Vitamin D deficiency disrupts your body’s ability to absorb calcium, weakens your bones, compromises your immune system, and raises your risk for a surprising range of health problems. A blood level below 20 ng/mL is classified as deficient, while levels between 20 and 30 ng/mL are considered insufficient. Globally, about 48% of people fall below that sufficiency threshold, making this one of the most common nutritional shortfalls in the world.
How Vitamin D Works in Your Body
Vitamin D isn’t just a vitamin. It functions more like a hormone. Your skin produces it when exposed to ultraviolet light, and your liver and kidneys convert it into its active form. Once activated, its most critical job is helping your intestines absorb calcium from the food you eat. Without enough vitamin D, your gut absorbs only a fraction of the calcium passing through it. The active form of vitamin D stimulates cells lining your intestine to actively pull calcium across the intestinal wall and into your bloodstream. It also helps regulate phosphate levels, which work alongside calcium to keep bones dense and strong.
When vitamin D drops too low, your body can’t maintain adequate calcium levels through diet alone. To compensate, it starts pulling calcium from your bones. This is the beginning of a cascade that affects far more than your skeleton.
Bone Pain and Soft Bones
The most direct consequence of vitamin D deficiency is what it does to your bones. In adults, prolonged deficiency causes a condition called osteomalacia, where bones gradually soften. People with osteomalacia often describe a dull, persistent aching in their bones, particularly in the spine, pelvis, and legs. Pressing even lightly on the breastbone or shinbone can produce noticeable pain, which is one of the ways doctors check for the condition during a physical exam.
In children, the equivalent condition is rickets. Because children’s bones are still growing, the effects are more dramatic: bowed legs, knock-knees, thickened wrists and ankles, and a breastbone that pushes outward. Children with rickets also experience delayed growth, delayed motor skills, and muscle weakness. In infants, low calcium from vitamin D deficiency can cause tight muscle tone and irregular breathing.
Over time, even milder deficiency contributes to osteoporosis, where bones become brittle and fracture-prone, particularly in older adults.
Muscle Weakness and Falls
Vitamin D deficiency causes measurable muscle weakness, not just in older adults but at any age. The weakness tends to affect the muscles closest to the center of your body, like your thighs and upper arms. You might notice it as difficulty climbing stairs, getting up from a chair, or a general sense of heaviness in your limbs. This muscle weakness, combined with bone pain, is one reason people with low vitamin D are significantly more likely to fall. For older adults, that increased fall risk compounds the fracture risk from weakened bones.
A Weakened Immune System
Vitamin D plays a hands-on role in your immune defenses. It activates key immune cells, including T cells, B cells, and macrophages, and it stimulates the production of natural antimicrobial compounds that help your body fight off pathogens. When levels are chronically low, the immune system becomes sluggish. This creates increased vulnerability to infections, particularly respiratory viruses like influenza and intracellular bacteria like tuberculosis.
The flip side is equally important. Vitamin D helps keep your immune system from overreacting. It shifts certain immune cells from a pro-inflammatory state to an anti-inflammatory one, which is essential for preventing the immune system from attacking your own tissues. When vitamin D is deficient, this regulatory function breaks down, and the risk of autoimmune responses increases. Maintaining blood levels above 50 ng/mL has been associated with significant reductions in risk for both infections and autoimmune conditions.
Blood Pressure and Heart Health
Vitamin D acts as a natural brake on the renin-angiotensin system, the hormonal pathway that controls blood pressure. Animal studies have shown this clearly: mice lacking the vitamin D receptor develop elevated renin activity and high blood pressure, and when vitamin D function is restored, blood pressure normalizes. In humans, the pattern is consistent. Large population studies in the United States, Germany, and the United Kingdom all show that lower vitamin D levels are associated with higher blood pressure.
The numbers are notable. People with levels below 15 ng/mL had roughly 2.7 times the risk of developing high blood pressure compared to those above 30 ng/mL. Even comparing the lowest quartile of vitamin D levels to the highest, the odds of developing hypertension were about 66% greater. Vitamin D also appears to protect blood vessels directly by improving the function of the inner lining, supporting nitric oxide production, and reducing inflammatory markers linked to atherosclerosis.
That said, clinical trials of vitamin D supplements have not consistently shown blood pressure reductions, though trials focused on people who already had high blood pressure and used higher doses did show modest decreases, around 6 points on systolic pressure.
Mood and Brain Health
Vitamin D receptors are found throughout the brain, and the vitamin plays a role in regulating calcium levels inside neurons. One leading theory, called the phenotypic stability hypothesis, proposes that vitamin D helps maintain the pumps and buffers that keep calcium at the right level inside brain cells. When vitamin D is low, neuronal calcium rises. That excess calcium appears to contribute to depressive symptoms. This mechanism is supported by the fact that some effective antidepressant treatments, like ketamine, work partly by blocking the same calcium-related pathways that vitamin D helps regulate.
The connection between elevated neuronal calcium and depression may also explain why people with depression have a higher likelihood of eventually developing Alzheimer’s disease. Chronically high calcium levels inside brain cells can trigger the formation of amyloid plaques, the hallmark of Alzheimer’s. While the research on this link is still developing, it underscores how vitamin D’s role extends well beyond bones.
Hair Loss
Vitamin D is essential for creating the cells that develop into hair follicles. When levels are low, the hair growth cycle can be disrupted. If you’re experiencing hair loss alongside other symptoms of deficiency, restoring your vitamin D levels may help, though hair loss has many possible causes and low vitamin D is just one piece of the puzzle.
Who Is Most at Risk
Your body’s ability to make vitamin D depends heavily on factors you may not be able to control. Melanin, the pigment that gives skin its color, acts as a natural sunscreen. Darker skin requires significantly more sun exposure to produce the same amount of vitamin D as lighter skin. Studies comparing the lightest and darkest skin types found that melanin reduces vitamin D production by a factor of about 1.3 to 1.4 under the same UV exposure. People who have immigrated from equatorial regions to higher latitudes face a compounded risk: their skin is adapted for stronger sunlight, but their new environment provides less of it.
Geography matters on its own, too. At higher latitudes, the angle of sunlight during fall and winter months is too low to trigger meaningful vitamin D synthesis in the skin, regardless of skin tone. People who spend most of their time indoors, cover most of their skin, or consistently use sunscreen also produce less.
Older adults face a double disadvantage. Aging skin produces vitamin D less efficiently, and the kidneys become less effective at converting it to its active form. Adults over 71 need 800 IU daily, compared to 600 IU for younger adults. Infants need 400 IU, and children and teens need 600 IU. These are the recommended daily amounts from the NIH, though many researchers argue that the true optimal intake is higher, particularly for people who get little sun exposure.
How Deficiency Is Detected
A simple blood test measuring 25-hydroxyvitamin D tells you where you stand. Below 20 ng/mL is deficient. Between 20 and 30 ng/mL is insufficient. Above 30 ng/mL is generally considered adequate, though some evidence suggests that levels above 50 ng/mL provide the strongest immune protection. In the Americas, about 5.5% of the population falls into the severely deficient category (below 12 ng/mL), but the proportion with suboptimal levels is much larger. Many people with mild to moderate deficiency have no obvious symptoms for years, which is part of what makes it so common.