Signs of vitamin D deficiency are easy to miss because they are quiet: persistent tiredness, dull aching in bones and muscles, muscle weakness with ordinary tasks like rising from a chair, more frequent colds and respiratory infections, and low mood — and roughly one in four US adults has a blood level below the 20 ng/mL threshold many laboratories use to define inadequacy, according to CDC's Second Nutrition Report. True deficiency, defined as serum 25-hydroxyvitamin D below 20 ng/mL by the NIH Office of Dietary Supplements, matters beyond symptoms, because vitamin D is required to absorb calcium and keep bone density intact.
This site publishes information, not medical advice. Symptoms like fatigue and aches are common to many conditions, so a 25-hydroxyvitamin D blood test — not guesswork — should decide whether you supplement.
What does vitamin D actually do?
Its first job is calcium management. Vitamin D promotes calcium absorption in the gut and maintains adequate blood calcium for bone mineralization, per the NIH Office of Dietary Supplements; without it, the body raids bone to keep blood calcium normal. In children that failure produces rickets; in adults it produces osteomalacia — soft, aching bones — and accelerates osteoporosis. Beyond bone, immune cells and muscle tissue carry vitamin D receptors, which is the biological basis for the infection and weakness associations. The hormone-like vitamin is also unusual in how it is made: skin synthesizes it from UVB sunlight, which is why latitude, sunscreen habits, season, and time spent outdoors all shape your blood level.
What are the most common signs of deficiency?
Per the NIH Office of Dietary Supplements, CDC guidance, and clinical reviews, the recognizable pattern includes:
- Fatigue that persists despite reasonable sleep, documented in case reports of adults whose exhaustion resolved after their levels were corrected
- Bone pain and lower back aches, the hallmark of osteomalacia in adults
- Muscle weakness and cramps, especially trouble with stairs or rising from a low chair
- More frequent respiratory infections; a 2017 BMJ meta-analysis of 25 trials found vitamin D supplementation reduced the risk of acute respiratory infections, with the strongest effect in people who started out deficient
- Low mood; a 2020 review in Depression and Anxiety found an association between low vitamin D and depressive symptoms, while noting the evidence does not prove supplementation treats depression
- Slow-healing fractures or, in severe long-standing cases, stress fractures from minimal force
- Hair shedding in prolonged deficiency, reported in case series rather than proven by trials
Many women feel nothing at all until a routine blood test surfaces the number — which is why risk factors, not symptoms, drive most screening decisions.
Related stories: Signs of Iron Deficiency: What to Watch For and When to Get Tested · Blood Pressure Numbers Explained: What 120/80 Actually Means for You.
Who is most at risk?
Per the NIH Office of Dietary Supplements, the groups with the highest rates of low levels include people with darker skin, because melanin reduces UVB-driven production; adults over 65, whose skin makes roughly a quarter of the vitamin D it did at 20; people with obesity, where the vitamin distributes into a larger fat compartment; those with malabsorption conditions such as celiac or Crohn's disease; and anyone with limited sun exposure, including women who cover their skin for cultural or religious reasons. Breastfed infants need supplemental vitamin D almost universally, which is why the American Academy of Pediatrics recommends 400 IU daily for exclusively and partially breastfed babies. Deficiency is also seasonal in northern states — levels bottom out at the end of winter, per CDC nutrition surveillance.
How much do you actually need?
The Recommended Dietary Allowance is 600 IU (15 mcg) daily for everyone aged 1 through 70, rising to 800 IU for adults over 70, per the NIH Office of Dietary Supplements — amounts intended to meet the needs of nearly all healthy people, not to correct a confirmed deficiency. Food sources are limited: fatty fish such as salmon and sardines, UV-treated mushrooms, and fortified milk, orange juice, and cereals carry most of the load, since few foods are naturally rich in the vitamin. When a blood test confirms deficiency, clinicians prescribe corrective doses individually; megadoses taken without testing are not a shortcut, because vitamin D is fat-soluble and levels above 100 ng/mL can cause toxicity with high blood calcium, kidney stones, and nausea, per the same NIH fact sheet.
When to talk to a clinician
Ask about a 25-hydroxyvitamin D test if you have persistent fatigue plus bone or muscle aches, if you are over 65 with a fracture after a minor fall, if you have darker skin and little sun exposure with any of the symptoms above, or if you have a malabsorption diagnosis. Seek care urgently for severe bone pain, muscle spasms, confusion, or unsteadiness, which can signal marked calcium abnormalities. And if you already supplement, mention the dose — more is not better, and the safe ceiling for self-directed use is 4,000 IU daily for adults, per the NIH Office of Dietary Supplements.
The bottom line
Low vitamin D shows up as fatigue, aching bones, muscle weakness, and more frequent respiratory infections — and about one in four US adults sits below the 20 ng/mL inadequacy line, per CDC's Second Nutrition Report. Most adults need 600 IU daily, and 800 IU after 70, per the NIH Office of Dietary Supplements, ideally from fatty fish and fortified foods plus sensible sun. Because the signs overlap with a dozen other conditions, get the blood test before supplementing, and treat a confirmed deficiency under a clinician's dosing.