testing in the general population, including people with obesity or darker skin, because no trial evidence shows that routine testing improves health, and no target level has been clearly defined.3
That advice applies to healthy people. Testing still makes sense when there is a clear medical reason, such as osteoporosis, a malabsorption condition, kidney or liver disease, abnormal calcium levels or symptoms like bone pain and muscle weakness. Your clinician decides that, not a winter calendar.
What the guidelines say about supplementing
For healthy adults, vitamin D from fortified milk, fatty fish, eggs and a standard daily intake is enough for most people. The Endocrine Society suggests taking more than the standard recommended intake, without testing, for adults over 75, people who are pregnant and people with high-risk prediabetes. For healthy adults under 75, it suggests against going above the standard intake to prevent disease.3
More is not better. Very high intakes over long periods can raise blood calcium and cause nausea, kidney stones and heart rhythm problems. If you take several products, add up the vitamin D in each, since it often appears in multivitamins, calcium tablets and fortified drinks.
Where specialty testing fits
Some newer panels report vitamin D alongside many other markers. TruHealth from TruDiagnostic, for example, estimates nutrient-related markers from DNA methylation patterns rather than measuring vitamin D in your blood directly.4 An estimate like that can be one input in a wider review, but a low result should be confirmed with a standard 25-hydroxyvitamin D test before anyone starts high-dose supplements.
For a broader view of which symptoms point to which nutrient, see our guide to the signs of nutrient deficiency.
Your next step
If you want to understand how a methylation-based nutrient estimate compares with a direct blood level, read how this test compares with blood micronutrient tests. The micronutrient testing hub gathers every guide on testing, results and supplement safety in one place.
Frequently asked questions
Can I make vitamin D through a window in winter?
No. Ordinary window glass blocks almost all UVB light, so sitting in a sunny room does not produce vitamin D, even in summer. Outdoor exposure on bare skin is what counts.
Do tanning beds help with vitamin D?
Tanning beds raise skin cancer risk, and dermatology groups advise against them. Food and, where appropriate, a supplement are safer ways to get vitamin D.
Should everyone in Pennsylvania take vitamin D in winter?
Not necessarily. Many healthy adults get enough from stored vitamin D and fortified foods. Older adults, people who are pregnant and people with risk factors are the groups most likely to benefit, and your clinician can tell you whether you are one of them.
Does low vitamin D cause fatigue or low mood?
Low levels often turn up in people with fatigue or low mood, but that does not prove vitamin D is the cause. In a large US trial of adults aged 50 and older, vitamin D supplements did not lower the risk of depression over about five years.5 Thyroid problems, poor sleep and other causes deserve a look first.
Related reading
This article is for general education and is not medical advice. Decisions about testing and treatment should be made with your clinician. If you have severe symptoms, call 911. For a suspected poisoning, call Poison Control at 1-800-222-1222.
