Low vitamin D weakens your bones, drains your energy, and can quietly raise your risk for several serious health problems. Many people with a deficiency don’t realize it because the symptoms develop gradually and mimic everyday tiredness or general aches. A blood level below 20 ng/mL is considered deficient, while levels between 20 and 30 ng/mL fall into an “insufficient” range. The goal is typically 30 ng/mL or above.
The Symptoms You’re Most Likely to Notice
Vitamin D deficiency in adults tends to be subtle rather than dramatic. The most common signs are fatigue, bone pain, muscle weakness or cramping, and mood changes like depression. You might notice that your legs feel heavy climbing stairs, or that getting out of a low chair takes more effort than it used to. Some people describe a persistent achiness in their back, hips, ribs, or pelvis that doesn’t seem tied to any injury.
In children, the effects are more visible. Severe deficiency causes rickets, a condition where bones become so soft they bow outward at the legs. Children with milder deficiency may just have sore, weak muscles and grow more slowly than expected. Their teeth may also come in later than usual.
What It Does to Your Bones Over Time
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 dense and hard. In adults, this leads to a condition called osteomalacia, where bones gradually soften. The result is pain that tends to concentrate in weight-bearing areas: your lower back, pelvis, hips, and feet. You may also fracture bones more easily, sometimes from something as minor as a stumble or a twist.
Osteomalacia is different from osteoporosis, though they can overlap. Osteoporosis means bones lose density and become brittle. Osteomalacia means they lose hardness and become flexible. Both make fractures more likely, and prolonged vitamin D deficiency can contribute to both. Tingling, cramping, or twitching in your muscles can also develop when calcium levels drop low enough, since your muscles depend on calcium to contract properly.
The Link to Depression and Mood
People with low vitamin D have roughly 60% higher odds of experiencing depression compared to those with adequate levels. That connection goes beyond correlation. A large meta-analysis of 20 randomized controlled trials found that vitamin D supplementation significantly reduced depressive symptoms, particularly at doses above 2,800 IU per day taken for eight weeks or longer.
The mechanism appears to involve inflammation and brain health. Vitamin D helps reduce inflammatory signals in the body and supports the production of proteins that protect nerve cells and promote healthy connections between them. It also plays a role in the kind of depression tied to cardiovascular problems, where improved blood vessel function may be part of the benefit. If you’ve noticed your mood consistently dipping during winter months, low vitamin D from reduced sun exposure could be a contributing factor.
Effects on Your Immune System
Vitamin D helps regulate the immune cells that detect and respond to infections. When levels are low, your body has a harder time mounting an effective defense against respiratory infections in particular. This connection was first noticed historically in children with rickets, who were significantly more prone to respiratory illness. More recent research confirms that deficiency increases susceptibility to infections of the lungs and airways.
The vitamin influences how your immune system balances its response, helping it fight pathogens without overreacting. When vitamin D is insufficient, that balance tips, which may partly explain why people who are deficient tend to get sick more often during cold and flu season.
Heart and Metabolic Risks
Low vitamin D is associated with stiffer arteries and higher blood pressure. The vitamin helps keep the lining of your blood vessels flexible and healthy. When levels drop, arteries can become rigid, making it harder for blood to flow and increasing the workload on your heart. Deficiency also appears to increase the likelihood of developing type 2 diabetes.
Together, these effects raise the overall risk of heart disease. However, there’s an important nuance: while people with very low levels seem to benefit from supplementation, it’s not yet clear that taking vitamin D supplements prevents heart attacks or strokes in people whose levels are only mildly low. The cardiovascular risk appears most relevant for people with significant, sustained deficiency.
Who’s Most Likely to Be Deficient
Your skin produces vitamin D when exposed to sunlight, so anything that limits that process raises your risk. People with darker skin need substantially more sun exposure to produce the same amount of vitamin D because melanin, the pigment that gives skin its color, acts as a natural sunblock against the UV rays that trigger vitamin D production. This is especially relevant for dark-skinned individuals living far from the equator, above 37°N latitude (roughly the line from San Francisco to Richmond, Virginia) or below 37°S.
Other major risk factors include:
- Age: Older adults produce vitamin D less efficiently through their skin and often spend less time outdoors.
- Limited sun exposure: Working indoors, wearing clothing that covers most skin, or living in northern climates during winter all reduce production.
- Obesity: Vitamin D is fat-soluble, and excess body fat can sequester it, keeping it out of circulation.
- Certain digestive conditions: Anything that impairs fat absorption (like celiac disease or Crohn’s disease) can reduce how much vitamin D you absorb from food.
How Much Vitamin D You Need
The recommended daily intake is 400 IU for infants up to 12 months, 600 IU for anyone ages 1 to 70, and 800 IU for adults over 70. These amounts are designed to maintain adequate levels in most people, but if you’re already deficient, your doctor will likely recommend a higher dose temporarily to bring your levels back up, then a maintenance dose afterward.
A simple blood test measuring your 25-hydroxyvitamin D level is the standard way to check. If your result comes back below 20 ng/mL, you’re deficient. Between 20 and 30 ng/mL is insufficient, and 30 ng/mL or above is the target.
A Note on Taking Too Much
Vitamin D toxicity is rare and generally only occurs at doses above 10,000 IU per day sustained over time. When it does happen, it causes dangerously high calcium levels in the blood, leading to symptoms like excessive thirst, frequent urination, nausea, confusion, and fatigue. You cannot get too much vitamin D from sunlight because your skin self-regulates production, but you can from supplements. Sticking to recommended doses or a level prescribed based on your blood work keeps you in safe territory.