Low vitamin D means your body doesn’t have enough of the nutrient it needs to absorb calcium, maintain bone strength, and support your immune system. On a blood test, a level below 20 ng/mL is considered inadequate for overall health, and below 12 ng/mL is classified as deficient. If your results came back low, it’s a common finding, and understanding what’s actually happening in your body can help you figure out what to do next.
What the Numbers on Your Test Mean
The standard blood test measures a form of vitamin D called 25-hydroxyvitamin D, which reflects how much vitamin D your body has stored. It’s reported in nanograms per milliliter (ng/mL). Here’s how the ranges break down:
- Below 12 ng/mL: Deficient. At this level, you’re at risk for bone-softening conditions like rickets in children or osteomalacia in adults.
- 12 to 19 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially harmful, especially above 60 ng/mL.
If your doctor ordered a different test measuring the “active” form of vitamin D (1,25-dihydroxyvitamin D), that result can actually be misleading. During a deficiency, levels of this active form often go up, not down, because your body compensates by producing more of it. The 25-hydroxyvitamin D test is the correct one for checking your vitamin D stores.
What Low Vitamin D Does to Your Body
Vitamin D’s most critical job is helping your gut absorb calcium from food. Without enough of it, your intestines absorb a fraction of the calcium you eat, and your body starts pulling calcium from your bones to keep blood calcium levels stable. Over time, this weakens your skeleton.
There are two distinct bone problems that can develop. Osteomalacia is a softening of existing bone caused by poor mineralization, meaning your bones have the right framework but not enough mineral packed into them. This shows up as widespread bone pain, tenderness, and unusual fractures, particularly in the ribs. It’s directly caused by prolonged, severe vitamin D deficiency. Osteoporosis is different: the bones lose mass overall but maintain a normal mineral-to-framework ratio. Low vitamin D can contribute to osteoporosis, but it’s not the sole cause.
Beyond bones, vitamin D plays a role in immune function. It helps strengthen your immune response while also preventing it from overreacting during infections. Studies on children receiving vitamin D supplements during winter months have shown a reduced risk of influenza A, which suggests that adequate levels help your body fight off respiratory infections more effectively.
Symptoms You Might Notice
Many people with low vitamin D have no obvious symptoms at all, which is why it often shows up unexpectedly on a blood test. When symptoms do appear, they tend to be vague enough that you might attribute them to stress, aging, or poor sleep. The most commonly reported signs include fatigue, bone pain, muscle weakness or cramps, and mood changes like irritability or feeling persistently down.
The connection to mood is real but nuanced. People with depression are more likely to have low vitamin D levels, and there’s a recognized relationship between the two. But the evidence isn’t strong enough to say that low vitamin D directly causes depression, and bringing levels up doesn’t reliably lift mood in everyone. It’s one piece of a larger puzzle.
In children, a severe deficiency can cause rickets, which leads to bowed legs, delayed growth, and bone deformities. Milder deficiency in kids may only show up as weak or sore muscles.
Why Your Levels Dropped
Your body makes vitamin D when ultraviolet B rays from sunlight hit your skin. Sufficient production requires exposing your face, hands, and arms to the sun (without sunscreen) for roughly half the time it would take your skin to start burning, two to three times a week. For someone with fair skin in strong summer sun, that’s about 12 minutes. But several factors make this harder than it sounds.
Geography matters enormously. If you live at a higher latitude, UV-B radiation from October through March is too weak to trigger meaningful vitamin D production, no matter how long you’re outside. Anyone living north of roughly the 37th parallel (think San Francisco or Richmond, Virginia) faces a months-long gap each year when sun exposure alone can’t maintain adequate levels.
Darker skin pigmentation reduces vitamin D synthesis because melanin, the pigment that gives skin its color, absorbs UV-B radiation before it can be used to produce the vitamin. People with darker skin need significantly more sun exposure to produce the same amount as someone with lighter skin.
Body weight plays a role too. Fat tissue absorbs and holds onto vitamin D, effectively pulling it out of circulation. This “trapping” effect means that people with obesity often have lower blood levels even when their total body stores of vitamin D are comparable to those of people at a lower weight. It also means they typically need higher doses of supplementation to bring their blood levels up to the same range.
Age is another factor. As you get older, your skin becomes less efficient at producing vitamin D, and your kidneys become less effective at converting it to its active form. This is why the recommended daily intake increases from 600 IU to 800 IU after age 70.
How Much Vitamin D You Need
The recommended daily intake for most adults aged 19 to 70 is 600 IU (15 micrograms). Adults over 70 should aim for 800 IU (20 micrograms). The tolerable upper limit for adults is 4,000 IU per day from all sources combined, including food, fortified beverages, and supplements. Going above this consistently raises the risk of toxicity, which can cause dangerously high calcium levels.
The Endocrine Society’s most recent guidelines suggest that adults 75 and older take vitamin D daily because of a potential benefit in lowering mortality risk. For this age group, the recommendation favors daily, lower-dose supplementation over large weekly or monthly doses. For healthy adults under 75, the guidelines don’t recommend routine supplementation beyond the standard daily allowance or routine blood testing. The reasoning is that for most healthy people, the standard intake through diet and moderate sun exposure is enough.
If your test already shows you’re low, your situation is different from the general population these guidelines address. Your doctor will likely recommend a higher dose for a period of weeks or months to bring your levels back up, then transition to a maintenance dose.
Raising Your Levels
Vitamin D3 (the form your skin produces naturally) is the most effective supplement form. Food sources include fatty fish like salmon and mackerel, fortified milk and orange juice, egg yolks, and fortified cereals, but it’s difficult to get enough from diet alone. A 3-ounce serving of salmon provides roughly 400 to 600 IU, which is close to the full daily recommendation, but most other foods contribute far less.
Sun exposure remains the most efficient natural source. Aiming for brief, regular exposure of your arms and face during midday hours in the warmer months can maintain healthy levels for many people. In winter months or northern climates, supplementation becomes more important since the sun simply can’t do the job.
If you’ve been told your levels are low, a retest after 8 to 12 weeks of supplementation is typical to check whether your levels have responded. Some people, particularly those with obesity, malabsorption conditions, or very low starting levels, may need higher doses or longer treatment periods to reach the adequate range.