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 are the signs to watch for, who's most at risk, what 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.

Signs of Vitamin D Deficiency in Children

In children, vitamin D deficiency can look like general muscle soreness or weakness in mild cases. Left untreated, more severe deficiency can lead to rickets — a condition that causes bowed or bent bones, joint deformities, and growth problems^[7]. It's rare in developed countries today, but it's part of why pediatricians often recommend vitamin D supplementation for breastfed infants specifically^[7].

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.

Who's Most at Risk

Some people are far more likely to run low on vitamin D, regardless of symptoms^[1]:

• Adults over 65 — skin naturally becomes less efficient at producing vitamin D from sunlight with age.
• People with darker skin tones — higher melanin levels make it harder for skin to synthesize vitamin D from sun exposure.
• Anyone with limited sun exposure — people who work indoors, live in northern climates, or cover most of their skin for cultural or medical reasons.
• Breastfed infants, since breast milk alone typically doesn't provide enough vitamin D.
• People with certain health conditions, including Crohn's disease, celiac disease, cystic fibrosis, kidney or liver disease, and obesity — all of which can affect how the body absorbs or processes vitamin D.

How Vitamin D Deficiency Is Diagnosed

If you recognize a few of these signs — or fall into a higher-risk group — the only way to know for sure is a simple blood test that measures 25-hydroxyvitamin D (25(OH)D), the storage form of vitamin D in your blood^[1]. It's a standard test most doctors can order during a routine visit. Most labs classify results below 20 ng/mL as deficient and 20–29 ng/mL as insufficient^[1].

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.

The honest summary

• Deficiency is defined as a blood level under 20 ng/mL — symptoms alone can't confirm it^[1].
• Fatigue, bone and muscle pain, low mood, and hair thinning are the most reported signs, but many people have none^[3].
• Sun exposure, diet, age, skin tone, and certain health conditions are the main drivers of low levels^[1].
• Higher-dose supplementation is for confirmed deficiency or specific groups (adults 75+, pregnancy, prediabetes) — not a routine recommendation for every healthy adult^[2].

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].

What does vitamin D deficiency pain feel like?

It's typically described as a dull, aching bone pain (often in the back, hips, or legs) rather than a sharp or localized pain, along with general muscle soreness or weakness^[3].

Can you have vitamin D deficiency with no symptoms?

Yes, and it's actually common. This is exactly why risk factors — age, skin tone, sun exposure, certain health conditions — matter even if you feel fine^[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.
7. American Academy of Pediatrics. Prevention of Rickets and Vitamin D Deficiency in Infants, Children, and Adolescents. Pediatrics, 2008.

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.