Vitamin D is a fat-soluble vitamin naturally present in very few foods. It's different from many vitamins in your supplement cupboard because it's actually considered a hormone. When your skin is exposed to sunlight, your body makes vitamin D (Chauhan, 2022).
Vitamin D for bones
When it comes to vitamin D for bones, the sunshine nutrient is a must-have. Bone health is primarily measured by bone density, or how strong your bones are. Bone density is affected by many genetic and lifestyle factors, but vitamin D plays a significant role by helping your body absorb calcium, which your bones need to stay strong (Bone Health and Osteoporosis Foundation, 2023).
Your bone health is at a significant disadvantage without this vitamin. Without enough vitamin D, you can't make the most of the calcium you eat or obtain from calcium supplements. Vitamin D also keeps your muscles strong and helps regulate the amount of calcium in your bloodstream. Low vitamin D levels are associated with an increased risk of osteoporosis, fractures, and falls (Holick, 2006).
What does vitamin D do?
Vitamin D receptors (VDRs) are found all over your body. Receptors act like a lock that opens up when the key is inserted. In this case, the key is vitamin D.
When vitamin D binds to a VDR, it affects the expression of hundreds of genes in the body. This means that the health effects of vitamin D include functions such as: (Sirajudeen, 2019; Chauhan, 2022; Yousefi, 2014)
- Cell growth and differentiation
- Inflammation regulation
- Immune response
- Blood sugar balance
Unlike most other essential vitamins and minerals, the most efficient way to get sufficient vitamin D (aside from dietary supplements) isn't necessarily from your food – it's from the sun. You can get some vitamin D from the food you eat (as you'll learn below), but the sun is a primary source of this important vitamin.
Vitamin D and calcium
Calcium and vitamin D are essential for promoting bone growth and minimizing bone loss, especially as you age. Optimal calcium absorption depends on vitamin D. Vitamin D increases the action of calcium transporters in your gut, so more calcium can be absorbed from the small intestine into your bloodstream. Without enough vitamin D, calcium absorption is minimized, even if you're eating enough of it (Christakos, 2011).
Vitamin D also helps ensure that calcium levels in your blood stay at the right concentration for bone building. It also helps to limit how much calcium is lost from your kidneys (Fleet, 2017).
Low calcium and vitamin D are risk factors for developing osteoporosis. Still, research is mixed on whether calcium and vitamin D supplements help prevent fractures or osteoporosis risk. Osteoporosis is characterized by low bone mineral density, meaning bones are weak and at risk for fractures.
Some research suggests that calcium and vitamin D supplements may increase bone mineral density and reduce the risk of fractures, especially at the hip (Chung, 2009, Yao, 2019). But other studies have found vitamin D supplementation has no benefit for fractures, falls, or improving bone density (Bolland, 2018).
A recent study found that older adults without osteoporosis who took 2,000 international units (IU) of vitamin D daily had the same risk of developing osteoporosis after five years as those who only took a placebo (Leboff, 2022).
Why the differences in results? One point to consider is that the participants in the recent study were all considered healthy adults without disease conditions that could affect nutrient absorption. The effects of vitamin D on your bone health also likely depend on the current levels in your bloodstream.
In other words, someone with low vitamin D levels may see more significant health benefits from vitamin D supplementation than someone with optimal levels. People with low baseline levels of vitamin D (and calcium) and those at increased risk of osteoporosis may still benefit from supplementing with extra vitamin D (Reid, 2014).
Vitamin D3 and vitamin D2
We're not trying to overload you, but this vitamin has two forms: vitamin D3 and vitamin D2. Although both forms can significantly increase vitamin D levels, vitamin D3 is more effective than D2 (Balachandar, 2021; Albarri, 2022).
That isn't a problem for most people since plenty of vitamin D supplements are made using vitamin D3. But vegans who need to raise low vitamin D levels may have a harder time since there are very few foods with this vitamin that don't come from animal sources. They should also be aware that they must shop carefully if they need a vitamin D supplement. Vitamin D3 is made by exposing lanolin derived from wool to UV light, so it doesn't fit a vegan lifestyle (Holick, 2007).
