Vitamin D deficiency can cause a surprisingly wide range of health problems, from soft bones and muscle weakness to a poorly regulated immune system and a higher risk of cardiovascular disease. A blood level below 12 ng/mL is considered deficient by the National Institutes of Health, while levels between 12 and 20 ng/mL are generally inadequate for good health. Because vitamin D receptors exist in nearly every tissue in the body, running low affects far more than just your skeleton.
Weakened Bones and Poor Mineralization
The most well-established consequence of vitamin D deficiency is its effect on bones. Vitamin D regulates how your body absorbs and uses calcium and phosphorus, the two minerals that make bone tissue hard. Without enough vitamin D, newly formed bone can’t mineralize properly. In children, this produces rickets, a condition where bones become soft and bend under the body’s weight, leading to bowed legs and other skeletal deformities. In adults, the same process is called osteomalacia, which causes deep bone pain, tenderness, and an increased risk of fractures.
Even levels that aren’t technically “deficient” can chip away at bone density over time. When blood levels sit in the inadequate range (12 to 20 ng/mL), the body compensates by pulling more calcium from existing bone to maintain normal blood calcium levels. Over years, this accelerates bone loss and raises the likelihood of osteoporosis, particularly in postmenopausal women and older adults.
Muscle Weakness and Fall Risk
Vitamin D deficiency causes a specific pattern of muscle weakness that primarily affects the muscles closest to your trunk: the thighs, hips, and upper arms. This is called proximal myopathy, and it can make everyday movements like climbing stairs, standing up from a chair, or lifting objects overhead feel disproportionately difficult. In children, it can delay motor milestones. In older adults, it significantly increases fall risk.
The good news is that this type of muscle weakness is reversible. Once vitamin D levels are restored, functional strength typically improves. Despite this, vitamin D deficiency is frequently overlooked as a cause of muscle problems in both children and the elderly, even though it has been recognized as a treatable cause of proximal weakness for decades.
A Less Regulated Immune System
Vitamin D plays a direct role in how your immune system behaves. It helps activate certain immune cells when you need them and, just as importantly, helps dial down the immune response when you don’t. Without adequate vitamin D, the body struggles to develop regulatory immune cells, the subset responsible for keeping inflammation in check and preventing the immune system from attacking the body’s own tissues.
Research using animal models that lack vitamin D receptors shows a more aggressive, less controlled immune response and a higher risk of autoimmune disease. In humans, low vitamin D levels have been associated with conditions like multiple sclerosis, type 1 diabetes, and rheumatoid arthritis, all of which involve the immune system mistakenly targeting healthy tissue. Vitamin D also helps prime certain immune cells to respond to infections, which is why deficiency has been linked to greater susceptibility to respiratory infections.
Depression and Mood Changes
People with depression consistently show lower vitamin D levels than those without, and a large meta-analysis found that vitamin D supplementation produced a small but statistically significant reduction in depression symptoms. The relationship is nuanced, though. The benefit appeared mainly in people whose vitamin D levels were already above 20 ng/mL, not in those with the most severe deficiency (below 20 ng/mL). This suggests that vitamin D’s role in mood regulation may depend on other biological factors being in place first.
The connection between vitamin D and mood makes intuitive sense to anyone who has noticed feeling worse during darker winter months. Seasonal affective disorder is more common at higher latitudes, where sunlight exposure drops sharply in winter, and vitamin D production in the skin slows or stops entirely. While vitamin D deficiency likely isn’t the sole cause of seasonal mood changes, it appears to be a contributing factor.
Higher Cardiovascular Risk
Vitamin D receptors are found in the heart muscle, blood vessel walls, and the smooth muscle cells that line arteries. When vitamin D is low, the body’s blood pressure regulation system (the renin-angiotensin system) becomes more active, which can raise blood pressure and strain the heart. Animal studies confirm that removing vitamin D receptor activity leads to high blood pressure and thickening of the heart’s left ventricle.
In a large prospective study published in Circulation, people with vitamin D levels below 15 ng/mL had a 62% higher risk of experiencing a cardiovascular event compared to those with levels at or above 15 ng/mL. Population-level data also shows that rates of heart disease and hypertension climb with increasing distance from the equator, where people get less sun exposure and vitamin D deficiency is more common. Lower vitamin D levels have been cross-sectionally linked to higher blood pressure, greater coronary artery calcification, and more prevalent cardiovascular disease overall.
Insulin Resistance and Type 2 Diabetes Risk
Vitamin D appears to influence how well your cells respond to insulin and how effectively the insulin-producing cells in your pancreas function. Research in people at risk for type 2 diabetes found that vitamin D levels were independently associated with both insulin sensitivity and the ability of the pancreas to secrete insulin appropriately, even after accounting for body weight, physical activity, and other factors.
The relationship weakens in people with obesity (BMI of 30 or above), likely because vitamin D is fat-soluble and gets sequestered in fat tissue, reducing its availability in the bloodstream. This means that people carrying excess weight may need more vitamin D to see the same metabolic benefits, and their blood levels may underrepresent how much vitamin D they’ve actually taken in.
Who Is Most at Risk
Certain groups are more likely to become deficient. People with darker skin produce vitamin D somewhat less efficiently from sunlight, though the difference is smaller than previously thought. Studies comparing the lightest and darkest skin types found that melanin reduces vitamin D synthesis by a factor of roughly 1.3 to 1.4, meaning darker-skinned individuals need modestly more sun exposure rather than dramatically more.
Digestive conditions that impair nutrient absorption pose a more significant risk. Crohn’s disease, ulcerative colitis, and celiac disease all reduce the gut’s ability to take in vitamin D from food or supplements. Gastric bypass surgery, which reroutes food past part of the small intestine where vitamin D is absorbed, also makes deficiency more likely. People who spend most of their time indoors, live at northern latitudes, consistently wear clothing that covers most of their skin, or use high-SPF sunscreen daily are also at elevated risk.
How Deficiency Is Measured
A simple blood test measuring 25-hydroxyvitamin D gives a reliable picture of your vitamin D status. The NIH defines the key thresholds as follows:
- Below 12 ng/mL (30 nmol/L): Deficient. Associated with rickets in children and osteomalacia in adults.
- 12 to 20 ng/mL (30 to 50 nmol/L): Inadequate for bone and overall health.
- 20 ng/mL or above (50 nmol/L): Adequate for most people.
- Above 50 ng/mL (125 nmol/L): Potentially harmful, especially above 60 ng/mL.
These thresholds matter because many of the health consequences described above begin in the inadequate range, not just at outright deficiency. If you’re in a higher-risk group, testing is straightforward and widely available. Correcting a deficiency typically involves supplementation, increased sun exposure, or both, with most people seeing improvement in symptoms within a few weeks to months depending on how low their levels were to begin with.