Vitamin D deficiency weakens bones, impairs immune function, saps muscle strength, and may contribute to mood disorders and cardiovascular problems. Blood levels below 12 ng/mL are classified as deficient, while levels between 12 and 20 ng/mL are considered inadequate for bone and overall health. The effects range from subtle (fatigue, frequent illness) to severe (bone deformities in children, fractures in adults), depending on how low levels drop and how long they stay there.
How Vitamin D Keeps Bones Strong
Vitamin D’s most critical job is helping your body absorb calcium from food. Without enough of it, your intestines absorb only 10% to 15% of the calcium you eat. When levels are adequate, that absorption rate jumps to 30% to 40%. This difference is enormous. Even if your diet is rich in calcium, a vitamin D deficiency means most of it passes through your body unused.
When calcium absorption drops, your body starts pulling calcium from your bones to maintain the blood calcium levels your heart and nerves need to function. Over time, this creates a cycle of progressive bone weakening. In children, this leads to rickets, a condition where growing bones become soft and bend under the body’s weight, causing bowed legs and other skeletal deformities. In adults, the equivalent condition is called osteomalacia: the bones soften, causing deep, aching bone pain (often in the hips, pelvis, and lower back) and increasing fracture risk from minor falls or everyday stress.
Muscle Weakness and Balance Problems
Vitamin D receptors are found throughout skeletal muscle, where they help muscle cells grow and mature into the fast-twitch fibers responsible for quick, powerful movements. These are the same fibers you rely on to catch yourself when you stumble. Vitamin D also plays a role in transporting calcium into the cellular machinery that drives muscle contraction, which directly affects your ability to maintain balance and posture.
When levels are chronically low, you may notice weakness in the muscles closest to your trunk: the thighs, hips, and upper arms. Climbing stairs feels harder. Getting up from a chair requires extra effort. In severe cases, deficiency can cause a noticeable change in gait, a waddling or unsteady walk that increases the risk of falls, particularly in older adults. This combination of weaker bones and weaker muscles is why vitamin D deficiency is so closely linked to fractures in people over 65.
Immune System Effects
Vitamin D acts as a regulator of both branches of your immune system. On the front lines, it helps your body’s first responders (cells called macrophages) produce a natural antibiotic protein that kills bacteria and other pathogens directly. This is one reason people with low vitamin D tend to get more respiratory infections, colds, and flu.
On the adaptive side, vitamin D fine-tunes how your immune system’s T cells respond to threats. It helps T cells activate fully when a real infection is present, improving their ability to migrate to the site of an infection. At the same time, it dials down certain inflammatory signals that can cause collateral damage to healthy tissue. This balancing act matters because an overactive immune response is the driver behind autoimmune conditions. Deficiency removes this brake, which is why low vitamin D has been associated with higher rates of autoimmune diseases like multiple sclerosis and type 1 diabetes.
Mood and Depression
A 2023 meta-analysis of 18 studies found that vitamin D supplementation produced a statistically significant reduction in depression symptom scores in adults with primary depression. But the results came with an important nuance: the benefit was concentrated in people whose blood levels were already at or above 20 ng/mL (50 nmol/L). For those with levels below that threshold, supplementation did not significantly improve depressive symptoms.
This suggests the relationship between vitamin D and mood isn’t as straightforward as “low levels cause depression.” It’s possible that a certain baseline level of vitamin D is needed for the brain to use it effectively in mood regulation, and that severe deficiency involves other metabolic disruptions that supplementation alone can’t fix. Still, the association between low vitamin D and depressive symptoms is consistent enough across research that it’s worth paying attention to, especially if you notice seasonal mood changes during winter months when sun exposure drops.
Cardiovascular and Blood Pressure Links
Vitamin D appears to influence blood pressure through its effects on blood vessel walls. It helps keep the lining of arteries flexible and healthy. When levels are low, arteries can stiffen, making it harder for blood to flow and raising blood pressure. Vitamin D also affects the chemical signaling pathways your body uses to regulate blood pressure.
Chronic deficiency has been linked to a higher likelihood of developing high blood pressure and diabetes. While this doesn’t mean low vitamin D directly causes heart disease, it does mean that deficiency adds to the overall burden on your cardiovascular system, particularly when combined with other risk factors like inactivity, poor diet, or obesity.
Who Is Most at Risk
Several groups are more likely to develop deficiency. People who spend most of their time indoors, cover most of their skin, or live at northern latitudes (above roughly 37 degrees, which includes most of the northern United States and all of Canada and the UK) produce significantly less vitamin D from sunlight during fall and winter months. Older adults produce vitamin D less efficiently in the skin even with the same sun exposure. People with obesity also tend to have lower circulating levels because vitamin D gets sequestered in fat tissue.
Darker skin does reduce the rate of vitamin D production from UV exposure, though recent research in the Journal of Investigative Dermatology found the effect is smaller than previously assumed: people with the darkest skin tones need roughly 30% to 40% more UV exposure to produce equivalent vitamin D compared to the lightest skin tones. That’s a real difference, but far less dramatic than older estimates suggested. The gap becomes clinically meaningful mainly during winter or for people with limited outdoor time.
What Healthy Levels Look Like
A simple blood test measuring 25-hydroxyvitamin D tells you where you stand. The National Academies of Sciences defines the key thresholds:
- Below 12 ng/mL: Deficient. Associated with rickets in children and osteomalacia in adults.
- 12 to 20 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially harmful, with toxicity risk increasing above 60 ng/mL.
Vitamin D toxicity is rare but real. It doesn’t happen from sun exposure or food, only from excessive supplementation. Toxic levels, typically above 150 ng/mL, cause dangerously high blood calcium, which can damage the kidneys, heart, and blood vessels. This is why more is not always better with vitamin D supplements, and why testing matters before taking high doses long term.
Addressing a Deficiency
Your body makes vitamin D when UVB rays from sunlight hit bare skin. For many people, 10 to 30 minutes of midday sun exposure on the face, arms, and legs a few times per week is enough to maintain adequate levels during summer. In winter, at higher latitudes, or for people who can’t get regular sun, food and supplements fill the gap.
Fatty fish like salmon and mackerel are the richest natural food sources. Fortified milk, orange juice, and cereals provide smaller amounts. For supplementation, the recommended daily amount is 600 IU for adults up to age 70 and 800 IU for adults over 70, though many clinicians recommend higher doses for people with confirmed deficiency. If your blood test shows deficiency, your provider will typically recommend a higher loading dose for several weeks before moving to a maintenance dose.
Vitamin D is fat-soluble, so taking supplements with a meal that includes some fat improves absorption. And because it accumulates in the body rather than being flushed out daily like water-soluble vitamins, consistency matters more than timing. A daily or even weekly dose works better than sporadic large doses.