Vitamin D is essential for bone health. Without it, your body absorbs only 10 to 15 percent of the calcium you eat. With adequate vitamin D, that number jumps to 30 to 40 percent. This single nutrient essentially controls whether the calcium in your diet actually reaches your skeleton or passes right through you.
How Vitamin D Strengthens Bones
Vitamin D’s primary job is to increase calcium absorption in your intestines. When you eat calcium-rich foods, your gut lining needs to actively transport that calcium into your bloodstream. The active form of vitamin D triggers this process in two ways: it opens dedicated calcium channels in the cells lining your intestine, and it loosens the junctions between those cells so calcium can slip through the gaps. Without vitamin D driving both of these pathways, most dietary calcium never makes it past your digestive tract.
Vitamin D also works in a feedback loop with parathyroid hormone (PTH), which your body releases when blood calcium drops too low. PTH signals your kidneys to activate more vitamin D, which then boosts calcium absorption from your gut. Once blood calcium levels recover, PTH secretion shuts off. This cycle keeps calcium in a tight, healthy range. But when vitamin D is chronically low, the system breaks down. PTH stays elevated and starts pulling calcium directly from your bones to maintain blood levels, gradually weakening your skeleton.
What Happens When You Don’t Get Enough
Severe vitamin D deficiency causes two well-known bone diseases. In children, it leads to rickets, where growing bones don’t mineralize properly and can bow or deform. In adults, the equivalent condition is called osteomalacia, literally “soft bones.”
Adults with osteomalacia typically experience diffuse bone pain, especially in the lower back, pelvis, hips, and legs, that gets worse with activity and weight bearing. Muscle weakness is common too, making it hard to walk or climb stairs. In severe cases, bones can fracture from minimal trauma. Other signs include a waddling gait, muscle spasms, fatigue, and bowed legs. Blood levels of 25(OH)D below 10 ng/mL are typical in people with nutritional osteomalacia.
Even moderate deficiency, well before it reaches the point of osteomalacia, contributes to gradual bone loss and higher fracture risk over time. That’s because your body compensates for low vitamin D by leaning harder on PTH, which quietly borrows calcium from bone tissue year after year.
How Much Vitamin D You Need
The NIH recommends the following daily amounts:
- Infants (birth to 12 months): 400 IU
- Children and teens (1 to 18 years): 600 IU
- Adults (19 to 70 years): 600 IU
- Adults over 70: 800 IU
- Pregnant or breastfeeding women: 600 IU
The bump to 800 IU for older adults reflects the fact that aging skin produces less vitamin D from sunlight, and kidneys become less efficient at activating it. This is also the population most vulnerable to fractures.
Blood Levels That Matter for Bones
A blood test measuring 25(OH)D is the standard way to check your vitamin D status. The National Academies of Sciences, Engineering, and Medicine define the thresholds this way:
- Below 12 ng/mL: Deficient. Associated with rickets in children and osteomalacia in adults.
- 12 to 20 ng/mL: Generally inadequate for bone and overall health.
- 20 ng/mL or above: Adequate for most people.
- Above 50 ng/mL: Linked to potential adverse effects.
You don’t need to aim as high as possible. Levels above 50 ng/mL offer no proven additional benefit for bones and may carry risks. The sweet spot for most people is somewhere at or above 20 ng/mL.
Vitamin D and Fracture Prevention
A meta-analysis looking at vitamin D supplementation and fracture risk found that a daily dose of 800 to 1,000 IU reduced the risk of osteoporotic fractures by 13 percent and lowered the risk of falls by 9 percent. Interestingly, doses below 800 IU or above 1,000 IU daily did not show the same protective effect. Very high intermittent doses (like a single large monthly or yearly dose) appear less effective than consistent daily intake, suggesting that keeping vitamin D levels steady matters more than occasionally flooding the system.
Why Vitamin K Matters Too
Vitamin D doesn’t work in isolation. It increases your body’s production of certain proteins that direct calcium where it needs to go, but those proteins require vitamin K to become active. Two of the most important are osteocalcin, which helps deposit calcium into bone, and matrix Gla protein, which prevents calcium from building up in blood vessel walls.
When vitamin D is high but vitamin K is low, these proteins can’t do their jobs properly. The result is a paradox: excess calcium that should be strengthening your bones may instead end up calcifying your arteries. This is why some researchers recommend paying attention to vitamin K intake (found in leafy greens, fermented foods, and some cheeses) alongside vitamin D, particularly if you supplement at higher doses. The two vitamins work as a team to get calcium into bone and keep it out of soft tissue.
Upper Limits and Safety
More vitamin D is not always better. Blood concentrations above 50 ng/mL have been linked to adverse effects, and levels above 60 ng/mL raise concern further. Toxicity from vitamin D causes too much calcium to accumulate in the blood, which can lead to nausea, kidney stones, and in extreme cases, kidney damage. This almost never happens from sun exposure or food alone. It’s a risk with high-dose supplements taken over time.
If you’re taking a standard daily supplement of 600 to 1,000 IU, toxicity is extremely unlikely. Problems typically arise at sustained intakes well above 4,000 IU per day, which is the tolerable upper limit set for adults. People with certain medical conditions, particularly kidney disease or granulomatous disorders, may be more sensitive to vitamin D and should be cautious with supplementation.