Vitamin D deficiency is serious, particularly when levels stay low for months or years. Nearly half the world’s population has levels below the threshold considered sufficient, and the consequences go well beyond bone health. Chronic deficiency raises the risk of fractures, weakens your immune system, and is closely linked to cardiovascular disease, diabetes, and depression.
What Counts as Deficient
Vitamin D is measured through a simple blood test that checks your level of 25-hydroxyvitamin D. Most experts define deficiency as a level below 20 ng/mL, while 21 to 29 ng/mL is considered insufficient. The recommended target is above 30 ng/mL. A pooled analysis of 7.9 million participants across studies from 2000 to 2022 found that about 48% of people worldwide fall below 50 nmol/L (roughly 20 ng/mL), the widely used cutoff for deficiency. So if you’ve been told your levels are low, you’re far from alone.
Despite how common it is, the Endocrine Society’s 2024 clinical practice guideline suggests against routine screening for the general population. There isn’t enough trial evidence to show that testing everyone and treating based on a number prevents disease. Screening is typically reserved for people with established risk factors: limited sun exposure, darker skin, obesity, digestive disorders that impair fat absorption, or conditions like osteoporosis.
What It Does to Your Bones
The most well-established consequence of vitamin D deficiency is damage to your skeleton. Vitamin D’s primary job is helping your intestines absorb calcium. When your levels drop, your body can’t pull enough calcium from food, and it starts borrowing from your bones instead. Over time, this leaves bones soft, weak, and prone to fractures.
In children, this process causes rickets, a condition where bones become so soft they bend and bow under the body’s weight. Growth patterns become abnormal, joints can develop visible deformities, and children experience muscle weakness and bone pain. In adults, the same underlying problem is called osteomalacia. Symptoms include diffuse bone pain and tenderness, fragility fractures (particularly in the ribs, hips, and spine), and muscle weakness that tends to hit the thighs and hips hardest. Between 44% and 100% of people with osteomalacia develop noticeable muscle problems, depending on how severe the deficiency is. In extreme cases, weakness becomes so pronounced that people are bedridden.
Long-term deficiency also accelerates osteoporosis, the gradual loss of bone density that makes fractures more likely as you age. A hip fracture in an older adult can be life-altering, so this isn’t a minor concern.
Immune System Effects
Vitamin D plays an active role in regulating your immune system. It helps shift immune responses away from excessive inflammation and supports the production of regulatory immune cells that keep your defenses balanced. When levels are low, two things tend to happen: you get sick more often, and your risk of autoimmune conditions rises.
A study of nearly 19,000 people found that those with vitamin D levels below 30 ng/mL were significantly more likely to report a recent upper respiratory infection, even after adjusting for season, age, sex, body weight, and race. A separate study of military recruits in Finland showed that those with lower levels lost more days of active duty to respiratory infections than recruits with adequate vitamin D. The pattern is consistent: low vitamin D means your body has a harder time fighting off common infections.
Heart Disease and Metabolic Risk
Large epidemiological studies have identified vitamin D deficiency as a marker of cardiovascular risk. Low levels promote vascular inflammation, damage to the lining of blood vessels, and the buildup of arterial plaque. Deficiency has been associated with a higher prevalence of hypertension, and people with very low levels show signs of accelerated atherosclerosis even when their coronary arteries appear otherwise normal.
The metabolic consequences are equally concerning. Chronic vitamin D deficiency triggers a chain reaction: your parathyroid glands go into overdrive, which increases insulin resistance and impairs the insulin-producing cells in your pancreas. This combination pushes your body toward metabolic syndrome, a cluster of conditions that includes high blood sugar, high blood pressure, and abnormal cholesterol. A systematic review of prospective studies found an inverse relationship between circulating vitamin D and the development of type 2 diabetes, meaning people with lower levels were more likely to develop the disease over time.
It’s worth noting that while observational evidence linking vitamin D to heart disease is strong and consistent, randomized clinical trials of supplementation haven’t produced conclusive results for cardiovascular outcomes. The association is real, but whether taking supplements reverses the risk isn’t fully settled.
Mental Health and Neurological Effects
Mood changes, particularly depression, are among the recognized symptoms of vitamin D deficiency. The connection goes beyond simply feeling tired. Vitamin D deficiency is considered a risk factor for neuropsychiatric disorders, and the fatigue it causes can be persistent and hard to explain through other means. If you’ve been experiencing low mood, brain fog, or unexplained tiredness alongside any of the physical symptoms, low vitamin D could be a contributing factor.
Symptoms to Recognize
Mild deficiency often produces vague symptoms that are easy to dismiss. In adults, the most common signs are fatigue, bone pain, muscle weakness or cramping, and mood changes like depression. Because these overlap with dozens of other conditions, many people live with low vitamin D for years without realizing it.
Children may show more obvious signs. A mildly deficient child might just have sore, weak muscles. More severe deficiency leads to the visible skeletal changes of rickets: bowed legs, swollen joints, and delayed growth. In adults, the progression from “a little tired” to “can’t walk normally” happens gradually, which is part of what makes this deficiency dangerous. By the time symptoms are severe, significant bone and muscle damage has already occurred.
How Long Recovery Takes
The good news is that vitamin D deficiency is treatable. A typical correction protocol involves high-dose supplementation for about eight weeks, followed by a recheck of blood levels. If levels haven’t risen above 20 ng/mL, the high-dose course is repeated. Once levels normalize, a daily maintenance dose of 1,000 to 2,000 IU is generally enough to keep them there.
How quickly you feel better depends on how depleted you were and what symptoms you had. Fatigue and mood often improve within a few weeks of starting supplementation. Bone pain and muscle weakness take longer, sometimes several months, especially if osteomalacia has developed. Bone density improvements can take a year or more to show up on scans. The key is consistency: sporadic supplementation won’t maintain the levels your body needs, and stopping too early is a common reason for relapse.
People at higher risk of staying deficient include those who spend most of their time indoors, live at northern latitudes, have darker skin (which produces less vitamin D from sunlight), are significantly overweight, or have conditions affecting nutrient absorption like celiac disease or Crohn’s disease. If any of these apply to you, ongoing supplementation rather than a one-time correction may be necessary.