A new in-depth study of bone structure in people with vitamin-D deficiency reveals changes that can dramatically increase the risk of fractures.

In a study published today in Science Translational Medicine, German researchers show that vitamin-D deficiency increases both the start and spread of bone fractures by up to 31 percent.

Björn Busse and colleagues analyzed bone samples from 30 subjects ages 57 to 60, half of whom were deficient in vitamin D. All of the people selected for the study were healthy and had died of unnatural or accidental causes.

The scientists used state-of-the-art technologies, including synchrotron radiation-based micro computed tomography (CT) and three-point stress tests, to characterize bone quality on the smallest of size scales, from nanometers to micrometers.

The researchers discovered that vitamin D-deficient patients had less mineralization on the surface of their bones. But underneath, the bones were actually more heavily mineralized, showing the structural characteristics of older and more brittle bones.

What this means is that vitamin-D deficiency is not just associated with a loss of bone density. It also affects bone quality by changing the way bone-building cells called osteoblasts and osteoclasts work.

Osteoclasts that normally model the bone cannot get through the surface layer. As a result, bone hidden underneath continues to age and mineralize, even as the overall bone mineral content progressively declines.

This type of bone damage can occur at any age as the result of vitamin-D deficiency.

“People deficient in Vitamin D can compensate with a diet with rich in vitamin D (seafood, fish, etc.) and/or food fortification with vitamin D,” Busse told Healthline.

Vitamin D is a nutrient found in very few foods. It is called the “sunshine” vitamin because it can also be absorbed through the skin. Its main function is to help the body absorb calcium, which is necessary to maintain strong bones.

Deficiencies in vitamin D and calcium can cause soft, thin, brittle bones—a condition known as rickets in children and osteoporosis in adults. More than 40 million people have either osteoporosis or low bone mass, according to the National Institutes of Health (NIH).

The condition can lead to a greater risk of bone fractures, especially as we age. According to the International Osteoporosis Foundation (IOF), worldwide one in three women and one in five men over the age of 50 will experience osteoporotic bone fractures in their lifetime.

But vitamin-D deficiency can be reversed, according to Busse. “Balancing vitamin-D levels not only improves mineralization of bone, but by reintegrating old bone into the physiological process of bone remodeling, an improvement of bone quality and fracture rates can be achieved,” he said.

The Office of Dietary Supplements (ODS) of the National Institutes of Health recommends different levels of vitamin D depending on your age:

  • Birth to 12 months: 400 International Units (IU)
  • Children and teens: 600 IU
  • Adults 19–70 years: 600 IU
  • Pregnant and breastfeeding women: 600 IU
  • Adults 71 years and older: 800 IU

The best way to find out if you’re getting enough vitamin D is to have a blood test that measures 25-hydroxyvitamin D, the metabolized form of vitamin D.

Levels below 30 nmol/L (nanomoles per liter) are considered too low for bone health, and levels of 50 nmol/L are sufficient for most people.

Certain people may not get enough vitamin D and may need to take supplements. These people include:

  • Breastfed infants, as human milk is a poor source of the nutrient
  • Older adults, as their skin doesn’t make vitamin D when exposed to sunlight as efficiently as when they were young, and their kidneys are less able to convert vitamin D to its active form
  • People with dark skin, because their skin is less able to produce vitamin D from the sun
  • People with disorders such as Crohn’s disease or celiac disease who don’t handle fat properly, because vitamin D needs fat to be absorbed
  • Obese people, because their body fat binds to some vitamin D and prevents it from getting into the blood
  • People who have had gastric bypass surgery may be unable to metabolize vitamin D because the area of the gut that processes it has been bypassed

The best food source of vitamin D is cod fish oil. Just 1 tablespoon provides 1,360 IUs. Fatty fish, including salmon, swordfish, and tuna, are also excellent sources.

Dairy products, such as fortified milk, yogurt, butter, cheese, and egg yolks, as well as beef liver, offer lower levels of vitamin D.

Supplemental vitamin D is available in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). According to the ODS, both metabolize equally well in the body and provide protection against rickets and osteoporosis.

In vitamin D-deficient bone, the researchers still found areas of high mineralization, from the core to the exterior. Based on this finding, the authors say that current tests to measure bone density may not provide an accurate assessment.

Current bone density tests measure how many grams of calcium and other bone minerals are packed into a segment of bone in the spine, hip, or forearm.

“It is not possible to differentiate between structure (mass) and mineralization (mineral content) using the common DXA bone density test,” Busse explained.

“Newer, high resolution 3D techniques, such as high resolution peripheral microcomputed tomography, may serve as an additional method to characterize patients bone status in more detail,” he added.

However, from the standpoint of cost, state-of-the-art technologies like this may not be feasible for clinical use—at least for the time being.

The best protection is to ensure that you get enough vitamin D every day, based on your age and other health requirements.