How much vitamin D do you need?
There is some disagreement about how much vitamin D is needed. Part of the reason is that vitamin D is required for many different functions, so the amount needed to prevent fracture risk and strengthen your bones may differ from that needed to optimize your immune health. Also, body size, age, health status, and genetics can all affect your ability to absorb vitamin D (Maurya, 2017).
The current recommended daily allowance (RDA) to prevent deficiency for women aged 50 to under 70 is 600 (IU) of vitamin D per day. The RDA jumps to 800 IU daily for women over 70.
The Endocrine Society also notes that it can take at least 1,500 to 2,000 IU a day to effectively raise blood vitamin D levels to optimal levels if someone is low (NIH, 2022, Holick, 2011). If you have vitamin D deficiency, your doctor may offer a higher dose vitamin D supplement to get your levels back within the normal range.
Over time, severe vitamin D deficiency can cause rickets in childhood, characterized by soft bones that are easily bent and misshapen. It can also cause osteomalacia, the adult form of rickets that affects bone density (Udey, 2020).
How common is vitamin D deficiency?
Vitamin D deficiency is common among American adults. Forty-two percent of adults in the U.S. have vitamin D levels that are too low, but that's only an average. Among Blacks and Hispanics, vitamin D deficiency rates are 82.1 and 69.2%, respectively (Forrest, 2011).
Where you live also dramatically affects your risk of vitamin D deficiency because it changes how much vitamin D your body can make from sun exposure. People who live in more Northern areas have a harder time getting the UV exposure needed to make enough vitamin D (Leary, 2017). If you drew a line through the U.S. that passed from central California to southern Virginia, anyone who lives above that line would have a higher risk of vitamin D deficiency (Holick, 2006).
Can you get too much vitamin D?
So if getting enough vitamin D for bones is crucial, is more vitamin D better?
Since it's a fat-soluble vitamin, it is possible to get too much vitamin D from high doses because it can be stored in your body. Dangerously high vitamin D levels can cause an increase in calcium levels, kidney problems, and other problems. A blood test to check vitamin D levels is the best way to know how much vitamin D you need through food and dietary supplements (if any) (Asif, 2022).
With all of the above in mind, it's best to work with a healthcare provider who can check your vitamin D levels and recommend an appropriate amount for your bone health.
How to get your vitamin D
Sun exposure is the best way to get vitamin D, but it is not always practical or safe due to UV exposure and skin cancer. Consult with your doctor if you want to try to increase your vitamin D levels with sunlight so they can help guide you safely. They may suggest tools like TK app, which uses the UV levels in your area, your skin type, and your clothing to determine how long you can safely be in the sun and set a timer for your session.
It isn't always practical to get vitamin D from sunlight, though. Skin pigment, sunscreen use, time of year, genetics, and where you live affect how much vitamin D you get from the sun. Your ability to create vitamin D from sunlight also decreases with age (Chalcraft, 2020; Maurya, 2017).
Unfortunately, very few foods contain this vitamin, but they do exist. Food sources that naturally contain vitamin D include: (USDA)
- Fatty fish: 3 ounces of sockeye salmon contains approximately 479 IU of vitamin D3
- Egg yolks: 1 egg contains approximately 50 IU of vitamin D3
- Mushrooms (exposed to UV light): Certain mushrooms exposed to ultraviolet light during cultivation can provide some dietary sources of vitamin D (vitamin D2), but the amount varies (Ozzard, 2008)
Although they don't naturally contain vitamin D, fortified foods are also an option. For example, one cup of fortified low-fat milk contains approximately 100 IU of Vitamin D (plant-based beverages may also be fortified). Fortified orange juice and cereals are also options, and the amount of vitamin D in each varies (USDA).
The bottom line on vitamin D for bone health
Getting enough vitamin D through food, sunlight, and supplements is critical for healthy bones. Still, it's just one part of a bone-supporting plan. Bone building exercise and other lifestyle habits are equally essential to ensure your bones are strong.