What are the most common signs of vitamin D deficiency?
The most common signs of vitamin D deficiency include persistent fatigue, bone pain or tenderness, muscle weakness, frequent infections, and mood changes including depression. Many people with low vitamin D levels experience these symptoms without realizing the underlying cause, since they overlap with many other conditions.
Vitamin D deficiency is remarkably prevalent worldwide, affecting an estimated 1 billion people according to a landmark 2007 review published in the New England Journal of Medicine by Dr. Michael Holick. The condition often develops gradually, making it easy to dismiss early symptoms as normal aging or stress. The Endocrine Society defines deficiency as serum 25-hydroxyvitamin D levels below 20 ng/mL (50 nmol/L), while insufficiency falls between 20-29 ng/mL. Understanding these clinical thresholds is critical for early detection and treatment.
Fatigue is perhaps the most frequently reported symptom, yet it is also the most commonly overlooked. A 2015 study published in the North American Journal of Medical Sciences found that 77.2% of patients reporting chronic fatigue had low vitamin D levels. Bone pain and tenderness, particularly in the lower back, hips, and legs, result from impaired calcium absorption — vitamin D is essential for calcium metabolism. The National Institutes of Health (NIH) notes that prolonged deficiency can lead to osteomalacia in adults, a condition characterized by soft, weakened bones.
Muscle weakness is another hallmark sign, especially in the proximal muscles of the thighs and upper arms. Research published in the Journal of Clinical Endocrinology & Metabolism has demonstrated that vitamin D receptors are present in skeletal muscle tissue, and deficiency can impair muscle function and increase fall risk in older adults. The World Health Organization (WHO) has identified vitamin D deficiency as a contributing factor to increased fracture risk in the elderly population.
A landmark review established that vitamin D deficiency affects approximately 1 billion people worldwide
What causes vitamin D deficiency?
Vitamin D deficiency is caused by insufficient sun exposure, inadequate dietary intake, malabsorption disorders, obesity, certain medications, and aging. Geographic location, skin pigmentation, and lifestyle factors like sunscreen use and indoor work all influence your body's ability to produce and absorb this essential nutrient.
The primary source of vitamin D for most people is cutaneous synthesis — your skin produces vitamin D3 when exposed to ultraviolet B (UVB) radiation from sunlight. However, multiple factors reduce this process. Living above 37 degrees latitude (roughly north of Atlanta, Georgia, in the United States) means UVB rays are insufficient for vitamin D synthesis during winter months. The CDC reports that people with darker skin require 3-6 times more sun exposure to produce the same amount of vitamin D as those with lighter skin, due to higher melanin levels acting as a natural sunscreen.
Dietary sources of vitamin D are limited. The USDA Dietary Guidelines for Americans 2020-2025 identifies fatty fish (salmon, mackerel, sardines), cod liver oil, fortified milk, fortified orange juice, and egg yolks as the primary food sources. However, even a diet rich in these foods typically provides only 200-400 IU daily — well below the amount many experts recommend. This dietary gap makes supplementation necessary for many individuals, particularly during winter months or for those with limited sun exposure.
Medical conditions that impair fat absorption can cause vitamin D deficiency because vitamin D is a fat-soluble vitamin. Celiac disease, Crohn's disease, ulcerative colitis, and chronic pancreatitis can all reduce vitamin D absorption from the gut. Obesity is another significant risk factor — the Endocrine Society notes that individuals with a BMI over 30 may need 2-3 times more vitamin D than normal-weight individuals because the vitamin is sequestered in adipose tissue, reducing its bioavailability.
Certain medications can interfere with vitamin D metabolism. Glucocorticoids, anticonvulsants (phenytoin, carbamazepine), and the weight-loss drug orlistat can all reduce vitamin D levels. Aging also plays a role — the NIH Office of Dietary Supplements notes that adults over 70 have a reduced capacity to synthesize vitamin D in the skin, approximately 75% less than younger adults, which is why the recommended intake increases to 800 IU daily for this age group.
