What Vitamin D Does in the Body
Vitamin D is not just about strong bones. It helps regulate calcium and phosphorus, supports muscle function, and plays a role in the immune system and nervous system. The body produces vitamin D when skin is exposed to sunlight, and it can be obtained in small amounts from certain foods and supplements. Modern life—staying indoors, sunscreen use, and living at higher latitudes—can limit this production, increasing the risk of a deficiency over time.
What Is Considered Vitamin D Deficiency?
Doctors measure a form of vitamin D in the blood called 25-hydroxyvitamin D (25(OH)D). Values can vary by laboratory and guideline, but common thresholds are used to classify status. Many experts describe <20 ng/mL as deficient, 20–29 ng/mL as suboptimal, and 30–50 ng/mL as a typical target for many adults. Some clinicians prefer higher targets, particularly for older adults or people with certain health conditions. If you’re unsure, discuss your results with a healthcare professional who can interpret them in context.
Who Is At Risk?
Several factors increase the likelihood of a vitamin D deficiency. These include:
- Limited sun exposure due to indoor lifestyles, long work hours, or cultural practices
- Wearing clothing that covers most of the skin for religious or cultural reasons
- Having darker skin, which can require longer sun exposure to make the same amount of vitamin D
- Aging, as the skin’s ability to synthesize vitamin D declines with age
- Digestive or liver/kidney conditions that impair absorption or activation of vitamin D
- Certain medications that affect vitamin D metabolism
In addition, people who are pregnant or breastfeeding, or infants and young children, can be more susceptible and should monitor their status with a clinician’s guidance.
Symptoms in Adults
Vitamin D deficiency can manifest in several ways. Common adult symptoms include:
- Bone pain, especially in the back, hips, pelvis, or legs
- Muscle weakness or a reduction in muscle function
- Fatigue and lower energy levels
- Frequent infections or a perceived drop in immune resilience
- Persistent mood changes, including low mood or irritability
Some studies link low vitamin D with higher blood pressure, type 2 diabetes risk, and certain cancer outcomes, but the relationships are complex and require more research for clear causation. A clinician can help interpret whether vitamin D may be contributing to these concerns in your case.
Symptoms in Children and Teens
Children rely on vitamin D for healthy bone growth and immunity. A severe deficiency can lead to rickets, with softened bones and skeletal deformities. Other potential signs include:
- Stunted or delayed bone growth
- Muscle weakness or poor motor development
- Increased susceptibility to infections
- Bone and dental development issues, including higher risk of dental problems
In infants, maternal deficiency can translate into low vitamin D in breast milk, potentially causing early signs of deficiency in the baby. Early detection and intervention are important to prevent long-term effects on growth and health.
Testing, Prevention, and Safe Supplementation
If you notice persistent bone or muscle symptoms, or if you’re in a risk group, talk to a healthcare provider about testing your vitamin D status. A simple blood test can reveal your 25(OH)D level and help tailor a plan to restore sufficiency.
Prevention focuses on a combination of safe sun exposure, diet, and, when needed, supplements. Food sources with vitamin D include fatty fish, fortified dairy or plant-based alternatives, and fortified cereals. Sensible sun exposure—short periods a few times per week and protective measures for skin health—can also support production without increasing skin cancer risk. Supplements may be recommended if levels remain insufficient after lifestyle changes, but dosages should be guided by a clinician to avoid toxicity.
Vitamin D toxicity is rare but possible with excessive supplement use. Signs of excess include nausea, vomiting, weakness, and very high calcium levels, which can damage organs. If supplementing, follow medical advice and report any unusual symptoms to your doctor.
Bottom Line
Vitamin D plays a crucial role far beyond bone health. Being aware of the risk factors and symptoms for deficiency can help you seek testing and take steps to maintain healthy levels. If you’re unsure about your risk or symptoms, schedule a conversation with a healthcare professional who can tailor recommendations to your health profile.
