Vitamin D deficiency symptoms affect a substantial portion of the global population, yet this condition remains widely underdiagnosed. The NIH Office of Dietary Supplements estimates that approximately 1 billion people worldwide have inadequate vitamin D levels, and in the United States, roughly 35% of adults have vitamin D deficiency 1. Vitamin D is a fat-soluble vitamin that the body produces when skin is exposed to ultraviolet B (UVB) sunlight.
It plays essential roles in calcium absorption and bone health, immune function, muscle function, and inflammation regulation. Unlike most nutrients, vitamin D functions more like a hormone — the body converts it into an active form (calcitriol) that regulates gene expression across dozens of tissues 1.
Because vitamin D deficiency can develop gradually and silently, many people do not recognize it until bone loss, muscle weakness, or other symptoms become apparent. A blood test measuring 25-hydroxyvitamin D is the standard diagnostic tool. For an overview of the broader nutrient needs by gender, see our guide to best vitamins for women.
What Are the Symptoms of Vitamin D Deficiency?
Vitamin D deficiency symptoms span multiple body systems and can range from subtle to severe depending on how low levels fall and how long the deficiency has persisted. Some people have significant deficiency without any noticeable symptoms — making routine blood testing important for at-risk groups 2. Others experience a cluster of nonspecific symptoms that are easy to attribute to other causes, such as stress or aging.
Vitamin D Deficiency Symptoms by Category
Bone Pain and Muscle Weakness
Bone pain and aching — particularly in the lower back, hips, and legs — are among the more recognizable vitamin D deficiency symptoms. In severe, prolonged deficiency, the mineralization of bone is impaired, leading to osteomalacia (soft bones) in adults or rickets in children 1. Muscle weakness and generalized aches that are not explained by activity levels or injury are also characteristic symptoms.
A 2016 study found that people with vitamin D deficiency were significantly more likely to report musculoskeletal pain and weakness 3.
Fatigue and Low Energy
Persistent fatigue — particularly fatigue that does not improve with rest — is frequently reported by people with vitamin D deficiency. Research suggests this may be related to vitamin D's role in mitochondrial function and its effects on serotonin and other neurotransmitters 3. A 2015 case-control study found that vitamin D supplementation in deficient individuals significantly improved fatigue scores compared with placebo.
However, fatigue has many causes, and low vitamin D levels are just one potential contributor. If fatigue persists despite correcting vitamin D levels, brain fog causes and how to clear it may also be worth reviewing.
Mood Changes and Depression
Research suggests an association between low vitamin D levels and depression and low mood, particularly seasonal affective disorder (SAD) — a pattern of depression linked to reduced sunlight exposure during winter months 4. Vitamin D receptors are present in brain regions involved in mood regulation, including the hippocampus and prefrontal cortex.
A 2017 meta-analysis of 15 observational studies found an inverse association between vitamin D levels and depression risk. However, whether supplementation effectively treats established depression is less clear — results of randomized trials have been mixed.
Impaired Immune Function
Vitamin D plays an important role in immune system regulation, supporting both innate and adaptive immune responses. Low vitamin D levels have been associated with increased susceptibility to respiratory infections, including influenza and other viral illnesses 1.
Research suggests that vitamin D deficiency may also contribute to the development of autoimmune conditions, including multiple sclerosis, rheumatoid arthritis, and type 1 diabetes, though the causal relationships are still being studied.
Hair Loss
Significant hair loss or thinning is sometimes associated with vitamin D deficiency. Vitamin D receptors are present in hair follicles, and animal research suggests vitamin D is involved in hair follicle cycling 3. Some small human studies have found lower vitamin D levels in people with alopecia areata (an autoimmune form of hair loss) compared with control groups. The evidence in humans is still developing.
Delayed Wound Healing
Slower-than-expected wound healing may be a symptom of vitamin D deficiency, as vitamin D is involved in the production of compounds needed for forming new skin and fighting infection during the healing process 1. Prolonged wound healing following minor injuries or surgery, particularly in combination with other symptoms, warrants evaluation.
Causes and Risk Factors
The most common causes of vitamin D deficiency include: limited sunlight exposure, which may result from indoor lifestyles, geography (living at higher latitudes), habitual use of sunscreen, or darker skin pigmentation (which reduces UVB absorption); inadequate dietary intake, since very few foods naturally contain vitamin D in significant amounts; malabsorption conditions such as Crohn's disease, celiac disease, and cystic fibrosis; obesity, as vitamin D is fat-soluble and may be sequestered in adipose tissue; kidney or liver disease, which impairs conversion of vitamin D to its active form; and older age, as skin's ability to produce vitamin D from sunlight declines with age 2.
Vitamin D deficiency also frequently co-occurs with iron deficiency and other nutrient shortfalls that affect energy and immune function.