The Dietary Guidelines for Americans 2020-2025 identifies primary food sources of vitamin D
How is vitamin D deficiency diagnosed?
Vitamin D deficiency is diagnosed with a simple blood test called serum 25-hydroxyvitamin D or 25(OH)D. This is the most accurate marker of vitamin D status because it reflects both dietary intake and cutaneous synthesis. Your doctor can order this test during a routine blood panel or when symptoms suggest deficiency.
The 25-hydroxyvitamin D blood test is considered the gold standard for assessing vitamin D status. The Endocrine Society and the NIH Office of Dietary Supplements define deficiency as levels below 20 ng/mL (50 nmol/L), insufficiency as 20-29 ng/mL, and sufficiency as 30 ng/mL or above. Some experts, including the Endocrine Society's Clinical Practice Guideline published in the Journal of Clinical Endocrinology & Metabolism, suggest that optimal levels for bone and overall health may be between 40-60 ng/mL, though this remains debated.
The US Preventive Services Task Force (USPSTF) does not currently recommend universal screening for vitamin D deficiency in asymptomatic adults. However, the Endocrine Society recommends testing for individuals at high risk, including those with osteoporosis, malabsorption syndromes, chronic kidney disease, liver failure, and those taking medications that affect vitamin D metabolism. If you fall into a high-risk category or experience symptoms like persistent fatigue, bone pain, or frequent infections, discuss testing with your healthcare provider.
It is important to note that 25(OH)D levels can fluctuate seasonally. A study published in the American Journal of Clinical Nutrition found that levels are typically lowest in late winter and highest in late summer. For this reason, some clinicians recommend testing at the end of winter when levels are at their nadir. Retesting after 8-12 weeks of supplementation helps confirm that treatment is effectively raising your levels to the target range.
The Endocrine Society Clinical Practice Guideline recommends specific diagnostic thresholds
What are the best food sources of vitamin D?
The best food sources of vitamin D include fatty fish like salmon (570 IU per 3 oz), canned tuna (240 IU), fortified milk and orange juice (120 IU per cup), sardines (165 IU per 3.75 oz can), egg yolks (44 IU each), and fortified cereals. Cod liver oil provides the highest concentration at 1,360 IU per tablespoon.
Fatty fish are the richest natural food sources of vitamin D. According to the NIH Office of Dietary Supplements, a 3-ounce serving of sockeye salmon provides approximately 570 IU of vitamin D, while rainbow trout offers about 645 IU per serving. Canned tuna provides roughly 240 IU per 3-ounce serving, and sardines offer about 165 IU per 3.75-ounce can. These marine sources contain vitamin D3, the same form produced by your skin, making them highly bioavailable.
Fortified foods are essential contributors to vitamin D intake in the United States. The USDA reports that fortified milk (both dairy and many plant-based alternatives) typically provides about 120 IU per cup. Fortified orange juice offers a similar amount. Many breakfast cereals are fortified with 40-80 IU per serving. The Dietary Guidelines for Americans 2020-2025 emphasizes that fortified foods serve as the primary dietary source of vitamin D for most Americans, given the limited number of foods that naturally contain the vitamin.
Mushrooms represent a unique plant-based source of vitamin D, though they contain vitamin D2 rather than D3. UV-exposed mushrooms can contain up to 400 IU per 3-ounce serving. The FDA approved labeling of UV-treated mushrooms as a vitamin D source in 2020. Egg yolks provide about 44 IU each, with pasture-raised eggs potentially containing higher levels due to increased sun exposure of the hens. Despite these options, the American Dietetic Association acknowledges that most people struggle to meet the recommended 600-800 IU daily through food alone.
The NIH Office of Dietary Supplements provides comprehensive vitamin D food source data
- Cod liver oil: 1,360 IU per tablespoon (highest food source)
- Rainbow trout: 645 IU per 3 oz serving
- Sockeye salmon: 570 IU per 3 oz serving
- Canned tuna: 240 IU per 3 oz serving
- Sardines: 165 IU per 3.75 oz can
- Fortified milk: 120 IU per cup
- Fortified orange juice: 120 IU per cup
- Egg yolk: 44 IU per large egg
How is vitamin D deficiency treated?
