You're exhausted despite getting enough sleep. Your bones ache. You catch every cold going around. Winter doesn't usually affect you this way, but something feels off this year. Low vitamin D might be the problem.

Vitamin D deficiency is incredibly common; about 1 in 4 adults in the U.S. have inadequate levels. Winter makes it worse because reduced sunlight exposure means your body produces less vitamin D. If you're experiencing persistent fatigue, bone pain, or frequent infections, testing might reveal a deficiency that's easy to fix.

At Summit Health, we help patients identify vitamin D deficiency and develop treatment plans that restore energy and well-being. Many people don't realize how much this single nutrient affects their overall health.

What is Vitamin D and Why It Matters

Vitamin D helps your body absorb calcium, support immunity, and regulate inflammation. Your body makes it from sunlight, but winter reduces sun exposure, lowering production.

Vitamin D deficiency weakens bones, impairs immunity, and increases inflammation, leading to fatigue, pain, and frequent infections. Let’s look at common low vitamin D symptoms.

Common Low Vitamin D Symptoms

Vitamin D deficiency develops gradually, which is why many people don't realize what's happening.

Symptoms can be subtle at first, then worsen over time.

• Fatigue and exhaustion: This is one of the most common complaints. You might feel persistent tiredness that doesn't improve with rest. Even simple tasks feel draining.
• Bone and muscle pain: Aching bones, muscle weakness, or generalized body pain often signal low vitamin D. Some people describe it as chronic soreness or stiffness, especially in the legs and lower back.
• Frequent infections: Vitamin D plays a key role in immune function. Low levels mean your body fights infections less effectively, leading to more colds, flu, and other illnesses.
• Mood changes: Deficiency is linked to depression, anxiety, and seasonal mood changes. Winter vitamin D deficiency often contributes to seasonal affective disorder (SAD).
• Slow healing: Cuts, bruises, and wounds take longer to heal when vitamin D is low because the nutrient supports skin health and tissue repair.
• Bone loss or fractures: Severe deficiency increases osteoporosis risk and fracture risk, especially in older adults.
• Headaches: Some people experience more frequent or severe headaches during the winter months when vitamin D production drops.

These symptoms overlap with many other conditions, which is why getting tested is important for an accurate diagnosis.

Vitamin D Deficiency Testing

Testing is straightforward and helps confirm whether symptoms are actually caused by low vitamin D.

• The 25-hydroxyvitamin D blood test: This is the standard test. A simple blood draw measures your vitamin D level. Results show whether you're deficient, insufficient, or sufficient.

What the numbers mean:

• Less than 30 nmol/L: Severe deficiency (requires immediate treatment)
• 30-50 nmol/L: Deficiency (likely causing symptoms, needs supplementation)
• 50-75 nmol/L: Insufficiency (borderline; may cause issues for some people)
• Above 75 nmol/L: Sufficient levels (normal for most people)

Who should get tested for low vitamin D?

• Anyone with symptoms of deficiency
• People with limited sun exposure
• Older adults
• Those with dark skin pigmentation living in northern climates
• People with digestive disorders affecting fat absorption
• Anyone taking medications that reduce vitamin D absorption

Timing matters: Winter testing often reveals lower levels than summer testing because sunlight exposure is minimal. If you're experiencing winter symptoms, that's a good time to test.

Treating Low Vitamin D

Treatment is simple and effective. Most people feel significantly better within weeks of supplementing.

• Vitamin D supplements: These are the main treatment. Dosage depends on how deficient you are and your individual needs. Most people take 1,000-4,000 IU daily, though some need higher doses.
• Food sources: Fatty fish (salmon, mackerel), egg yolks, fortified milk, and mushrooms exposed to sunlight all contain vitamin D. For most people, food alone isn't enough to correct a deficiency, but it helps maintain levels.
• Sunlight exposure: Spending 10-30 minutes outside in midday sun several times per week helps your body produce vitamin D. In winter or in northern climates, this may not be enough alone.
• Treating the underlying cause: If a digestive disorder impairs absorption, treating that condition helps. Some medications interfere with vitamin D; your doctor may adjust these if appropriate.
• Follow-up testing: After starting treatment, blood tests at 8-12 weeks show whether your levels are improving. Your doctor adjusts doses based on results.
• Timeline: Most people notice improvement in energy and mood within 2-4 weeks. Bone-related improvements take longer, usually a few months.

Who's Most at Risk for Vitamin D Deficiency?

Certain groups are more likely to develop a deficiency in vitamin D:

• People in northern climates with limited winter sunlight
• Older adults who spend less time outdoors
• People with dark skin pigmentation (requires more sun exposure to produce vitamin D)
• Those with digestive disorders like Crohn's disease or celiac disease
• Vegans and vegetarians (fewer dietary sources)
• People who consistently use sunscreen or protective clothing
• Individuals taking medications that reduce vitamin D absorption

Winter increases risk for everyone, but especially those already at higher risk.

What's the Worst That Can Happen With Low Vitamin D?

Severe or long-term vitamin D deficiency causes bone density loss, increasing fracture risk. In children, it causes rickets and permanent bone damage. 

Chronic vitamin D deficiency is linked to heart disease, cancer risk, and autoimmune disorders. Your immune system weakens significantly.

The takeaway: catching it early and supplementing prevents serious consequences.

When to Contact Your Doctor About Vitamin D Deficiency

Reach out to Summit Health if you experience persistent fatigue, bone pain, frequent infections, or mood changes, especially during winter. There's no downside to testing; vitamin D is inexpensive, safe, and deficiency is easy to correct.

If winter fatigue, bone pain, or frequent infections are affecting your quality of life, ask your doctor about vitamin D testing.

Summit Health's primary care physicians can order testing, interpret results, and recommend appropriate supplementation, whether through in-person visits, virtual appointments, or home-based care.

Contact your nearest Summit Health practice to get started.