Low vitamin D often causes fatigue, bone pain, muscle weakness, and mood changes, but many people with a deficiency have no obvious symptoms at all. That’s what makes it tricky: the signs tend to be vague and easy to blame on stress, aging, or poor sleep. A simple blood test is the only reliable way to confirm whether your levels are low, and understanding what to look for can help you decide whether it’s worth asking for one.
The Most Common Symptoms
The classic signs of low vitamin D in adults are fatigue, bone pain, muscle weakness or aches, muscle cramps, and mood changes like depression. These symptoms overlap with dozens of other conditions, which is one reason vitamin D deficiency goes undiagnosed so often. You might notice that your legs feel heavy after climbing stairs, that a general tiredness persists no matter how much you sleep, or that your mood dips noticeably during winter months.
In children, the picture can look different. Mild deficiency may show up as sore or weak muscles. Severe deficiency causes rickets, a condition where bones soften and bend during growth, leading to bowed legs, joint deformities, and delayed development. Rickets is rare in developed countries but still occurs, particularly in breastfed infants who don’t receive supplementation.
Bone Pain and Fracture Risk
Vitamin D is essential for absorbing calcium from food. When your levels drop too low, your body pulls calcium from your bones to keep blood calcium stable, and over time this weakens your skeleton. The severity depends on how deficient you are.
At very low levels (below 12 ng/mL in a blood test), adults can develop osteomalacia, a condition where bones become soft and painful. You might feel a deep, aching pain in your shins, pelvis, or lower back that worsens with activity. At moderately low levels, the damage is subtler: your body increases bone turnover to compensate, gradually thinning your bones and raising your risk of osteoporosis and fractures. This process is silent until a bone breaks, which is why many people don’t realize their vitamin D has been low for years.
Muscle Weakness and Cramps
Vitamin D plays a direct role in muscle repair and contraction. When levels are low, muscles don’t contract as efficiently, which can feel like generalized weakness rather than the kind of fatigue you get from a hard workout. You might struggle with tasks that used to feel easy, like standing up from a low chair or carrying groceries. Cramps, particularly in the legs, are another common complaint. Research from Harvard has confirmed the link between low vitamin D and measurable loss of muscle strength, especially in older adults where the consequences (falls, fractures) are most serious.
Getting Sick More Often
Vitamin D supports both the immediate and longer-term branches of your immune system. People with severely low levels tend to catch respiratory infections more frequently. In one cross-sectional study, 70% of people in the severely deficient group reported respiratory illnesses, averaging about three per year, compared to 50% of those with higher levels. The correlation was clear: as vitamin D levels rose, respiratory illness frequency dropped. If you find yourself cycling through colds, sinus infections, or bronchitis more than seems normal, low vitamin D is worth investigating.
Mood Changes and Depression
Depression is listed among the recognized symptoms of vitamin D deficiency, and it’s one of the more disruptive ones. The connection is strongest during fall and winter, when reduced sunlight means your skin produces less vitamin D naturally. This doesn’t mean low vitamin D causes all seasonal mood dips, but it’s a contributing factor that’s relatively easy to test for and address. Some people describe a persistent low mood, difficulty concentrating, or a loss of motivation that doesn’t match anything else going on in their lives.
Hair Loss
Thinning hair or increased shedding is a less well-known symptom, but the research supports a connection. A meta-analysis in Frontiers in Nutrition found that about 52% of people with alopecia areata (patchy hair loss) and roughly 54% of people with telogen effluvium (widespread shedding) were vitamin D deficient. People with alopecia areata were nearly three times more likely to be deficient than people without hair loss. The link with general shedding was weaker statistically, but deficient individuals still had vitamin D levels about 5.7 ng/mL lower than controls on average. Hair loss has many causes, but if you’re experiencing it alongside other symptoms on this list, vitamin D deficiency becomes a stronger suspect.
Who Is Most at Risk
Some people are more likely to become deficient regardless of diet or lifestyle. People with darker skin need up to ten times longer in the sun to produce the same amount of vitamin D as someone with fair skin, because melanin competes with the skin’s vitamin D precursor for UV absorption. This makes deficiency significantly more common in Black and South Asian populations living at higher latitudes.
Other high-risk groups include people who spend most of their time indoors, those who live in northern climates with limited winter sunlight, adults over 65 (whose skin becomes less efficient at synthesis), people with obesity (vitamin D gets sequestered in fat tissue), and anyone with conditions that impair fat absorption, like celiac disease or Crohn’s disease. If you fall into more than one of these categories and recognize the symptoms above, a blood test is a straightforward next step.
What Your Blood Test Numbers Mean
The standard test measures 25-hydroxyvitamin D in your blood, reported in ng/mL (or nmol/L outside the U.S.). According to the National Institutes of Health, the cutoffs break down like this:
- Below 12 ng/mL: Deficient. This is the level associated with rickets in children and osteomalacia in adults.
- 12 to 20 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially harmful, especially above 60 ng/mL.
Many people fall into that 12 to 20 ng/mL gray zone where symptoms may be mild or absent but long-term bone health is compromised. If your result is in this range, supplementation typically makes sense even if you feel fine.
How Long Recovery Takes
Once you start supplementing, vitamin D levels in your blood generally rise within a few weeks. How quickly you feel better depends on how deficient you were. Mild fatigue and muscle aches may improve within a month or two. Severe deficiency, particularly if it has caused bone softening or rickets, can take several months of consistent supplementation to fully resolve. Immune benefits also take time to build; research suggests that reaching optimal levels through supplementation typically takes about three months.
The recommended daily intake for most adults under 70 is 600 IU per day, rising to 800 IU for adults over 70. These are baseline recommendations. If you’re already deficient, your provider will likely suggest a higher dose for a set period to bring your levels up before switching to a maintenance dose. The goal is to get your blood level above 20 ng/mL and keep it there consistently.