Vitamin D Deficiency: Causes, Symptoms, and How to Fix It

What Is Vitamin D and Why Does It Matter?

Vitamin D is a fat-soluble nutrient that acts more like a hormone than a typical vitamin. It plays a critical role in helping your body absorb calcium and phosphorus, keeping your bones strong. Beyond bone health, vitamin D supports your immune system, muscle function, and even your mood.

Unlike most nutrients, your body can produce vitamin D on its own — but only when your skin is exposed to sunlight. That is why vitamin D deficiency has become one of the most common nutritional problems worldwide, especially among people who spend most of their time indoors.

Who Is at Risk?

Vitamin D deficiency can affect anyone, but some groups are more vulnerable:

• People who spend little time outdoors — office workers, students, and those in cold or cloudy climates
• Older adults — skin becomes less efficient at producing vitamin D with age
• People with darker skin — higher melanin reduces the skin's ability to synthesize vitamin D from sunlight
• Obese individuals — vitamin D gets stored in fat tissue and becomes less available in the bloodstream
• People with certain medical conditions — such as Crohn's disease, celiac disease, or kidney disease that affect absorption

Common Symptoms of Vitamin D Deficiency

Vitamin D deficiency often develops slowly and silently. Many people have no obvious symptoms at first. Over time, however, the following signs may appear:

• Fatigue and low energy — feeling tired even after a full night's sleep
• Bone pain or tenderness — especially in the back, legs, and hips
• Muscle weakness — difficulty climbing stairs or performing everyday tasks
• Frequent illness — getting colds or infections more often than usual
• Depression or low mood — vitamin D receptors exist in areas of the brain linked to mood regulation
• Hair loss — in severe cases, deficiency has been linked to increased hair shedding
• Slow wound healing — vitamin D plays a role in the production of compounds that help repair skin

If you experience several of these symptoms together, it is worth speaking with a doctor and getting a blood test.

How Is Vitamin D Deficiency Diagnosed?

A simple blood test measuring the level of 25-hydroxyvitamin D (25(OH)D) in your blood can determine your vitamin D status. The general reference ranges are:

Level	Interpretation
Below 20 ng/mL	Deficiency
20–29 ng/mL	Insufficiency
30–100 ng/mL	Sufficient
Above 100 ng/mL	Potentially toxic

Your doctor will interpret your results in the context of your overall health and symptoms.

How to Treat and Prevent Vitamin D Deficiency

1. Get More Sunlight

Spending 10–30 minutes in midday sunlight several times a week can meaningfully boost vitamin D levels. Expose your arms and legs without sunscreen for short periods. Duration depends on your skin tone and location.

2. Eat Vitamin D-Rich Foods

While few foods contain high amounts of vitamin D naturally, the following are good sources:

• Fatty fish (salmon, mackerel, sardines)
• Egg yolks
• Beef liver
• Fortified foods (milk, orange juice, breakfast cereals)
• Mushrooms exposed to UV light

3. Take a Vitamin D Supplement

For people who cannot get enough from sunlight and food alone, supplements are an effective solution. Vitamin D3 (cholecalciferol) is generally recommended over D2 as it raises blood levels more effectively. Common supplementation doses range from 1,000 to 4,000 IU per day, but always follow your doctor's advice before starting.

When to See a Doctor

See a healthcare professional if you suspect a deficiency, are in a high-risk group, or experience persistent symptoms like bone pain or extreme fatigue. Self-supplementing with very high doses without guidance can lead to vitamin D toxicity, which causes symptoms such as nausea, weakness, and kidney problems.

Key Takeaways

Vitamin D deficiency is widespread but largely preventable. A combination of sensible sun exposure, a balanced diet, and supplementation when needed can help you maintain healthy levels. If you are unsure about your vitamin D status, a routine blood test is a simple and reliable first step.

Prepared by: Punyanut Angkhanawin, Pharm.D.

Sources:

1. Holick, M.F. (2007). Vitamin D Deficiency. New England Journal of Medicine, 357(3), 266–281.
2. National Institutes of Health – Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals
3. World Health Organization (WHO). Vitamin D Supplementation in Infants and Children