Vitamin D is essential for bone health because it controls how much calcium your body can actually absorb from food. Without enough vitamin D, you could eat plenty of calcium-rich foods and still not get enough into your bones. When vitamin D levels drop too low (below 12 ng/mL in a blood test), the result can be soft, weak bones that are prone to fractures, deformity, and pain.
How Vitamin D Gets Calcium Into Your Bones
Calcium doesn’t just pass through your gut wall on its own. Your intestinal cells need specific molecular machinery to pull calcium from digested food into your bloodstream, and vitamin D is what builds that machinery. When your body activates vitamin D (converting it to its active form in the kidneys), it triggers cells lining your small intestine to produce two key components: a channel on the cell surface that lets calcium in, and a ferry protein inside the cell that shuttles calcium across to the other side.
In vitamin D-deficient animals, levels of these transport proteins drop significantly. When vitamin D is restored, the gene for the calcium channel switches on first, and intestinal calcium absorption increases shortly after. Without this system running properly, your gut absorbs only a fraction of the calcium passing through it, no matter how much dairy or leafy greens you eat.
What Happens When Vitamin D Runs Low
When your body senses that not enough calcium is making it into the blood, it doesn’t just accept the shortage. Your parathyroid glands (four tiny glands in your neck) ramp up production of parathyroid hormone, which acts as an emergency signal. This hormone pulls calcium directly out of your bones and into your bloodstream to keep vital functions like heartbeat and nerve signaling running normally.
This rescue system keeps your blood calcium levels looking relatively normal on a lab test, which is why a calcium blood test alone won’t reveal the problem. But the cost is real: your bones are being quietly dismantled from the inside. The parathyroid hormone also causes your kidneys to dump phosphorus into your urine, and since bones need both calcium and phosphorus to stay hard, this double loss accelerates the damage. Doctors call this chain of events secondary hyperparathyroidism, and it’s one of the most common ways chronic vitamin D deficiency silently weakens the skeleton.
Rickets in Children, Osteomalacia in Adults
The bone diseases caused by vitamin D deficiency look different depending on age, because children’s and adults’ skeletons work differently.
In children, the growth plates at the ends of long bones are still active. These are the zones where cartilage gradually converts into hard bone, making a child taller over time. Vitamin D deficiency disrupts both the mineralization and the organization of cartilage at these growth plates. The result is rickets: bones that bend and bow under the child’s own weight, delayed growth, and in severe cases, visible deformity of the legs, wrists, or ribcage.
Adults no longer have active growth plates, so rickets isn’t possible. Instead, vitamin D deficiency causes osteomalacia, a condition where newly formed bone tissue fails to harden properly. The bone structure itself is still there, but it doesn’t calcify the way it should. People with osteomalacia often experience widespread bone pain, tenderness when pressing on the ribs or shins, and unusual fractures in places like the ribs that wouldn’t normally break.
Osteomalacia vs. Osteoporosis
These two conditions are easy to confuse because both involve weakened bones, but they’re fundamentally different problems. In osteoporosis, you’re losing bone mass overall, but whatever bone remains has a normal mineral-to-tissue ratio. Think of it as having less bone, but the bone you have is properly built. In osteomalacia, you may still have a normal amount of bone tissue, but the mineral content within it is too low. The bone is there, but it’s soft.
Vitamin D deficiency can contribute to both. It directly causes osteomalacia through failed mineralization, and it contributes to osteoporosis indirectly through the parathyroid hormone cycle described above, which increases bone breakdown over time. Osteomalacia should be suspected when someone has bone pain, unusual fractures, and consistently very low vitamin D levels. In practice, many older adults with fragile bones have elements of both conditions happening simultaneously.
Vitamin D, Muscle Strength, and Falls
Bone fractures in older adults usually start with a fall, and vitamin D plays a role here too. Vitamin D deficiency is associated with a lower proportion of fast-twitch muscle fibers, the type responsible for quick movements like catching yourself when you stumble. Active vitamin D increases expression of the proteins that build these fast-twitch fibers.
A meta-analysis published in Frontiers in Endocrinology found that active vitamin D treatment reduced the risk of falls by 19% and significantly improved quadriceps (thigh muscle) strength. Since quadriceps strength is one of the strongest predictors of whether someone will fall, this connection matters enormously for fracture prevention. Keeping vitamin D levels adequate protects bones in two ways at once: it keeps them dense and hard, and it helps maintain the muscle strength needed to stay upright.
How Much Vitamin D You Need
The standard blood test measures 25-hydroxyvitamin D, the circulating form your body stores. The NIH classifies levels like this:
- Below 12 ng/mL: Deficient. Associated with rickets in children and osteomalacia in adults.
- 12 to 19 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Generally adequate for most people.
The National Academies of Sciences, Engineering, and Medicine set the recommended daily intake at 600 IU for adults up to age 70 and 800 IU for adults over 70, with 400 IU recommended for infants. These amounts are designed to get most people to the 20 ng/mL threshold. People with darker skin, limited sun exposure, obesity, or conditions that impair fat absorption (like celiac disease) often need more. Certain medications, including some anti-seizure drugs, can also interfere with vitamin D metabolism and increase the risk of deficiency.
Your body produces vitamin D when ultraviolet B rays hit your skin, and you can get it from fatty fish, fortified milk, and supplements. For people who are already deficient, dietary sources alone are rarely enough to correct the problem, and a supplement is typically needed to bring levels back into the adequate range.