Not getting enough vitamin D disrupts your body’s ability to absorb calcium, weakens your bones, and leaves your immune system less equipped to fight infections. A blood level below 20 ng/mL is considered deficient, and the consequences range from subtle fatigue and muscle aches to serious skeletal diseases depending on how low your levels drop and how long they stay there.
Your Body Can’t Absorb Calcium Without It
Vitamin D’s most critical job is helping your intestines absorb calcium from food. When you eat something containing calcium, your gut uses two routes to pull it into the bloodstream: a passive route where small amounts trickle through, and an active route that moves much larger quantities. Vitamin D’s hormonal form is the major driver of that active route. It triggers cells lining your intestine to produce specialized transport proteins that shuttle calcium across the gut wall and into circulation.
When vitamin D is low, that active transport system slows dramatically. Your body still absorbs some calcium passively, but not nearly enough to maintain healthy bones, support muscle contractions, or keep your nerves firing properly. To compensate, your parathyroid glands ramp up production of a hormone that pulls calcium directly out of your bones. This works as a short-term fix, but over months and years it steadily hollows out your skeleton.
What It Does to Bones in Adults
Prolonged vitamin D deficiency in adults causes a condition called osteomalacia, where new bone tissue forms but never hardens properly. This is different from osteoporosis, though the two are easy to confuse. In osteoporosis, you lose bone mass overall but what remains is normally mineralized. In osteomalacia, the ratio of mineral to bone tissue is low, meaning the bones themselves are soft. The result is a deep, aching bone pain, particularly in the hips, lower back, and legs, that often gets misdiagnosed as fibromyalgia or chronic fatigue.
Over time, osteomalacia increases fracture risk and can make weight-bearing activities painful. Because vitamin D deficiency also accelerates bone density loss, many people end up with both osteomalacia and osteoporosis simultaneously, compounding the damage.
How It Affects Children Differently
In children, severe vitamin D deficiency causes rickets. Because a child’s skeleton is still growing, the consequences are more visible and more structural. Mineralization breaks down at the growth plates of long bones in the arms and legs, leaving them soft and unable to support a child’s increasing body weight. Toddlers who begin walking on bones weakened by rickets often develop noticeably bowed legs.
Rickets also delays overall bone growth, leaving affected children shorter than their peers. The wrists, skull, and ends of the ribs can become visibly enlarged. The weakened bones fracture more easily and are often painful. While rickets is now rare in countries that fortify milk and infant formula, it still occurs in breastfed infants who don’t receive supplemental vitamin D and in children with very limited sun exposure.
Muscle Weakness and Fatigue
Vitamin D receptors exist throughout your muscle tissue, and deficiency impairs muscle function in ways that go beyond simple tiredness. People with low levels commonly report a general heaviness in the limbs, difficulty climbing stairs, and trouble getting up from a seated position. This type of weakness tends to affect the muscles closest to your trunk (thighs, upper arms, hips) more than your hands and feet.
The fatigue associated with low vitamin D is persistent and doesn’t resolve with extra sleep. It’s one of the most common early symptoms, but also one of the easiest to dismiss or attribute to stress, poor sleep, or aging.
A Weaker Immune Response
Vitamin D plays a direct role in your innate immune system, the first-response defense that activates before your body produces targeted antibodies. It boosts the production of antimicrobial peptides, natural proteins that strengthen the protective barriers in your airways and gut lining. When vitamin D is low, that frontline defense weakens.
The clinical data on respiratory infections is fairly consistent. A large analysis of 25 randomized controlled trials covering nearly 11,000 people found that vitamin D supplementation reduced the risk of experiencing at least one acute respiratory infection by about 12%. Smaller analyses have found even stronger effects, with reductions in the range of 36% to 42%. The protection appears strongest in people who were deficient to begin with. Notably, this benefit has been demonstrated for respiratory infections broadly but not specifically for pneumonia, where trials have shown no clear difference.
Links to Chronic Disease
Observational studies have consistently linked low vitamin D levels to higher rates of cardiovascular disease, type 2 diabetes, metabolic syndrome, and several autoimmune conditions. People with deficiency show up more often in populations with hypertension, abnormal cholesterol, stroke, and heart failure.
The important caveat here is that observational data can’t prove cause and effect. People with low vitamin D may share other risk factors (less outdoor activity, higher body fat, poorer diets) that independently raise disease risk. Clinical trials of vitamin D supplementation for cardiovascular prevention have so far been inconclusive. The association is real and repeatedly demonstrated, but whether raising your vitamin D level directly lowers your heart disease risk remains an open question.
Who Is Most Likely to Be Deficient
Your skin produces vitamin D when UVB rays from sunlight convert a cholesterol compound into a precursor form of the vitamin. Anything that reduces UVB exposure reduces your production. Geography matters enormously: if you live at latitudes above about 40 degrees north (think Boston, Madrid, Beijing) or the equivalent south, there simply isn’t enough UVB radiation from roughly November through early March to produce meaningful vitamin D. Move ten degrees farther north, to places like Edmonton or London, and that “vitamin D winter” stretches from October through April.
Skin pigmentation is another major factor. Melanin, the pigment that gives darker skin its color, competes with the cholesterol compound in your skin for UVB absorption. People with dark skin may need up to ten times as long in the sun to produce the same amount of vitamin D as someone with fair skin. This is one reason vitamin D deficiency rates are disproportionately high in Black and South Asian populations living in northern climates.
Other high-risk groups include older adults (the skin’s ability to synthesize vitamin D declines with age), people who are homebound or institutionalized, those who cover most of their skin for religious or cultural reasons, people with obesity (vitamin D gets sequestered in fat tissue), and anyone with conditions that impair fat absorption, since vitamin D is fat-soluble.
How Much You Need
The recommended daily intake for most adults ages 19 to 70 is 600 IU (15 mcg). Adults over 70 need 800 IU (20 mcg), reflecting the age-related decline in skin synthesis. Infants from birth through 12 months should get 400 IU daily, and children ages 1 through 18 need 600 IU.
These recommendations are designed to maintain blood levels above 20 ng/mL for the general population, but some researchers argue that optimal levels for calcium absorption and bone health may be closer to 30 ng/mL or above. Vitamin D is found naturally in fatty fish (salmon, mackerel, sardines), egg yolks, and beef liver, and is added to fortified milk, orange juice, and many cereals. For people with limited sun exposure or dietary intake, a supplement is often the most reliable way to maintain adequate levels. A simple blood test measuring 25-hydroxyvitamin D can tell you exactly where you stand.