Vitamin D deficiency is primarily treated with vitamin D3 (cholecalciferol) supplementation. For severe deficiency, the Endocrine Society recommends a loading dose of 50,000 IU weekly for 8 weeks, followed by a maintenance dose of 1,500-2,000 IU daily. Mild deficiency may only require daily supplementation of 1,000-2,000 IU.
Treatment protocols depend on the severity of the deficiency and underlying causes. The Endocrine Society Clinical Practice Guideline, published in the Journal of Clinical Endocrinology & Metabolism, provides the most widely referenced treatment framework. For adults with serum 25(OH)D levels below 20 ng/mL, the guideline recommends 50,000 IU of vitamin D2 or D3 once weekly for 8 weeks, followed by maintenance therapy of 1,500-2,000 IU daily. A 2012 meta-analysis in the American Journal of Clinical Nutrition demonstrated that vitamin D3 is approximately 87% more potent than D2 at raising serum levels.
For individuals with malabsorption conditions such as celiac disease, Crohn's disease, or gastric bypass surgery, higher doses may be required. The Endocrine Society suggests these patients may need 6,000-10,000 IU daily to achieve levels above 30 ng/mL, followed by maintenance doses of 3,000-6,000 IU daily. Obese individuals (BMI over 30) may also require 2-3 times the standard dose because vitamin D is stored in fat tissue, reducing circulating levels available for use by the body.
Monitoring is an essential component of treatment. The NIH recommends rechecking 25(OH)D levels after 8-12 weeks of supplementation to ensure adequate response. It is also important to ensure adequate calcium intake during treatment, as vitamin D enhances calcium absorption. The National Osteoporosis Foundation recommends 1,000-1,200 mg of calcium daily for adults, preferably from dietary sources. Toxicity from vitamin D supplementation is rare but possible at very high doses — the tolerable upper intake level set by the Institute of Medicine is 4,000 IU daily for adults, though therapeutic doses under medical supervision may exceed this.
A meta-analysis demonstrated vitamin D3 is approximately 87% more potent than D2
What is the link between vitamin D and bone health?
Vitamin D is essential for bone health because it regulates calcium and phosphorus absorption in the intestines. Without adequate vitamin D, the body absorbs only 10-15% of dietary calcium compared to 30-40% with sufficient levels. Chronic deficiency leads to osteomalacia in adults and rickets in children, both characterized by weakened, soft bones.
The relationship between vitamin D and bone health is one of the most well-established nutritional connections in medicine. Vitamin D promotes calcium absorption in the gut and maintains serum calcium and phosphate concentrations needed for normal bone mineralization. The National Osteoporosis Foundation emphasizes that without sufficient vitamin D, bones can become thin, brittle, or misshapen. A landmark study by the Women's Health Initiative found that calcium plus vitamin D supplementation reduced hip fracture risk in women who adhered to the regimen.
Osteomalacia, the adult form of rickets, develops when chronic vitamin D deficiency impairs bone mineralization. Symptoms include diffuse bone pain, muscle weakness, and an increased risk of fractures. The WHO has identified vitamin D deficiency as a major public health concern contributing to the global burden of osteoporotic fractures, particularly in elderly populations. A 2014 meta-analysis in the BMJ analyzing over 30,000 participants found that vitamin D supplementation at doses of 800 IU or more daily reduced fracture risk by approximately 14%.
The interplay between vitamin D and parathyroid hormone (PTH) is critical to understanding bone health. When vitamin D levels drop, PTH rises to maintain blood calcium levels by pulling calcium from bones. This secondary hyperparathyroidism accelerates bone loss over time. The American Association of Clinical Endocrinologists recommends maintaining 25(OH)D levels above 30 ng/mL to suppress excess PTH secretion and protect bone density, particularly in postmenopausal women and older adults.
