Vitamin D Deficiency: The Symptoms Most People Attribute to Something Else

Vitamin D deficiency is one of the most common laboratory findings I see in internal medicine, and also one of the most clinically underappreciated. Patients come in with fatigue that does not resolve with rest, unexplained muscle weakness, bone pain that gets dismissed as arthritis, and low mood that does not fully meet criteria for depression. Many have seen multiple providers. When I check their vitamin D level, it is often critically low. Repletion does not always produce dramatic results, but when it does, the improvement in how patients feel is significant enough that I now check vitamin D routinely in anyone presenting with these symptoms.

Estimates suggest that more than one billion people worldwide have vitamin D deficiency, defined as a serum 25-hydroxyvitamin D level below 20 ng/mL. Another 50 percent of the general population has insufficiency, defined as levels between 20 and 30 ng/mL. Given that vitamin D influences over 200 genes and plays roles in immune function, bone metabolism, muscle function, mood regulation, and cardiovascular health, these numbers have real clinical implications.

Why Vitamin D Deficiency Is So Common

Vitamin D is unique among vitamins because the primary source for most people is not food but sun exposure. When UV-B radiation hits the skin, it converts a cholesterol precursor into previtamin D3, which is then processed by the liver and kidneys into the active form. Most foods contain minimal vitamin D naturally; fatty fish, egg yolks, and fortified dairy and cereals are the main dietary sources, but in amounts that rarely meet daily needs without supplementation.

The problem is that the conditions required for adequate sun synthesis are not available to most people most of the time. Living above 35 degrees latitude (which includes most of Europe, Canada, and large portions of the United States) means that UV-B rays are insufficient for vitamin D synthesis for several months of the year. Spending most of the day indoors, wearing sunscreen, having darker skin tone (which requires longer UV exposure for equivalent synthesis), being older (skin becomes less efficient at synthesis with age), and having obesity (vitamin D is fat-soluble and sequestered in adipose tissue) all significantly increase the risk of deficiency.

This explains why deficiency cuts across populations in ways that seem inconsistent: a person living in Los Angeles can be just as deficient as someone in Manchester if they spend their days in an office.

What Vitamin D Actually Does

Vitamin D functions more like a hormone than a vitamin. Virtually every tissue in the body has vitamin D receptors, which gives some sense of how broadly it operates. Its most studied role is in calcium and phosphorus absorption in the gut, which is fundamental to bone mineralization. But its functions extend well beyond bone health.

The immune system relies on vitamin D to regulate both innate and adaptive immune responses. Vitamin D activates macrophages, the immune cells that engulf pathogens, and modulates T-cell function in ways that are relevant to both infection resistance and autoimmune disease. Low vitamin D has been associated with increased susceptibility to respiratory infections, and the correlation between vitamin D levels and COVID-19 severity was observed early in the pandemic, though establishing causality is more complex.

Vitamin D also influences serotonin synthesis and signaling, which provides a plausible mechanism for the association between deficiency and mood disorders. Seasonal affective disorder, which peaks in winter months when sun exposure is minimal, may partly reflect seasonal variation in vitamin D status alongside circadian disruption from reduced daylight hours.

Symptoms of Vitamin D Deficiency

Fatigue

Unexplained fatigue is the most common complaint associated with vitamin D deficiency. The mechanism involves impaired mitochondrial function in muscle cells, disrupted sleep architecture, and effects on serotonin signaling. The challenging thing about fatigue as a symptom is that it is non-specific; it can reflect anemia, thyroid dysfunction, depression, sleep apnea, or many other conditions. Vitamin D is one of several things worth checking when fatigue is the primary complaint and has no obvious explanation.

Bone and Back Pain

Bone pain, particularly in the lower back, hips, and legs, is a classic feature of severe vitamin D deficiency. When deficiency is prolonged, the bones become inadequately mineralized, a condition called osteomalacia in adults. This is different from osteoporosis, which is a loss of bone density; osteomalacia is a failure of the mineralization process itself. Diffuse bone tenderness, especially in the ribs, shins, and lower back, in combination with a low vitamin D level, is a clinically significant finding that requires treatment rather than monitoring.

Muscle Weakness and Pain

Muscle weakness, particularly in the proximal muscles (hips and thighs), is associated with vitamin D deficiency and can affect gait and balance, particularly in older adults. Muscle pain that is diffuse and does not follow the pattern of specific muscle or joint injury is also reported. Some patients with fibromyalgia-like symptoms are found to have severe vitamin D deficiency, and supplementation can produce meaningful improvement.

Frequent Illness

Recurrent upper respiratory infections, particularly in people who seem to get every circulating cold or flu, are worth investigating from a vitamin D perspective. Several randomized controlled trials have shown that vitamin D supplementation reduces the risk of acute respiratory infection, particularly in those who are deficient at baseline.

