Vitamin D deficiency most commonly causes fatigue, bone pain, muscle weakness, and mood changes like depression. Many people with low levels don’t notice obvious symptoms for months or even years, which is part of what makes deficiency so widespread. A simple blood test measuring your 25-hydroxyvitamin D level can confirm whether you’re deficient, with levels below 30 ng/mL generally considered insufficient.
Fatigue and Low Energy
Persistent tiredness is one of the most reported symptoms, and one of the easiest to dismiss. When vitamin D drops too low, your body struggles to absorb calcium properly. This triggers a chain reaction: your parathyroid glands work overtime to compensate, and the resulting hormonal imbalance contributes to a deep, hard-to-explain fatigue that doesn’t improve much with sleep. If you’ve been feeling drained for weeks without an obvious cause, low vitamin D is worth investigating.
Bone and Joint Pain
Your body needs vitamin D to absorb calcium and phosphorus, the two minerals that keep bones hard and strong. Without enough of it, bones don’t mineralize properly and begin to soften, a condition called osteomalacia. The pain tends to show up most in your hips, pelvis, and legs, often as a dull ache that worsens with activity or pressure.
This is different from osteoporosis, where bones become brittle and porous. Osteomalacia is about softening. The bones haven’t lost their structure so much as they’ve lost their rigidity. Lower back pain that seems to come from nowhere is a common early complaint.
Muscle Weakness and Cramps
Vitamin D receptors are present throughout your muscle tissue, and they play a direct role in muscle fiber development, size, and repair. When vitamin D levels drop, these receptors can’t do their job. The result is weakness that often affects the legs and hips first, making it harder to climb stairs, stand up from a chair, or carry things that didn’t used to feel heavy.
This matters especially for older adults. The number of vitamin D receptors in muscle tissue decreases naturally with age, which may partly explain age-related loss of strength. Meta-analyses of clinical trials in older adults with low vitamin D levels have found that supplementation reduces the risk of falls, likely because vitamin D helps maintain the fast-twitch muscle fibers responsible for balance and quick movements. In people who were deficient, treatment increased the size of these fibers significantly.
Mood Changes and Depression
Depression is one of the more established links to vitamin D deficiency, though the relationship isn’t always straightforward. Low levels are consistently associated with depressed mood, and the connection is strong enough that screening for vitamin D deficiency is reasonable for anyone dealing with persistent low mood, particularly during winter months when sun exposure drops. Seasonal patterns of depression align closely with seasonal drops in vitamin D production.
Getting Sick More Often
Vitamin D plays a direct role in immune function. It helps your body produce antimicrobial proteins that strengthen the lining of your respiratory tract, your first line of defense against airborne infections. When levels are low, this defense weakens.
A meta-analysis of randomized controlled trials found that people taking vitamin D supplements had 42% fewer episodes of respiratory infections (colds, flu, pneumonia) compared to those taking a placebo. If you seem to catch every cold that goes around, or your respiratory infections linger longer than they should, deficiency could be a contributing factor.
Hair Loss
While some hair shedding is normal, noticeable thinning or patchy hair loss has been linked to low vitamin D. The vitamin plays a role in stimulating hair follicle cycling, and deficiency can push more follicles into their resting phase prematurely. This symptom is more common in women and often overlaps with other nutrient deficiencies like iron.
Symptoms in Children
Children with mild deficiency typically present with sore, weak muscles and general fatigue. Severe deficiency causes rickets, which involves incorrect growth patterns, bowed or bent bones, bone pain, and joint deformities. Rickets is rare in developed countries but still occurs, particularly in breastfed infants who don’t receive supplementation (breast milk contains very little vitamin D).
Who Is Most at Risk
Several factors make deficiency more likely. People with darker skin produce less vitamin D from sunlight because melanin, the pigment responsible for skin color, acts as a natural shield against UV radiation. The same UV rays that cause sunburn are the ones your skin uses to synthesize vitamin D, so higher melanin levels mean slower production. People with very dark skin may need several times more sun exposure than those with light skin to produce the same amount.
Other high-risk groups include people who spend most of their time indoors, those living at northern latitudes (above roughly 37 degrees, which includes most of the United States north of Los Angeles), people over 70 whose skin synthesizes vitamin D less efficiently, and anyone with obesity, since vitamin D gets sequestered in fat tissue and circulates less freely in the blood.
How Deficiency Is Diagnosed
A blood test measuring 25-hydroxyvitamin D is the standard. Levels below 20 ng/mL are widely considered deficient, while 20 to 29 ng/mL is typically classified as insufficient. Research looking across multiple health outcomes suggests that the most beneficial levels start at 30 ng/mL, with optimal levels falling between 36 and 40 ng/mL.
How Long Recovery Takes
Once you start supplementing, blood levels typically begin rising within a few weeks of consistent daily use. For mild insufficiency, levels can normalize within about 12 weeks. Symptom relief often follows a similar timeline, though severe deficiency takes longer. Conditions like rickets in children may take months to fully resolve.
The recommended daily intake for adults under 70 is 600 IU, rising to 800 IU for those over 70. The safe upper limit is set at 4,000 IU per day. Many people with confirmed deficiency are prescribed higher doses for an initial loading period before stepping down to a maintenance dose. Vitamin D is fat-soluble, so taking it with a meal that contains some fat improves absorption.