A BMJ meta-analysis found vitamin D supplementation reduced fracture risk by approximately 14%
Can vitamin D deficiency affect your immune system?
Yes, vitamin D plays a significant role in immune function. Vitamin D receptors are present on most immune cells, including T cells, B cells, and macrophages. Research shows that adequate vitamin D levels support both innate and adaptive immune responses, and deficiency is associated with increased susceptibility to infections, including respiratory tract infections.
The immunomodulatory effects of vitamin D have been a major area of research over the past two decades. A 2017 meta-analysis published in the BMJ, analyzing 25 randomized controlled trials with over 11,000 participants, found that vitamin D supplementation reduced the risk of acute respiratory infections by 12% overall. The protective effect was particularly pronounced in those with baseline 25(OH)D levels below 10 ng/mL, where supplementation reduced infection risk by 70%. Daily or weekly dosing was more effective than large bolus doses.
Vitamin D influences the immune system through multiple mechanisms. It stimulates the production of antimicrobial peptides like cathelicidin and defensins, which are the body's natural antibiotics. The National Institutes of Health notes that vitamin D also modulates the inflammatory response by reducing the production of pro-inflammatory cytokines and increasing anti-inflammatory cytokines. This balancing act is crucial — excessive inflammation can damage tissues, while insufficient immune activation leaves the body vulnerable to pathogens.
The relationship between vitamin D and autoimmune conditions has also garnered significant attention. Observational studies published in the journal Nutrients have found associations between low vitamin D levels and increased risk of multiple sclerosis, type 1 diabetes, rheumatoid arthritis, and inflammatory bowel disease. While causation has not been definitively established, a large randomized trial (VITAL study) published in the BMJ in 2022 found that vitamin D supplementation of 2,000 IU daily reduced autoimmune disease risk by 22% over a 5-year follow-up period.
A BMJ meta-analysis of 25 RCTs found vitamin D supplementation reduced respiratory infection risk
What is the connection between vitamin D deficiency and depression?
Research suggests a significant association between low vitamin D levels and depression. Vitamin D receptors are found throughout the brain, including regions involved in mood regulation. A 2013 meta-analysis in the British Journal of Psychiatry found that low vitamin D levels were associated with a significantly higher risk of depression, though the relationship may not be purely causal.
The link between vitamin D and mental health has been investigated extensively. A comprehensive meta-analysis published in the British Journal of Psychiatry, analyzing 31,424 participants across 14 studies, found that individuals with low vitamin D levels had a significantly higher risk of depression compared to those with adequate levels. The association persisted after adjusting for confounding factors such as age, sex, BMI, and physical activity. Vitamin D receptors and the enzyme needed to convert vitamin D to its active form (1-alpha-hydroxylase) are present in brain regions implicated in depression, including the prefrontal cortex, hippocampus, and cingulate gyrus.
However, intervention studies have produced mixed results. A 2019 meta-analysis in the Journal of Affective Disorders, reviewing 14 randomized controlled trials, found that vitamin D supplementation had a small but statistically significant positive effect on depressive symptoms, particularly in participants who had clinically diagnosed depression. The effect was more pronounced with doses above 2,000 IU daily and in study durations longer than 8 weeks. The American Psychiatric Association has not yet included vitamin D screening in routine depression workup guidelines, though many clinicians check levels as part of a comprehensive evaluation.
Seasonal affective disorder (SAD) provides an interesting lens for understanding the vitamin D-depression connection. SAD occurs predominantly in winter months when sunlight exposure and consequently vitamin D synthesis are reduced. The National Institute of Mental Health (NIMH) notes that while light therapy remains the primary treatment for SAD, some researchers hypothesize that reduced vitamin D production during winter months may contribute to the seasonal mood changes. Maintaining adequate vitamin D levels through supplementation during winter may be a reasonable adjunct strategy.
A meta-analysis in the British Journal of Psychiatry found low vitamin D associated with higher depression risk
How can you prevent vitamin D deficiency?
