Vitamin D deficiency affects an estimated 42% of American adults. One billion people worldwide have insufficient vitamin D levels. Most of them do not know it. The symptoms are diffuse, non-specific, and easy to attribute to other causes. Here is how to recognize the signs and who is most at risk.
Why Deficiency Is So Common
The human body makes vitamin D from sunlight, but modern life systematically blocks this process:
- Most adults spend 90% or more of their time indoors
- UVB rays (the ones that make vitamin D) do not penetrate glass
- SPF 15 sunscreen reduces vitamin D synthesis by approximately 99%
- At latitudes above 35 degrees north, UVB is insufficient for vitamin D production from October through March
- Dark skin requires 3 to 6 times more sun exposure for the same D3 synthesis as light skin
Add to this that very few foods naturally contain meaningful vitamin D (fatty fish and egg yolks are the main sources), and you have conditions perfectly suited to widespread deficiency.
Common Symptoms of Vitamin D Deficiency
Fatigue and Low Energy
Chronic fatigue is one of the most common reported symptoms of vitamin D deficiency. Vitamin D receptors (VDRs) are expressed throughout the body including in muscle tissue and mitochondria. Inadequate D3 impairs mitochondrial efficiency, reducing energy production at the cellular level. A 2016 study in Medicine found significant improvement in fatigue scores after vitamin D supplementation in people with confirmed deficiency.
Bone Pain and Muscle Weakness
Vitamin D deficiency causes inadequate calcium absorption, leading to bone pain (particularly in the lower back, hips, and legs) and muscle weakness. Severe, prolonged deficiency causes osteomalacia in adults (soft, weak bones) and rickets in children. Mild deficiency produces subtle bone pain that many people attribute to aging or overuse.
Frequent Illness and Infections
Vitamin D plays a direct role in immune function. VDRs are expressed on virtually every immune cell. D3 supports the production of antimicrobial peptides (including cathelicidins and defensins) that are the body’s first-line defense against pathogens. People with low vitamin D levels consistently show higher rates of respiratory infections, including influenza and the common cold. A 2017 meta-analysis in the BMJ found that vitamin D supplementation reduced the risk of acute respiratory infections by 12% overall and by 70% in people who were severely deficient.
Depression and Mood Changes
VDRs are expressed in multiple brain regions, and vitamin D is involved in the synthesis of serotonin, dopamine, and other neurotransmitters. Low vitamin D is consistently associated with elevated rates of depression in population studies. A 2020 meta-analysis in the Journal of Affective Disorders found a significant inverse relationship between vitamin D levels and depression scores. While the relationship is correlational and deficiency correction does not cure clinical depression, mood improvement is among the most commonly reported benefits of vitamin D supplementation in people who were deficient.
Impaired Wound Healing
Vitamin D plays a role in the inflammatory and proliferative phases of wound healing. People with low D levels show delayed healing of cuts and surgical incisions. If you notice that minor wounds take longer to heal than they should, vitamin D status is worth checking.
Hair Loss
VDRs are expressed in hair follicles and play a role in the hair growth cycle. Vitamin D deficiency has been associated with alopecia areata and non-scarring alopecia in some research. This is a less established connection than the above symptoms but worth noting for people experiencing unexplained hair thinning.
Brain Fog and Cognitive Difficulties
Adequate vitamin D supports neurological function, including memory and processing speed. Population studies consistently show lower cognitive performance in people with vitamin D deficiency compared to people with adequate levels. Correction of deficiency has been shown to improve cognitive function in older adults.
Who Is Most at Risk
Certain groups are at substantially higher risk of deficiency:
- Indoor workers: Minimal sun exposure throughout the workday
- People with darker skin: Higher melanin content reduces UVB absorption, requiring longer sun exposure for equivalent D3 synthesis
- Adults over 65: Skin produces D3 at about 25% the efficiency of younger adults; kidneys also convert D3 to active form less efficiently
- Overweight individuals: Vitamin D is sequestered in fat tissue, reducing its availability in blood
- People in northern latitudes: Insufficient UVB radiation during fall and winter months
- People who consistently use sunscreen: Even moderate SPF dramatically reduces D3 synthesis
- People with gastrointestinal conditions: Crohn’s disease, celiac disease, and short bowel syndrome reduce D3 absorption
How to Know for Sure
The definitive answer is a 25(OH)D blood test. This is a standard, inexpensive blood test. Most labs offer it for $30 to $60 direct-to-consumer. Optimal levels are generally considered 40 to 60 ng/mL by most researchers. Below 20 ng/mL is clinical deficiency; 20 to 30 ng/mL is insufficiency.
If you have several of the symptoms listed above and fall into one of the at-risk categories, testing is the most efficient next step. If testing is not accessible, supplementing with 2,000 to 5,000 IU of D3 daily is unlikely to cause harm and addresses the most common root cause. For dosage specifics, see our guide on how much vitamin D3 to take per day.