Depression and Mood Changes

The relationship between vitamin D and mood is bidirectional and complex. Low vitamin D is associated with higher rates of depression in population studies, but supplementation trials in non-deficient people have not consistently shown antidepressant effects. In people who are genuinely deficient, correcting the deficiency often improves mood as part of the broader improvement in how they feel. I do not consider vitamin D supplementation a substitute for evidence-based depression treatment, but it is a variable worth addressing when deficiency is present.

Hair Loss

Hair follicle cycling requires vitamin D signaling, and severe deficiency has been associated with telogen effluvium, a diffuse type of hair loss in which a large proportion of hairs simultaneously enter the shedding phase. This is not the most common cause of hair loss, but it is one worth checking if hair loss is occurring alongside other deficiency symptoms.

Who Is at Highest Risk

Certain groups have substantially higher rates of deficiency and should be checked routinely. Older adults produce vitamin D less efficiently from sun exposure and are more likely to be housebound. People with dark skin require significantly longer sun exposure for equivalent synthesis. Exclusively breastfed infants are at risk because breast milk has minimal vitamin D content; infant supplementation is recommended across all major pediatric guidelines. People with fat malabsorption syndromes, including Crohn's disease, celiac disease, and cystic fibrosis, absorb dietary and supplemental vitamin D poorly. People who have had bariatric surgery, particularly malabsorptive procedures, require aggressive supplementation and monitoring.

Testing and Interpreting Your Levels

The blood test for vitamin D is a 25-hydroxyvitamin D (25-OH vitamin D) level. This is the standard clinical test and reflects body stores. The active form, 1,25-dihydroxyvitamin D, is not a useful routine marker because it can be normal or even elevated in the presence of deficiency due to compensatory mechanisms.

Level interpretation by most guidelines: below 20 ng/mL is deficiency, 20 to 30 ng/mL is insufficiency, above 30 ng/mL is sufficient, and above 50 ng/mL is generally considered optimal by some authorities though the evidence for benefits above 30 is less robust. Toxicity occurs at very high levels, typically above 150 ng/mL, and is almost always the result of excessive supplementation rather than sun exposure.

How to Treat Vitamin D Deficiency

For mild insufficiency in an otherwise healthy adult, a daily supplement of 1,000 to 2,000 IU of vitamin D3 (cholecalciferol, the form better absorbed than D2) is typically adequate. For clinical deficiency with symptoms or levels below 20 ng/mL, higher doses are used, often 4,000 to 6,000 IU daily or weekly high-dose protocols, depending on the severity and the clinical context. Vitamin D3 should ideally be taken with a meal containing fat for best absorption.

Follow-up testing at three months allows assessment of response. In people with malabsorption or obesity, much higher doses may be required to achieve adequate levels. Most people who supplement appropriately see their levels normalize within eight to twelve weeks.

When to See a Doctor

If you are experiencing persistent fatigue, bone or muscle pain, frequent illness, or mood changes and have not had your vitamin D checked, it is a simple blood test worth requesting. If you are in a high-risk group, routine annual testing is reasonable. JourneyDoctors physicians can order lab work, interpret your results in context, and guide appropriate supplementation. Consultations start at $19.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment of any medical condition.

Frequently Asked Questions

Can I get enough vitamin D from food alone?

For most people, no. The vitamin D content of food is too low to meet daily requirements without sun exposure, particularly if you live at higher latitudes, spend most of your time indoors, or have darker skin. Fatty fish (salmon, mackerel, sardines) and fortified foods provide some vitamin D, but supplementation is usually necessary to maintain adequate levels in people who cannot get sufficient sun exposure.

Is vitamin D toxicity a real concern?

Yes, though it is uncommon at standard supplementation doses. Toxicity occurs at sustained levels above 150 ng/mL and typically requires chronic ingestion of 10,000 IU or more per day. Symptoms include nausea, weakness, frequent urination, kidney stones, and in severe cases, calcification of soft tissues. At the doses typically recommended (1,000 to 4,000 IU daily), toxicity is not a practical concern for most adults.

Does sunscreen block vitamin D synthesis?

SPF 30 sunscreen applied correctly reduces vitamin D synthesis by approximately 95 percent. In practice, most people do not apply sunscreen to all exposed skin or reapply consistently, so real-world synthesis reduction is less complete. For people who are deficient, supplementation is the more reliable strategy than modifying sun protection, which carries skin cancer risk.

Does vitamin D help with COVID-19?

There is observational evidence associating low vitamin D with worse COVID-19 outcomes, and plausible mechanistic reasons involving immune function. However, randomized trial data on vitamin D supplementation for COVID-19 treatment or prevention is mixed. Correcting genuine deficiency is reasonable for general immune health; supplementation beyond sufficiency in non-deficient individuals has not been shown to provide additional benefit for COVID-19 specifically.

How long does it take for vitamin D supplements to work?

Lab levels typically normalize within eight to twelve weeks of adequate supplementation. Symptom improvement varies: fatigue and mood often improve within four to eight weeks of reaching sufficient levels. Bone pain from osteomalacia may take longer to resolve. Testing at three months after starting supplementation gives a good picture of whether the dose is adequate.