Low vitamin D weakens your bones, drains your energy, and quietly undermines your immune system and mood. A blood level below 12 ng/mL is classified as deficient, while levels between 12 and 20 ng/mL are considered inadequate for overall health. Because symptoms develop gradually and overlap with so many other conditions, many people live with low vitamin D for months or years without realizing it.
Common Symptoms in Adults
The signs of low vitamin D are frustratingly vague. Fatigue is the most frequently reported symptom, and it’s the kind that doesn’t improve much with extra sleep. Bone pain, particularly in the lower back, hips, and legs, is another hallmark. You may also notice muscle weakness, aching, or cramps that seem disproportionate to your activity level. Mood changes are common too, especially persistent low mood or a general sense of feeling “off.”
What makes this tricky is that none of these symptoms scream vitamin D on their own. Fatigue could be poor sleep. Bone pain could be aging. Depression could be situational. That’s why deficiency often goes undetected until a blood test picks it up or until complications become more serious.
What Happens to Your Bones
Vitamin D’s primary job is helping your body absorb calcium from food. Without enough of it, your bones don’t get the minerals they need to stay hard and strong. In adults, this leads to a condition called osteomalacia, where bones become soft and prone to pain and unusual fractures, especially in the ribs. The ratio of mineral to bone tissue drops, meaning there’s physically less calcium packed into each unit of bone. This is distinct from osteoporosis, where the total amount of bone shrinks but the remaining bone is still properly mineralized.
The practical difference matters. Osteomalacia causes a deep, aching bone pain and tenderness when you press on your shins or sternum. It also raises your risk for fractures in places that don’t typically break, like the ribs or pelvis. Osteoporosis, by contrast, is often silent until a fracture happens. Both conditions can coexist, and prolonged vitamin D deficiency contributes to both.
Effects on Children
In infants and children, severe vitamin D deficiency causes rickets, a condition where growing bones soften and bend under the body’s weight. The most recognizable sign is bowed legs, but rickets can also cause widening of the knees and ankles in children who walk, or swollen wrists in infants who crawl. The ends of the ribs can swell visibly, the spine may curve abnormally, and the skull can develop an unusual shape.
Growth delays are common. Children with even mild deficiency may have sore, weak muscles without any visible bone changes. Because vitamin D is critical during periods of rapid skeletal growth, pediatricians typically recommend supplementation for breastfed infants, since breast milk alone doesn’t provide enough.
Immune System and Infection Risk
Vitamin D plays a hands-on role in how your immune system responds to threats. It helps activate T cells, the white blood cells that identify and destroy pathogens. It also acts as a thermostat for inflammation, dialing down the aggressive, tissue-damaging side of the immune response while boosting the regulatory cells that prevent your immune system from overreacting.
When vitamin D is low, this balance tips. Research shows that low blood levels are positively correlated with higher rates of upper respiratory tract infections, including the flu. One analysis found that for every 10 nmol/L increase in blood vitamin D, infection risk dropped by 7%. Low levels may also reduce your body’s antiviral defenses by interfering with a key signaling pathway that helps cells fight off viruses in their earliest stages.
This immune connection also runs in the other direction. By promoting regulatory immune cells and suppressing overactive inflammatory responses, adequate vitamin D helps protect against autoimmune conditions where the body mistakenly attacks its own tissues.
Depression and Mood
People with depression tend to have lower vitamin D levels than the general population, and the link has been studied extensively. A large meta-analysis of supplementation trials found that vitamin D produced a statistically significant reduction in depression symptom scores in adults with primary depression.
There’s an important nuance, though. The benefit appeared only in people whose blood levels were already at or above 20 ng/mL (50 nmol/L). For those with the most severe deficiency, supplementation alone didn’t meaningfully improve depressive symptoms. This suggests that vitamin D plays a supporting role in mood regulation rather than acting as a standalone treatment, and that relatively adequate baseline levels may be needed before supplementation makes a noticeable difference.
Muscle Weakness and Fall Risk
Low vitamin D doesn’t just make muscles sore. It weakens them, particularly the large muscle groups in your hips and thighs. This proximal weakness shows up as difficulty standing from a chair, climbing stairs, or maintaining your balance. For older adults, this translates directly into a higher risk of falls, which in combination with weakened bones creates a dangerous setup for fractures.
The mechanism is straightforward. When vitamin D drops low enough, blood calcium levels fall too. Your body compensates by pulling calcium from your bones and ramping up parathyroid hormone production. Both the low calcium and the elevated parathyroid hormone contribute to muscle cramps, weakness, and fatigue. It’s a cascade where one deficiency creates multiple overlapping problems.
Understanding Your Blood Levels
Vitamin D status is measured through a simple blood test that checks your level of 25-hydroxyvitamin D. The NIH uses these 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 and above: Generally adequate for most people.
- Above 50 ng/mL: Potentially harmful, particularly above 60 ng/mL.
If you’re experiencing persistent fatigue, unexplained bone or muscle pain, or frequent infections, asking for this blood test is a reasonable first step. It’s inexpensive, widely available, and gives you a clear number to work with rather than guessing based on symptoms alone.
Who Is Most at Risk
Several groups are more likely to run low. People with darker skin produce less vitamin D from sunlight because melanin reduces UV absorption. Those who live at northern latitudes, spend most of their time indoors, or consistently wear sunscreen get less UV exposure. Older adults are at higher risk because the skin becomes less efficient at synthesizing vitamin D with age, and kidney function (which is needed to convert vitamin D to its active form) declines over time.
People with conditions that impair fat absorption, such as celiac disease, Crohn’s disease, or a history of gastric bypass surgery, may struggle to absorb vitamin D from food or supplements since it’s a fat-soluble vitamin. Obesity also plays a role: vitamin D gets sequestered in fat tissue, making less of it available in the bloodstream.