Signs of Vitamin D Deficiency: How to Know If You'…

Vitamin D is produced in the skin when it's exposed to UVB sunlight, and it functions more like a hormone than a vitamin — regulating hundreds of genes involved in immune function, bone metabolism, muscle function, mood, and cell growth. Deficiency is one of the most common nutrient shortfalls globally, with estimates ranging from 40% to 80% of adults depending on latitude, skin tone, indoor work patterns, and sun avoidance.

The problem with vitamin D deficiency is that its symptoms are vague, overlap with dozens of other conditions, and develop slowly over months or years. Most people don't realize they're low until they get a blood test.

Persistent fatigue and low energy

Fatigue is the most commonly reported symptom of vitamin D deficiency, and it's often attributed to stress, poor sleep, or just "getting older" rather than investigated nutritionally. Multiple observational studies show a strong association between low 25-OH vitamin D levels and fatigue severity. Intervention trials in deficient populations consistently show improved energy levels after repletion.

Key distinction: This is not the kind of fatigue that resolves with a good night's sleep. It's a baseline, chronic low-energy state that persists regardless of rest.

Bone pain and back pain

Vitamin D is essential for calcium absorption in the gut. Without adequate vitamin D, the body cannot maintain normal blood calcium levels, and it responds by leaching calcium from bones — a process that over time produces osteomalacia (soft bones) in adults, characterized by aching, deep bone pain, and tenderness.

Low back pain, hip pain, and rib pain are common presentations. Bone pain from vitamin D deficiency is often diffuse and difficult to localize, unlike the sharp, acute pain of injury. If you have unexplained chronic aching in multiple locations, vitamin D status is worth checking.

Muscle weakness and reduced physical performance

Vitamin D receptors are found throughout skeletal muscle tissue, and vitamin D directly influences muscle protein synthesis and calcium transport in muscle cells. Deficiency is associated with proximal muscle weakness — difficulty climbing stairs, getting up from a chair, or lifting objects overhead — and reduced grip strength, balance, and athletic performance.

This pattern of weakness in large, proximal muscles (hips, thighs, shoulders) without pain is a useful distinguishing feature. In older adults, low vitamin D is one of the strongest modifiable risk factors for falls and fracture.

Mood changes and depression

Vitamin D receptors are present throughout the brain, including regions involved in mood regulation. Low vitamin D is associated with depression, seasonal affective disorder (SAD), and generalized negative mood. The association is particularly strong for people in northern latitudes who experience dramatic seasonal variation in sunlight exposure.

Meta-analyses of supplementation trials show modest but consistent improvements in depression scores in deficient populations. The effect is more pronounced in people who were genuinely deficient at baseline rather than those in the normal range.

Frequent illness and slow infection recovery

Vitamin D activates T-cells and other immune cells and plays a critical role in the innate immune response — the first line of defense against pathogens. Low vitamin D is associated with increased susceptibility to respiratory infections, longer illness duration, and poorer immune response to influenza and other viruses.

If you seem to catch every cold that circulates, take longer than most people to recover from illness, or get sick multiple times per season, low vitamin D is a plausible contributor worth testing.

Hair loss

Hair follicles contain vitamin D receptors, and vitamin D plays a role in the hair growth cycle. Low vitamin D has been associated with alopecia areata (patchy hair loss) and with non-scarring hair loss more broadly. The relationship is not as well-established as some other symptoms, but several studies show lower vitamin D levels in people experiencing significant hair shedding.

Worth noting: Hair loss has many causes — iron deficiency, thyroid dysfunction, stress, hormonal shifts, and genetics. Vitamin D is one piece to check, not the only one.

Slow wound healing

Vitamin D is involved in the production of compounds necessary for skin repair and immune control of wound sites. Studies in patients with poorly healing surgical wounds and leg ulcers have found associations with low vitamin D status. Experimental evidence shows vitamin D accelerates keratinocyte migration, which is necessary for skin regrowth.

If minor cuts, scrapes, or surgical incisions seem to heal unusually slowly, vitamin D status is relevant to investigate.

Who is at highest risk

People who spend most of their time indoors
People who live above 37 degrees north latitude (roughly: above San Francisco, Denver, Richmond) — especially in winter, when UVB angle is insufficient
People with darker skin tones (higher melanin requires more sun exposure to produce equivalent vitamin D)
Older adults (skin converts vitamin D less efficiently with age)
People who cover their skin for religious or other reasons
Obese individuals (vitamin D is fat-soluble and sequestered in adipose tissue)
People with fat malabsorption conditions (Crohn's, celiac, cystic fibrosis)
People on certain medications (some anticonvulsants, glucocorticoids, weight-loss drugs)

How to test

Order 25-hydroxyvitamin D (25-OH D), also written as 25(OH)D or calcidiol. This is the storage form and the clinically relevant measurement for deficiency assessment. Do not order 1,25(OH)D (the active hormone form) — it does not reflect overall status.

Interpreting results:

Below 20 ng/mL: Deficient — supplementation is urgent
20–29 ng/mL: Insufficient — supplementation recommended
30–50 ng/mL: Technically "normal" but many functional medicine practitioners target higher
50–80 ng/mL: Optimal range for most adults according to many experts
Above 100 ng/mL: Warrants caution; above 150 ng/mL: toxicity risk

Testing is available through your doctor, or directly through Ulta Lab Tests, LabCorp, Quest Diagnostics, or Life Extension Labs without a prescription in most states.

What to do about it

Supplementation: Vitamin D3 (cholecalciferol) is the preferred form — it raises blood levels roughly twice as effectively as D2. Take with a fat-containing meal.

General dosing guidance:

For maintenance in people without deficiency: 1,000–2,000 IU/day
For insufficiency (20–29 ng/mL): 2,000–4,000 IU/day
For confirmed deficiency (below 20 ng/mL): 4,000–8,000 IU/day until levels normalize, then reduce to maintenance

Cofactors: Take vitamin D with vitamin K2 (MK-7, 100–200mcg) and magnesium — magnesium is required for vitamin D conversion to its active form, and K2 directs calcium into bones rather than arteries.

Retest after 3 months of supplementation to confirm levels have normalized.

The bottom line

Vitamin D deficiency is common, underdiagnosed, and responsible for a wide range of symptoms including fatigue, bone pain, muscle weakness, mood changes, frequent illness, hair loss, and slow wound healing. None of these symptoms is unique to vitamin D deficiency, which is precisely why testing is the most reliable approach. Order a 25-OH vitamin D test, check your level against optimal ranges (not just laboratory reference ranges), and supplement with D3 plus K2 if you're low.

Log your vitamin D supplementation and track symptom changes over time. Use Optimize free.

Signs of Vitamin D Deficiency: How to Know If You're Low