Having low vitamin D means your blood levels of this nutrient have dropped below what your body needs to properly absorb calcium, maintain strong bones, and support muscle and immune function. A simple blood test measures your level, and below 20 ng/mL is generally considered insufficient, while below 12 ng/mL is classified as deficient. Low vitamin D is common, often produces vague symptoms that are easy to overlook, and is usually straightforward to correct.
What the Numbers on Your Blood Test Mean
The standard test measures a form called 25-hydroxyvitamin D, which is the most accurate snapshot of how much vitamin D your body has available. Your results will typically fall into one of four categories:
- Below 12 ng/mL: Deficient. At this level, your risk of bone disease and other health problems rises significantly.
- 12 to 20 ng/mL: Insufficient. Your bones and overall health may be affected even if you feel fine.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially too high, with risk of adverse effects increasing above 60 ng/mL.
These ranges come from the National Academies of Sciences, Engineering, and Medicine. Some labs and professional societies use slightly different cutoffs, which is why you may see 30 ng/mL listed as the threshold for “optimal” on certain lab reports. The key takeaway: the lower your number, the more urgently your body needs vitamin D replenished.
Why Vitamin D Matters in Your Body
Vitamin D’s most fundamental job is helping your intestines absorb calcium from food. Without enough of it, your body absorbs only a fraction of the calcium you eat, no matter how much dairy or calcium-rich food you consume. When calcium absorption drops, your body compensates by pulling calcium from your bones to keep blood calcium levels stable. Over time, this weakens your skeleton.
Your body actually manufactures most of its vitamin D in the skin when ultraviolet rays from sunlight trigger a chemical reaction. The liver and kidneys then convert that raw material into the active form your cells can use. This is why vitamin D is sometimes called a hormone rather than a traditional vitamin: your body produces it, and it acts on tissues throughout the body, not just bones.
How Low Vitamin D Feels
Many people with low vitamin D have no obvious symptoms at all, which is part of what makes it tricky. When symptoms do appear, they tend to be nonspecific, meaning they overlap with dozens of other conditions. The most commonly reported signs in adults include fatigue, bone pain (especially in the back, hips, or legs), muscle weakness or cramps, and mood changes like persistent low mood or depression.
Children tend to show more dramatic effects. Severe deficiency causes rickets, a condition where bones become soft and bend during growth, leading to bowed legs, joint deformities, and muscle weakness. In milder cases, children may simply have sore or weak muscles without any visible bone changes.
In adults, the bone equivalent of rickets is called osteomalacia, where bones soften and ache. This is distinct from osteoporosis, which makes bones brittle and porous. Prolonged vitamin D deficiency can contribute to both.
Who Is Most Likely to Be Low
Certain groups face a much higher risk of deficiency because of how vitamin D is made, absorbed, or processed in the body:
- People with darker skin: Higher melanin levels reduce the skin’s ability to produce vitamin D from sunlight.
- Older adults: Aging skin produces vitamin D less efficiently, and aging kidneys are less able to convert it to its active form.
- People with obesity: Body fat binds to vitamin D and prevents it from circulating in the blood, effectively trapping it.
- People with digestive conditions: Crohn’s disease, ulcerative colitis, and celiac disease can all impair nutrient absorption in the gut.
- People who have had gastric bypass surgery: The procedure bypasses the section of the small intestine where vitamin D is absorbed.
- People with chronic kidney or liver disease: Both organs play a role in activating vitamin D, so disease in either one can leave you functionally deficient even if your intake seems adequate.
- Breastfed infants: Breast milk contains very little vitamin D, which is why supplements are recommended for nursing babies.
- People taking certain medications: Some cholesterol drugs, anti-seizure medications, steroids, and weight-loss drugs interfere with vitamin D metabolism.
Geography matters too. If you live at a northern latitude, spend most of your time indoors, or consistently wear sunscreen (which blocks the UV rays needed for production), your skin may produce very little vitamin D for months at a time.
What Happens If It Stays Low
Short-term, low vitamin D may only cause fatigue or vague aches. But when levels remain low for months or years, the consequences become more serious. The most direct risk is to your bones. Without adequate calcium absorption, bone density gradually declines, raising your risk of fractures, particularly in the hip, spine, and wrist. In older adults, this combination of weak bones and muscle weakness also increases the risk of falls.
There is also growing evidence linking chronic low vitamin D to other health concerns. Adults with prediabetes who supplement with vitamin D may reduce their risk of progressing to type 2 diabetes. In pregnant women, low vitamin D has been associated with higher rates of preeclampsia, preterm birth, and low birth weight. In children, adequate vitamin D levels appear to lower the frequency of respiratory tract infections.
How Low Vitamin D Is Corrected
Treatment depends on how low your levels are and what’s causing the deficiency. For people with confirmed deficiency, a common approach is a high-dose prescription of 50,000 IU taken once a week for eight weeks, followed by a maintenance dose. Some clinicians recommend a daily dose of around 6,000 IU as an alternative during the correction phase. People with obesity or absorption problems often need two to three times the standard dose to reach adequate levels.
For general prevention, the picture is more nuanced than “everyone should take vitamin D.” The Endocrine Society’s 2024 guidelines suggest routine supplementation for a few specific groups: children and adolescents (to prevent rickets and reduce respiratory infections), adults 75 and older (because it may lower mortality risk), pregnant women, and adults with prediabetes. For healthy adults under 75, the guidelines actually recommend against routine supplementation or routine blood testing, suggesting that most people in this age range get enough from a combination of sunlight and diet.
Food sources of vitamin D include fatty fish like salmon and mackerel, fortified milk and orange juice, egg yolks, and fortified cereals. However, diet alone rarely provides enough to correct a true deficiency, which is why supplementation or increased sun exposure is usually necessary when levels are low. Even 10 to 15 minutes of midday sun on bare arms and legs a few times a week can make a meaningful difference for people with lighter skin, though the exact amount varies by latitude, season, and skin tone.
After a treatment course, most providers will recheck your blood level to confirm it has risen above the sufficient range and then recommend an ongoing maintenance dose to keep it there.