Vitamin D deficiency can cause a surprisingly wide range of problems, from brittle bones and muscle weakness to a higher risk of infections and mood changes. Because vitamin D acts more like a hormone than a typical vitamin, influencing hundreds of processes throughout the body, running low affects far more than just your skeleton. Blood levels below 12 ng/mL are classified as deficient by the National Academies of Sciences, while levels between 12 and 20 ng/mL are considered inadequate for overall health.
Weakened Bones and Skeletal Damage
The most well-established consequence of vitamin D deficiency is bone disease. Vitamin D controls how much calcium your gut absorbs from food. When calcium intake is low, vitamin D ramps up a specialized absorption pathway in the intestines, boosting the maximum amount of calcium your body can pull in. Studies show that people eating a low-calcium diet naturally increase their absorption efficiency by roughly 43% compared to those eating a high-calcium diet, but that process depends on having enough vitamin D to drive it.
Without adequate vitamin D, your body simply cannot absorb enough calcium to keep bones strong, no matter how much dairy or calcium-rich food you eat. In children, this leads to rickets, a condition where bones become soft and bend under the body’s weight, causing bowed legs, delayed growth, and skeletal deformities. In adults, the equivalent condition is called osteomalacia: bones gradually lose mineral density, becoming painful and prone to fractures. Even moderate insufficiency over years contributes to osteoporosis, where bones thin silently until a minor fall causes a serious break.
Muscle Weakness and Fall Risk
Vitamin D plays a direct role in muscle repair and contraction, so deficiency doesn’t just weaken your bones. It weakens the muscles that protect them. A study tracking older adults over four years found that people with deficient vitamin D levels (below 30 nmol/L) were 70% more likely to develop clinically significant muscle weakness than those with normal levels. That muscle weakness, called dynapenia, shows up as difficulty rising from a chair, climbing stairs, or gripping objects firmly.
For older adults especially, this combination of fragile bones and weak muscles is dangerous. It increases both the likelihood of falling and the chance that a fall results in a fracture. The muscle effects often appear gradually, so many people attribute the weakness to aging itself rather than a correctable deficiency.
More Frequent Respiratory Infections
Vitamin D helps activate key immune cells that fight off bacteria and viruses, particularly in the respiratory tract. Large analyses pooling data from multiple trials have found that people who supplement with vitamin D experience a 12% lower risk of developing at least one acute respiratory tract infection compared to those who don’t. Some analyses focusing on people with the lowest starting levels show even larger reductions, with infection risk dropping by roughly 36 to 42%.
This means that if you seem to catch every cold and flu that goes around, low vitamin D could be a contributing factor. The effect is most pronounced in people who are already deficient. For those with adequate levels, adding extra vitamin D doesn’t appear to provide much additional immune protection.
Depression and Mood Changes
The link between vitamin D and mental health has been studied extensively. A meta-analysis of 18 trials found that vitamin D supplementation produced a modest but statistically significant reduction in depression symptoms. Interestingly, the benefit was strongest in people whose blood levels were already above 50 nmol/L (20 ng/mL), suggesting the relationship between vitamin D and mood isn’t a simple “more is better” equation.
For people with levels below that threshold, supplementation alone didn’t significantly improve depressive symptoms in the studies reviewed. This doesn’t mean vitamin D is irrelevant to mood at low levels, but rather that severe deficiency likely involves multiple overlapping problems that a single supplement can’t fully resolve. Still, persistently low vitamin D is consistently associated with higher rates of depression in population studies, and correcting a deficiency is a reasonable piece of any broader approach to mood support.
Cardiovascular and Metabolic Effects
Low vitamin D levels are linked to stiffer arteries and a greater likelihood of high blood pressure and diabetes, though the exact mechanisms are still being studied. Vitamin D appears to help keep the lining of blood vessels flexible and healthy. When levels drop, arteries can become rigid, making it harder for blood to flow and increasing the workload on the heart.
The relationship between vitamin D and cardiovascular disease is real in population data, but it’s worth noting that supplementation trials haven’t consistently shown that raising vitamin D levels prevents heart attacks or strokes. The current understanding is that deficiency is a contributing risk factor, particularly when combined with other cardiovascular risks like inactivity, obesity, or poor diet.
Who Is Most Likely To Be Deficient
Your body manufactures vitamin D when ultraviolet B rays from sunlight hit your skin, which means several factors can dramatically reduce your natural production. Geography is one of the biggest: if you live at latitudes around 40 degrees north or south (think Boston, Madrid, or Beijing), there isn’t enough UVB radiation from November through early March to produce meaningful vitamin D. Move 10 degrees farther from the equator, to places like Edmonton or London, and that “vitamin D winter” stretches from October through April.
Skin pigmentation also matters significantly. Melanin, the pigment that gives darker skin its color, competes with the molecule that starts vitamin D production by absorbing the same UV wavelengths. People with dark skin may need up to ten times longer in the sun to produce the same amount of vitamin D as someone with fair skin. This helps explain why vitamin D deficiency rates are disproportionately high in Black and South Asian populations, even in sunnier climates.
Other high-risk groups include older adults (the skin’s ability to synthesize vitamin D declines with age), people who spend most of their time indoors, those who consistently wear sunscreen or covering clothing, and people with higher body fat (vitamin D is fat-soluble and gets sequestered in fat tissue, reducing the amount available in the bloodstream).
Understanding Your Vitamin D Levels
A simple blood test measuring 25-hydroxyvitamin D tells you where you stand. The NIH classifies the ranges as follows:
- Below 12 ng/mL (30 nmol/L): deficient, associated with rickets in children and osteomalacia in adults
- 12 to 19 ng/mL (30 to 49 nmol/L): inadequate for bone and overall health
- 20 ng/mL or above (50 nmol/L): adequate for most people
- Above 50 ng/mL (125 nmol/L): potentially harmful, particularly above 60 ng/mL
That upper limit is worth noting. Because vitamin D is fat-soluble, your body stores it rather than flushing out excess amounts the way it does with vitamin C. Very high levels can cause calcium to build up in the blood, leading to nausea, kidney problems, and other serious issues. This is essentially impossible from sun exposure alone but can happen with aggressive supplementation over time.