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Last updated January 4, 2020. 6 minute read

Vitamin D and bone health: the relationship explained

Many people know that vitamin D helps build strong bones, support bone health, and prevent conditions like osteoporosis—and that all of this has something to do with calcium. In fact, vitamin D works in the gut to boost the absorption of calcium. It also regulates how much calcium and phosphorous are in our blood, both of which work with vitamin D to keep your bones strong.

Linnea Zielinski Written by Linnea Zielinski
Reviewed by Dr. Mike Bohl, MD, MPH

There are some things that we incorrectly label, just to make our lives easier. Look how messy the tomato vegetable-fruit debate is—still. But that’s far from the only example. We also call avocados vegetables (they’re berries), people who use autotune singers (we’re not sure what they are), and vitamin D a, well, vitamin when it’s actually a pro-hormone.

Vitamin D (calciferol) is actually a group of steroid hormones in your body, the two most important of which for humans are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). (Both offer the same health benefits.) And, as you likely know, this “sunshine vitamin” can be made from cholesterol by our bodies if we get enough sun exposure. There’s just one hitch: You don’t make it if you’re wearing effective sunscreen. And since healthcare professionals no longer feel comfortable suggesting any time in the sun without sunscreen due to the risk of skin cancer, we’re more reliant than ever on dietary sources of this vitamin.

Getting enough and avoiding vitamin D deficiency is crucial. A considerable 41.6% of adults in the United States don’t get enough of the “sunshine vitamin.” And people with darker skin are at an even higher risk; 82.1% of African Americans and 69.2% of Hispanics don’t get enough vitamin D (Forrest, 2011). At those rates, it’s clearly a public health concern. Vitamin D is vital to your body, and low blood levels have been associated with increased risk of cardiovascular disease, asthma in children (Ali, 2017), and cognitive impairment in older adults (Kent, 2009). Low levels of vitamin D in childhood can also cause rickets, a serious condition characterized by stunted growth and bone deformity.

Vitals

  • Vitamin D refers to a group of hormones.
  • Vitamin D2 and vitamin D3 are both important for human health, though they offer the same health benefits.
  • The Ds don’t just help keep bones strong; they also help build strong bones from the beginning when they form.
  • Getting enough vitamin D can help prevent conditions that weaken bone strength.
  • Most people should be able to hit their daily needs through dietary intake, though supplements are another good option.
  • A healthcare professional may treat low vitamin D levels with short-term high-dose supplementation.

Vitamin D and bone health

Bone health is complicated. We don’t just form healthy bones and then, hopefully, maintain them in the same quality. Our bones go through several processes: growth, modeling or consolidation, and remodeling—the latter of which happens throughout our lifetime. Remodeling refers to the process that happens when bone is broken down and formed again, not always in the same way—or at the same quality. These internal supports also go through mineralization, a process in which mineral crystals are deposited in a structure. The strength of our bones is determined by two factors: bone mineral density (also referred to as bone density), which is how many of those mineral crystals there are in an area of bone, and bone matrix quality, which is basically everything about the bone’s structure and composition that doesn’t involve minerals.

Many people know that vitamin D helps build strong bones, support bone health, and prevent conditions like osteoporosis—and that all of this has something to do with calcium. In fact, vitamin D works in the gut to boost the absorption of calcium. It also regulates how much calcium and phosphorous are in our blood, both of which work with vitamin D to keep your bones strong. And all three have to be balanced in your blood for this to work. Excess phosphorus, for example, will cause your body to pull calcium from your bones in order to achieve this equilibrium.

But the Ds also play a role in how our body remodels bone tissue. There’s a strong link between lower levels of vitamin D in blood serum and low bone mineral density in menopausal and postmenopausal women (Bener, 2015). And osteoporosis, a condition characterized by progressive bone loss, isn’t the only condition it can help prevent. Low D levels can also cause osteomalacia, a softening of the bones, in older adults, which can help prevent dangerous bone fractures (Sitta, 2009).

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Additional benefits of vitamin D

As if playing a critical role in multiple phases of bone development wasn’t enough, vitamin D also has a vital function in many other processes throughout your body. The Ds are even essential for critical cellular functions from cell growth to death, and potentially even diabetic health. But here are just some of the potential health benefits of vitamins D2 and D3 that you should know about:

  • May protect against the flu
  • May reduce risk of heart disease
  • May reduce depression symptoms
  • May help protect against certain cancers
  • May help improve cognitive function

Sources of vitamin D

You’ve likely heard a lot of praise for the Mediterranean Diet, which is known for its abundance of fruit and vegetables and quality sources of healthy fats. Vitamin D is one of the reasons why it’s so highly regarded. Fatty fish, like those common in this style of eating, are some of the best food sources of the sunshine vitamin. Egg yolks and dairy products (which are also good sources of dietary calcium) are other good ways to boost your dietary intake. Fish liver oil, like cod liver oil, sits somewhere between food source and supplement, but a serving of most kinds easily meets your daily needs of this vitamin. 

Luckily for vegans, many foods are fortified with vitamin D like certain breakfast cereals and orange juice, so it’s not impossible to get your Ds through food sources if you follow a plant-based diet. Your vitamin D intake may be sufficient if you’re eating several of these foods, as the recommended dietary allowance (RDA) for adults (600 international units or IU for those between 1 and 70, 800 IU of vitamin D for those 71 and older) can be hit through from food sources alone.

Vitamin D supplementation is another solid option, and it may even be necessary for certain people. Before starting any supplements, though, you should talk to a healthcare professional and get your blood levels tested. Too much vitamin D can also be dangerous. But those with medical conditions such as Crohn’s disease, celiac disease, or irritable bowel syndrome (IBS) suffer from malabsorption and may need vitamin D supplements to hit their RDAs. You can buy vitamin D as its own supplement, but some brands sell joint vitamin D-calcium supplements.

Vitamin D levels

Vitamin D deficiency and toxicity are both serious, so it’s important to work with a medical professional who can test your blood levels. Once you take in vitamin D, your body converts it to a chemical called 25-hydroxyvitamin D, or calcidiol. The best blood test checks your serum 25-hydroxyvitamin D (which you may see abbreviated as 25(OH)D). If your blood test comes back showing >50 nmol/L of this essential nutrient, you have vitamin D deficiency. Anything below 30 nmol/L is considered a severe deficiency (Iolascon, 2009). You may notice health problems such as hair loss, fatigue, and increased anxiety if your levels dip too low. You may also get sick often since the Ds may boost immunity. Some of the symptoms of vitamin D deficiency come on quickly, while others are the byproduct of a long-term lack of this essential nutrient.

Although a general practitioner may run these tests, it’s possible you may get referred to an endocrinologist to help correct low vitamin D and investigate what is causing it. Sometimes high-dose vitamin D is used by medical professionals to treat low levels, but you should always follow medical advice when taking higher doses. Taking more than what they suggest or higher doses for longer than they suggest may lead to vitamin D toxicity.

Vitamin D toxicity generally only happens with high doses of supplements—not with sun exposure or dietary intake. And it usually causes hypercalcemia, excess calcium in the blood, which can cause symptoms, including weakness, nausea, vomiting, and frequent urination. In most cases, it’s easy to fix. Patients are put on a low- or no-calcium diet and stop taking vitamin D supplements until their blood levels of the Ds are back to normal.