Vitamin D deficiency is one of the most common nutrient gaps worldwide, but it's easy to miss because the early symptoms are subtle. Here's what actually causes it, how it's diagnosed, and what treatment looks like in practice.

Written by Vita Globe Editorial Team | Reviewed against peer-reviewed research and federal health guidance

What Is Vitamin D Deficiency?

Vitamin D deficiency occurs when the body doesn't get or produce enough vitamin D to maintain healthy levels of calcium and phosphate — the two minerals it depends on for strong bones, muscle function, and immune support. Most clinicians define deficiency as a blood level of 25-hydroxyvitamin D below 20 ng/mL, while levels between 20–29 ng/mL are generally considered insufficient rather than deficient^[1]. Because vitamin D is produced mainly through sun exposure and found in relatively few foods, deficiency and insufficiency together are common in the U.S. — national health survey data has found a substantial share of adults fall short of sufficient levels^[5].

Signs and Symptoms of Vitamin D Deficiency

Vitamin D deficiency is easy to miss because early symptoms are subtle and often mistaken for normal fatigue or aging. The most commonly reported signs include^[3]:

• Persistent tiredness or low energy, even after adequate sleep
• Bone pain or a dull, generalized ache
• Muscle weakness, cramping, or soreness
• Frequent illness or infections due to reduced immune function
• Low mood or symptoms resembling seasonal depression
• Slow wound healing
• Hair thinning or hair loss^[4]
• Back or joint pain

Because these symptoms overlap with dozens of other conditions, the only reliable way to confirm a deficiency is a simple blood test (25-hydroxyvitamin D) ordered by your doctor.

What Causes Vitamin D Deficiency?

Vitamin D deficiency usually comes down to one or more of these factors^[1]:

• Limited sun exposure — office jobs, northern latitudes, winter months, sunscreen use, and covering clothing all reduce the skin's vitamin D synthesis.
• Darker skin tone — higher melanin levels reduce the skin's ability to produce vitamin D from sunlight.
• Age — skin becomes less efficient at synthesizing vitamin D as we get older.
• Limited diet — few foods naturally contain vitamin D (fatty fish, egg yolks, fortified milk), making deficiency common among people who eat a restricted or plant-based diet.
• Malabsorption conditions — Crohn's disease, celiac disease, and gastric bypass surgery can all interfere with vitamin D absorption.
• Obesity — vitamin D is fat-soluble and can become sequestered in body fat, lowering how much circulates in the blood.
• Kidney or liver conditions — both organs are required to convert vitamin D into its active form.

How to Treat Vitamin D Deficiency

Treatment depends on how low your levels are, but most plans combine three approaches:

• Supplementation — Most adults with confirmed deficiency are advised to take between 1,000–4,000 IU of vitamin D3 daily, though your doctor may prescribe a higher short-term dose (like 50,000 IU weekly) to correct a severe deficiency quickly^[1].
• Sensible sun exposure — 10–30 minutes of midday sunlight several times a week helps the skin produce vitamin D naturally.
• Diet adjustments — Adding fatty fish, egg yolks, fortified dairy or plant milk, and mushrooms exposed to UV light can support your levels alongside supplementation.

A 2024 Endocrine Society guideline update adds an important nuance: it recommends higher-dose vitamin D specifically for adults over 75, people who are pregnant, and people with prediabetes, but it does not recommend routine high-dose supplementation above the standard RDA for other healthy adults — supplementation at that level is meant for confirmed deficiency, not general prevention^[2].

Most people see blood levels improve within 8–12 weeks of consistent supplementation; your doctor will typically recheck levels via a follow-up blood test before adjusting the dose.

Frequently Asked Questions

What causes vitamin D deficiency?

Vitamin D deficiency is most often caused by limited sun exposure, a diet low in vitamin D–rich foods, darker skin, older age, obesity, or a health condition (like Crohn's, celiac disease, or kidney/liver disease) that interferes with absorption or conversion of vitamin D^[1].

What are the symptoms of vitamin D deficiency?

Common symptoms include fatigue, bone or muscle pain, muscle weakness, frequent infections, low mood, slow wound healing, and hair thinning. Symptoms are often mild or absent, which is why a blood test is the most reliable way to check your levels^[3].

Can vitamin D deficiency cause hair loss?

A 2024 systematic review and meta-analysis found vitamin D levels were significantly lower in people with non-scarring hair loss conditions like telogen effluvium and alopecia areata compared with people without hair loss^[4]. That's an association, not proof that correcting a deficiency reverses hair loss — but it's a common, low-risk thing for a doctor to check when hair thinning is otherwise unexplained.

Does vitamin D deficiency cause fatigue?

Yes, fatigue is one of the most frequently reported symptoms of low vitamin D. Because vitamin D supports muscle function and plays a role in energy metabolism, deficiency can leave people feeling persistently tired even with adequate sleep^[1].

Can vitamin D deficiency cause weight gain?

The evidence is mixed. Low vitamin D is more strongly associated with obesity than proven to directly cause weight gain — vitamin D is fat-soluble and gets sequestered in higher body fat, which lowers the amount circulating in blood^[1]. Correcting a deficiency isn't a standalone weight-loss fix.

How long does it take to recover from vitamin D deficiency?

Most people see meaningful improvement in blood levels within 8–12 weeks of consistent daily supplementation. Severe deficiencies treated with high-dose prescription regimens may normalize faster, but your doctor will confirm with a follow-up blood test^[1].

How do you test for vitamin D deficiency?

A simple blood test called the 25-hydroxyvitamin D test (25(OH)D) measures your circulating vitamin D level. Most labs classify results below 20 ng/mL as deficient and 20–29 ng/mL as insufficient^[1].

Sources

1. National Institutes of Health, Office of Dietary Supplements. Vitamin D — Health Professional Fact Sheet.
2. Demay MB, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. 2024.
3. Cleveland Clinic. Vitamin D Deficiency: Causes, Symptoms & Treatment.
4. Frontiers in Nutrition. Vitamin D Deficiency in Non-Scarring and Scarring Alopecias: A Systematic Review and Meta-Analysis. 2024.
5. National Center for Health Statistics (CDC). Vitamin D Status in the United States, 2011–2014.
6. Vita Globe. Vitamin D3 2,000 IU Gummies — product formulation and Supplement Facts panel.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for professional medical advice. Talk to your doctor before starting any new supplement.