Preventing vitamin D deficiency requires a combination of sensible sun exposure, dietary sources, and often supplementation. The NIH recommends 600 IU daily for adults under 70 and 800 IU for those over 70, though many experts suggest 1,000-2,000 IU daily for optimal prevention, especially for people with limited sun exposure or higher risk factors.
A proactive approach to vitamin D maintenance involves three pillars: sunlight, food, and supplements. For sun exposure, the WHO suggests 5-15 minutes of midday sun on exposed skin (face, arms, hands) 2-3 times per week during months when UVB radiation is sufficient. In the United States, people living above 37 degrees latitude (roughly the line from San Francisco to Richmond, Virginia) cannot produce adequate vitamin D from sunlight between October and March. During these months, supplementation becomes particularly important according to guidelines from the American Academy of Family Physicians.
Regular monitoring through annual blood tests is advisable for individuals at higher risk of deficiency. The Endocrine Society recommends that high-risk groups — including older adults, those with dark skin, obese individuals, and people with malabsorption conditions — have their 25(OH)D levels checked regularly. Maintaining levels between 30-50 ng/mL is considered optimal by most endocrinology guidelines. The CDC's National Health and Nutrition Examination Survey (NHANES) data shows that approximately 42% of American adults are vitamin D deficient, making this a widespread public health issue that warrants proactive prevention.
For daily supplementation, most clinicians recommend vitamin D3 (cholecalciferol) over D2 (ergocalciferol) for prevention. Taking vitamin D with a meal containing fat improves absorption, as it is a fat-soluble vitamin. A study in the Journal of the Academy of Nutrition and Dietetics found that taking vitamin D with the largest meal of the day increased blood levels by approximately 50% compared to taking it on an empty stomach. Combining vitamin D with calcium-rich foods or a calcium supplement can enhance the bone-protective effects of both nutrients.
NHANES data shows approximately 42% of American adults are vitamin D deficient
What do the latest guidelines say about vitamin D supplementation?
Current guidelines vary by organization. The NIH recommends 600-800 IU daily, the Endocrine Society suggests 1,500-2,000 IU for at-risk adults, and some researchers advocate for even higher doses. The 2024 updated Endocrine Society guideline shifted toward recommending supplementation for specific high-risk populations rather than universal supplementation for all adults.
The landscape of vitamin D supplementation guidelines has evolved significantly. The Institute of Medicine's 2011 recommendation of 600 IU daily for adults aged 19-70 (800 IU for those over 70) was designed to meet the needs of 97.5% of the population for bone health. However, the Endocrine Society's more recent clinical practice guidelines have taken a more targeted approach, recommending higher doses for individuals at risk of deficiency. The updated 2024 guideline particularly emphasizes supplementation for premenopausal women, adults over 75, pregnant women, and individuals with prediabetes at high risk for type 2 diabetes.
The debate between population-level and individual recommendations continues. The US Preventive Services Task Force concluded in 2021 that evidence was insufficient to recommend universal vitamin D screening or supplementation for preventing fractures, cancer, or cardiovascular disease in the general adult population. However, this position contrasts with the Endocrine Society and the National Osteoporosis Foundation, which recommend maintaining levels above 30 ng/mL for musculoskeletal health. The WHO maintains that daily intake of 200-600 IU is adequate for most populations but acknowledges regional variation.
Emerging research continues to inform these guidelines. The VITAL trial (Vitamin D and Omega-3 Trial), a landmark randomized controlled trial of over 25,000 participants published in the New England Journal of Medicine, found that 2,000 IU daily of vitamin D3 did not significantly reduce the risk of cancer or cardiovascular events in the general population. However, subgroup analyses suggested benefits for individuals with lower baseline vitamin D levels and for cancer mortality reduction over extended follow-up. These nuanced findings underscore the importance of personalized supplementation based on individual risk factors rather than a one-size-fits-all approach.
The VITAL trial published in the NEJM evaluated vitamin D supplementation in over 25,000 participants