Diagnosing Vitamin D Deficiency
The standard diagnostic test is a blood measurement of serum 25-hydroxyvitamin D (25(OH)D), reported in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). According to the NIH and the Endocrine Society, levels below 20 ng/mL (50 nmol/L) are generally considered deficient; levels between 20 and 29 ng/mL are considered insufficient; and levels at or above 30 ng/mL are generally considered sufficient for most people 2.
The optimal level for specific health outcomes beyond bone health remains an active area of research. Testing is recommended for people with risk factors or symptoms rather than as routine population-wide screening.
Treatment and Supplementation
The primary treatment for vitamin D deficiency is supplementation. Vitamin D3 (cholecalciferol) is generally considered more effective than vitamin D2 (ergocalciferol) at raising and maintaining blood levels 1. Increasing sunlight exposure and dietary intake of vitamin D-containing foods (fatty fish, egg yolks, fortified milk, and fortified cereals) can help, but supplementation is typically required to correct established deficiency.
Typical doses studied in clinical research for correction of deficiency range from 1,500 to 2,000 IU (international units) per day for maintenance, to higher loading doses under medical supervision for rapid repletion — always consult your healthcare provider for a dosage appropriate to your specific level and health status, as excessive supplementation can cause toxicity.
Women with hormonal imbalances may also want to review whether hormonal imbalance symptoms are contributing to fatigue alongside any vitamin deficiency.
Vitamin D Toxicity
Vitamin D toxicity (hypervitaminosis D) is rare but possible from excessive supplementation — it cannot occur from sun exposure alone, as the body self-regulates production through skin. The NIH sets the Tolerable Upper Intake Level at 4,000 IU/day for adults, though toxicity is unlikely below 10,000 IU/day in most people 1.
Symptoms of toxicity include nausea, vomiting, weakness, frequent urination, and in severe cases, elevated blood calcium (hypercalcemia) leading to kidney damage. This is why supplementation doses should be guided by blood test results and monitored by a healthcare provider.
When to See a Doctor
Request a blood test and medical evaluation if you experience: persistent bone pain, particularly in the back, hips, or legs; muscle weakness that is not explained by recent activity; fatigue that does not improve with adequate sleep; you have known risk factors for deficiency (limited sun exposure, darker skin, malabsorption condition, obesity, kidney or liver disease, or older age); you have recently been diagnosed with osteoporosis or experienced a fragility fracture; or you experience a seizure, severe confusion, excessive thirst, or frequent urination after starting a vitamin D supplement, which may signal hypercalcemia and requires immediate evaluation.
Our vitamin deficiency quiz can help you evaluate whether vitamin D — alongside other common shortfalls — may be contributing to your symptoms before speaking with your doctor. To ensure you're not missing other key nutrients, our Nutrition & Weight hub covers the most important dietary supplements for women's health.
Frequently asked questions
What is a normal vitamin D level?
Most clinical guidelines define vitamin D sufficiency as a serum 25(OH)D level of 20 ng/mL (50 nmol/L) or higher. The Endocrine Society recommends maintaining levels of at least 30 ng/mL for optimal health, particularly for bone health in older adults. Levels above 100 ng/mL may carry risk of toxicity over time. Testing provides the baseline needed to determine whether and how much supplementation is appropriate.
How much sun exposure is needed for vitamin D?
The amount of sun needed to produce adequate vitamin D depends on multiple factors: skin color, time of day, season, latitude, use of sunscreen, cloud cover, and the area of skin exposed. A general estimate often cited is 10–30 minutes of midday sun exposure to the face, arms, and legs several times per week for lighter-skinned individuals — but this varies widely and may not be achievable at higher latitudes in winter. Sunscreen use, which reduces UV-related skin cancer risk, also substantially reduces vitamin D synthesis. Sun exposure alone may be insufficient for many people, making dietary and supplemental sources important.
Can vitamin D deficiency cause weight gain?
Research suggests an association between low vitamin D levels and higher body weight and obesity, but the causal direction is not fully established. Vitamin D is fat-soluble and may be stored in fat tissue, making it less bioavailable in the body — so obesity may contribute to lower circulating vitamin D, rather than deficiency causing weight gain. Whether correcting vitamin D deficiency independently leads to weight loss is not supported by current evidence.
Which foods are highest in vitamin D?
Natural dietary sources of vitamin D are limited. The richest sources include fatty fish (salmon, mackerel, tuna, and sardines), cod liver oil, and egg yolks. Many foods are fortified with vitamin D, including cow's milk, some plant-based milks, orange juice, and breakfast cereals. Even with a vitamin D-rich diet, it may be difficult to obtain adequate amounts from food alone, particularly for those with significant deficiency.
Is vitamin D3 or D2 better?
Vitamin D3 (cholecalciferol), derived from animal sources or produced synthetically, is generally preferred over vitamin D2 (ergocalciferol, derived from plant sources) because research suggests D3 raises and sustains blood 25(OH)D levels more effectively. Both forms are converted to the active form in the body, but D3 appears to be more potent and has a longer half-life. For strict vegans or vegetarians, plant-derived D3 (from lichen) is available.