What Does Vitamin D Do?
Vitamin D helps your body absorb calcium, and calcium is needed for normal bone formation. Vitamin D also has a role in your nerve, muscle and immune systems. Few foods naturally contain it, so much of your supply depends on sunlight reaching your skin, and in a Northeast winter that source largely switches off.
That matters if you are asking whether your winter fatigue has a cause you can measure. Below, you will find why levels fall, who is most at risk, what the blood test shows, and what large trials found when people took extra vitamin D. The goal is simple: test, don't guess.
Why Your Skin Makes Less Vitamin D in Winter
Your skin makes vitamin D when ultraviolet B (UVB) rays in sunlight reach it. In winter at northern latitudes, the sun sits low and far less UVB gets through.
A 1988 study in the Journal of Clinical Endocrinology and Metabolism measured this directly. On cloudless days in Boston (42.2 degrees N), sunlight from November through February produced no previtamin D3 in skin. In Edmonton (52 degrees N), the ineffective period ran October through March. The authors described a "vitamin D winter" that lengthens with latitude, a period when supplementation may be advisable.
Westchester County and Fairfield County sit at a similar latitude to Boston, so the same seasonal pattern is likely to apply here. That study did not measure our area.
Latitude is not the only factor. The Institute of Medicine's 2011 report found that skin synthesis varies with season, skin pigmentation, genetic factors, sunscreen use, latitude and outdoor activity. Its recommended intakes assume minimal or no sun exposure.
Who Is Most Likely to Run Low
MedlinePlus notes that people with dark skin, people who often spend time in shade or on cloudy days, and people who live far from sunny areas make less vitamin D. The Institute of Medicine adds people with poor nutrition and people living in institutions to the list of those who may be at increased risk of not meeting their needs.
MedlinePlus also says some groups may need extra vitamin D:
- Older adults
- Breastfed infants
- People with dark skin
- People with conditions that limit fat absorption, such as Crohn's disease, celiac disease and ulcerative colitis
- People with obesity or who have had gastric bypass surgery
Signs of Low Vitamin D
MedlinePlus lists bone pain, muscle weakness or aches, soft or deformed bones, and weak bones with fractures as signs of low vitamin D. Long-term deficiency can lead to osteomalacia and osteoporosis in adults and rickets in children.
These signs are not specific. Muscle aches and fatigue have many possible causes, so symptoms alone cannot tell you whether vitamin D is the problem. A blood test is how you know.
The 25-Hydroxyvitamin D Blood Test and What the Numbers Mean
The Institute of Medicine describes serum 25-hydroxyvitamin D as a useful integrated marker of vitamin D exposure. It combines what your skin makes with what you get from food, fortified products and supplements.
The CDC's National Center for Health Statistics, using Institute of Medicine thresholds, groups results like this:
- Under 30 nmol/L (12 ng/mL): at risk of deficiency
- 30 to 49 nmol/L (12 to 19 ng/mL): at risk of inadequacy
- 50 to 125 nmol/L (20 to 50 ng/mL): sufficient
The Institute of Medicine reports that a level of 20 ng/mL (50 nmol/L) covers the needs of at least 97.5% of the population, and that levels above 50 ng/mL (125 nmol/L) should raise concern about potential adverse effects. Higher is not better.
Should Everyone Get Tested? Where Experts Disagree
The experts do not agree on this, and you should know that before you order a test.
The Endocrine Society's 2024 guideline suggests against routine 25-hydroxyvitamin D testing in the general population. It says blood levels that provide outcome-specific benefits have not been established in clinical trials, and it sets no numeric target. It does not apply to people with established reasons for vitamin D treatment or testing.
MedlinePlus also says routine testing is not recommended for everyone. A provider may order it for people with bone problems, symptoms such as bone pain, malabsorption or weight-loss surgery, or other high risk.
A functional medicine view fits between these positions. Test when your history gives a reason, rather than guessing in either direction. Bone pain, an absorption problem, limited time outdoors or a history of low levels are reasons to look.
Food First, and Why It Rarely Covers Winter Needs
MedlinePlus names fatty fish such as tuna, salmon and mackerel among the best food sources. Beef liver, cheese and egg yolks contain smaller amounts. Some mushrooms exposed to ultraviolet light are higher, and many milks, cereals, soy drinks, orange juices, yogurts and margarines are fortified.
