A low vitamin D level is generally defined as a blood concentration below 20 ng/mL (50 nmol/L). Levels between 20 and 30 ng/mL are considered insufficient, meaning your body has enough to function but not enough for optimal health. Above 30 ng/mL is where calcium absorption and bone metabolism work best.
How Vitamin D Levels Are Measured
The standard test is called a 25-hydroxy vitamin D test, and it’s a simple blood draw. This test captures both the vitamin D your body produces from sun exposure and the vitamin D you absorb from food or supplements, giving a single total number that reflects your overall status. It’s considered the most accurate way to measure how much vitamin D is in your body.
Your result will typically come back in nanograms per milliliter (ng/mL). Here’s how to read it:
- Below 20 ng/mL: Deficient
- 20 to 30 ng/mL: Insufficient
- Above 30 ng/mL: Sufficient
It’s worth noting that the Endocrine Society’s 2024 guidelines recommend against routine vitamin D screening for healthy adults of any age, including those with darker skin or obesity. Testing is most useful when there’s a specific reason to suspect deficiency, such as unexplained bone pain, a history of fractures, or a condition that impairs nutrient absorption.
What Low Vitamin D Feels Like
Mild deficiency often produces no obvious symptoms at all, which is part of why it can go undetected for years. When symptoms do appear, they tend to be vague enough to blame on other things: persistent fatigue, general muscle weakness, or aching bones that don’t have a clear cause. Some people notice they get sick more frequently, since vitamin D plays a role in immune function.
In more severe or prolonged deficiency, the symptoms become harder to ignore. Adults can develop a condition called osteomalacia, where bones soften and weaken, causing deep bone pain (often in the legs, pelvis, and lower back) along with noticeable muscle weakness. In children, severe deficiency causes rickets, a rare but serious condition where bones become so soft they bend and deform. Loss of bone density is one of the most significant consequences, contributing to osteoporosis and a higher risk of fractures over time.
Why Levels Drop
Your body manufactures vitamin D when ultraviolet B (UVB) rays from sunlight hit your skin. Anything that reduces that exposure can lower your levels. Geography is a major factor: the farther you live from the equator, the fewer months of the year your skin can actually produce vitamin D, because sunlight travels through more atmosphere and loses its UVB component. During winter at higher latitudes, it’s possible to get zero vitamin D from sun exposure regardless of how much time you spend outside.
Skin pigmentation matters too. Melanin, the pigment that gives skin its color, acts as a natural sunscreen and slows vitamin D production. People with darker skin can still produce adequate vitamin D with regular sun exposure, but in regions with limited UVB radiation, they’re at a practical disadvantage. Spending most of the day indoors, wearing clothing that covers most skin, and consistent sunscreen use all reduce production as well.
Diet is another piece. Few foods naturally contain meaningful amounts of vitamin D. Fatty fish, egg yolks, and beef liver have some, and many dairy products and cereals are fortified with it, but it’s difficult to reach adequate levels through food alone. Conditions that impair fat absorption, such as celiac disease, Crohn’s disease, or gastric bypass surgery, can also interfere with your ability to absorb vitamin D from food and supplements.
Long-Term Risks of Staying Low
Left untreated, chronic vitamin D deficiency takes a real toll on your skeleton. The most direct consequences are osteomalacia in adults and rickets in children, both characterized by bones that soften, weaken, and become prone to fractures and deformity. Even before reaching that point, low levels contribute to gradual bone density loss, raising the risk of osteoporosis, particularly in older adults who are already losing bone mass naturally.
Beyond bones, research has suggested that vitamin D plays a protective role against high blood pressure, certain cancers, and some autoimmune diseases. These connections are still being clarified, but they add weight to the idea that maintaining adequate levels matters for more than just skeletal health.
Correcting a Deficiency
The recommended daily intake for most adults is 600 to 800 IU (15 to 20 micrograms), an amount designed to maintain bone health and normal calcium metabolism. This is a maintenance dose for people whose levels are already adequate, not necessarily enough to correct a significant deficiency. If your levels are very low, your provider may recommend a higher dose for a set period to bring your numbers up before transitioning to a standard daily amount.
Vitamin D supplements come in two forms. D3 is the type your body makes from sunlight and is also found in animal-based foods. D2 comes from plant sources and fortified foods. Both work similarly in the body. Over-the-counter supplements are widely available, inexpensive, and effective for most people.
Getting more sunlight helps too, though the amount needed varies dramatically based on where you live, the season, your skin tone, and how much skin is exposed. In practical terms, supplementation is the most reliable way to raise and maintain your levels, especially if you live in a northern climate or spend most of your time indoors.
Too Much Vitamin D Is Also a Problem
Toxicity from vitamin D is rare and generally only occurs at doses above 10,000 IU per day taken over time. You cannot overdose from sun exposure, because your skin self-regulates production. The danger comes from supplements.
Excess vitamin D causes calcium to build up in your blood, a condition called hypercalcemia. Symptoms include nausea, vomiting, excessive thirst, frequent urination, constipation, fatigue, confusion, and muscle weakness. In severe cases it can lead to high blood pressure and kidney damage. Sticking to recommended supplement doses eliminates this risk for the vast majority of people.