Low vitamin D means your blood levels of this nutrient have dropped below what your body needs to properly absorb calcium and maintain healthy bones. A level below 20 ng/mL is classified as deficient, while 20 to 30 ng/mL is considered insufficient. The target is generally 30 ng/mL or above. If your blood work came back low, understanding why it matters and what to do about it is straightforward.
What the Numbers on Your Blood Test Mean
The standard test measures a form of vitamin D called 25-hydroxyvitamin D, which reflects how much vitamin D your body has stored from sunlight, food, and supplements combined. There’s a different form (the active hormone your kidneys produce), but that test isn’t useful for checking your overall status because its levels can appear normal even when your stores are depleted.
Here’s how the ranges break down:
- Below 20 ng/mL (50 nmol/L): Deficient. Your body doesn’t have enough to support normal calcium absorption and bone health.
- 20 to 30 ng/mL (50 to 75 nmol/L): Insufficient. You’re not critically low, but you’re below the level most guidelines consider optimal.
- 30 ng/mL and above (75 nmol/L): Sufficient. This is the minimum target for healthy bone metabolism.
If your result came in nmol/L instead of ng/mL, divide by 2.5 to convert. A reading of 50 nmol/L, for example, equals 20 ng/mL.
What Low Vitamin D Does Inside Your Body
Vitamin D’s primary job is helping your intestines absorb calcium from food. The active form of vitamin D is the main regulator of calcium transport across the gut wall. Without enough of it, you absorb significantly less calcium from whatever you eat, no matter how much dairy or calcium-rich food is in your diet.
When your body can’t pull enough calcium from food or retain enough through the kidneys, it turns to the only backup supply available: your bones. Your body actually breaks down bone tissue to release stored calcium into the bloodstream, keeping blood calcium levels stable at the expense of bone strength. Over time, this means less calcium gets incorporated into bone and overall bone quality declines.
In adults, prolonged and severe deficiency leads to a condition called osteomalacia, where bones become soft and weak. This causes bone pain (often in the lower back, hips, and legs), muscle weakness, and difficulty with movements like climbing stairs or getting up from a chair. In children, the same process causes rickets, which can visibly deform growing bones.
Symptoms You Might Notice
Low vitamin D is tricky because mild deficiency often causes no obvious symptoms at all. Many people only discover it through a blood test ordered for another reason. When symptoms do appear, they tend to develop gradually and are easy to attribute to other causes.
The most well-established symptoms are bone pain and muscle weakness. The bone pain is typically a dull ache rather than sharp, and it may feel worse when you press on your shinbone or breastbone. Muscle weakness tends to affect the larger muscles closest to the trunk of your body, making it harder to stand from a seated position or walk up stairs. Fatigue is commonly reported alongside low levels, though the connection is less precisely defined than the bone and muscle effects.
Why Your Levels Might Be Low
Your skin produces vitamin D when exposed to ultraviolet B rays from the sun, and this is the body’s primary source. Several factors can reduce or block that production:
Limited sun exposure is the most common culprit. If you live at a northern latitude, work indoors, wear covering clothing, or consistently use sunscreen, your skin produces less vitamin D. Season, time of day, cloud cover, and length of daylight all affect how much UV radiation actually reaches your skin. During winter months at higher latitudes, UV intensity drops so low that vitamin D production essentially stops.
Darker skin contains more melanin, which acts as a natural sunscreen. This reduces the skin’s ability to produce vitamin D from the same amount of sun exposure, meaning people with darker complexions need more time in sunlight to generate the same amount.
Age plays a significant role. The skin’s ability to synthesize vitamin D declines as you get older, putting adults over 65 at higher risk even with adequate sun exposure.
Digestive conditions can interfere with absorption. Because vitamin D is fat-soluble, your gut needs to be able to absorb dietary fat for vitamin D to come along with it. Conditions like celiac disease, Crohn’s disease, ulcerative colitis, cystic fibrosis, and some forms of liver disease all impair fat absorption and can lead to deficiency regardless of intake.
Obesity is another risk factor. Vitamin D gets sequestered in fat tissue, which means more of it is locked away and unavailable for use in the bloodstream.
How Low Vitamin D Gets Corrected
Treatment depends on how low your levels are. For moderate deficiency (levels between 8 and 24 ng/mL), studies show that daily doses in the range of 1,160 to 2,200 IU of vitamin D3, taken for about eight weeks, can bring levels above 30 ng/mL in an average-weight adult. The lower your starting level, the higher the dose needed. Someone starting at 10 ng/mL needs roughly 2,200 IU daily, while someone at 25 ng/mL may only need around 1,160 IU daily to reach the target.
For more severe deficiency, a higher-dose weekly regimen is sometimes used: 50,000 IU once a week for eight weeks, followed by a lower maintenance dose. This approach brings levels up faster and can be easier to stick with since it’s only one dose per week. After levels normalize, a daily maintenance dose prevents them from dropping again.
Vitamin D3 (cholecalciferol) is generally preferred over D2 (ergocalciferol) because it raises blood levels more effectively per unit dose. Since vitamin D is fat-soluble, taking supplements with a meal that contains some fat improves absorption.
Getting Vitamin D From Food and Sunlight
Very few foods naturally contain significant vitamin D. Fatty fish like salmon, mackerel, and sardines are the best natural sources. Cod liver oil is extremely concentrated. Egg yolks contain small amounts, and some mushrooms provide vitamin D2 when exposed to UV light. In many countries, milk, orange juice, and cereals are fortified with vitamin D, which helps but rarely provides enough on its own to correct a deficiency.
Sunlight remains the most efficient source. Exposing your arms and legs to midday sun for 10 to 30 minutes several times a week can produce substantial amounts, though the exact time needed varies enormously based on your skin tone, latitude, and season. This isn’t realistic for everyone, which is why supplementation is the practical solution for most people with low levels.
Do You Need Routine Testing?
The Endocrine Society’s most recent guidelines recommend against routine vitamin D screening for the general population, including healthy adults under 75, pregnant women, people with darker skin, and people with obesity. This doesn’t mean testing is never useful. It means that for most healthy people, the evidence doesn’t support checking levels just to guide whether to take a supplement. The guidelines note that the specific blood level needed for benefits beyond bone health hasn’t been clearly established in clinical trials.
Testing makes more sense when there’s a clinical reason for it: unexplained bone pain, a known malabsorption condition, osteoporosis, or abnormal calcium levels. If you already have a result showing low levels, the path forward is supplementation to bring your levels into the sufficient range, with a recheck after the treatment course to confirm it worked.