Because few foods naturally contain vitamin D, and because recommended intakes assume little or no sun, diet alone may fall short in the months when your skin makes none. A nutrition review can show where your diet stands. Our nutrition counseling team can help you build meals around the foods that do contain it.
Supplements: How Much Is Enough and How Much Is Too Much
The Institute of Medicine's recommended dietary allowance is 600 IU a day for ages 1 to 70 and 800 IU a day for ages 71 and older. The tolerable upper intake level is 4,000 IU a day for adults.
More is not better. Doses above that upper limit belong under a clinician who rechecks your blood levels.
The Endocrine Society suggests against routine supplementation for healthy adults under 75. It suggests empiric supplementation for ages 1 to 18, adults 75 and older, pregnancy, and adults with high-risk prediabetes. If a supplement is right for you, a physician-formulated Super Vitamin D with K is one option to discuss with a clinician who knows your history and medications.
What the Big Trials Found
Large trials give a more cautious picture than headlines suggest.
- Cancer and heart disease: In the VITAL trial, 25,871 adults took vitamin D3 at 2,000 IU a day for a median of 5.3 years. It did not lower the incidence of invasive cancer or major cardiovascular events compared with placebo.
- Fractures: A VITAL follow-up study found 2,000 IU a day did not significantly lower total, nonvertebral or hip fractures in generally healthy adults who were not selected for deficiency, low bone mass or osteoporosis.
- Respiratory infections: A meta-analysis of 25 randomized trials (11,321 people) found supplementation modestly reduced the risk of acute respiratory infection overall. The strongest effect was in people whose starting levels were very low (under 25 nmol/L) and in those taking daily or weekly doses rather than large single doses.
For cancer, cardiovascular disease, diabetes and autoimmune disorders, the Institute of Medicine judged the evidence inconsistent and inconclusive.
The honest takeaway: correcting a deficiency matters. Taking large amounts when your levels are normal has not shown benefit in these trials.
How SeeBeyond Medicine Approaches Vitamin D
See beyond your symptoms. When a patient tells us they feel run down every winter, we start with their history, then test before treating, so the plan addresses the cause rather than the symptom.
That can mean a comprehensive blood panel that includes a 25-hydroxyvitamin D level, or an Individual Optimal Nutrition profile when the question is broader than one nutrient. If a supplement makes sense, the dose is personalized to your results and your medications, and we recheck afterward. Our care is directed by a physician board-certified in Family Medicine, Integrative Holistic Medicine, and Obesity Medicine, and it follows a functional medicine approach. You can be seen in person or by telemedicine.
If you read a winter slump as a mood problem, see seasonal affective disorder. The two can overlap, and a clinician can help sort them out.
Key Takeaways
- At Northeast latitudes, winter sunlight is too weak for your skin to make meaningful vitamin D for several months.
- Older adults, people with darker skin, obesity, gut absorption problems or little time outdoors are more likely to run low.
- Low vitamin D signs such as bone pain and muscle aches are not specific. A 25-hydroxyvitamin D blood test is how you know.
- Adults need 600 to 800 IU a day, and the upper limit for adults is 4,000 IU unless a clinician is monitoring you.
- Large trials found that extra vitamin D does not prevent cancer, heart disease or fractures in people who are not deficient, so test, don't guess.
Related Resources
- Read what a standard annual physical misses to see why a routine visit may not include a vitamin D level.
- Learn how to strengthen your immune system through sleep, food and daily habits.
- Explore immunity IV therapy if you want to discuss nutrient support with a clinician.
- Read about seasonal affective disorder if your winter slump includes low mood.
Find Out Where You Stand
You do not have to guess whether this winter's fatigue has a measurable cause. A conversation about your history, and testing when it is warranted, is a clear next step. We see patients at our Scarsdale, NY office in Westchester County, at our Greenwich, CT office in Fairfield County, and by telemedicine.
This article is for general education and does not replace a medical evaluation. Treatment decisions depend on your history, medications, and lab work. Speak with a qualified clinician before starting, stopping, or changing any therapy.
