Contrary to what you've likely heard countless times, vitamin D isn't really beamed into your skin straight from the sun. In fact, the "sunshine vitamin" isn't even a vitamin at all — it's actually a group of hormones, according to the Hormone Health Network.
What remains true, though? The nutrient we know as vitamin D is essential to your health. Unfortunately, many people don't get enough, and a deficit can affect everything from your energy levels to your risk for certain serious diseases.
Here's what you need to know about vitamin D and the important role it plays in your body.
So, What Is Vitamin D?
At the scientific level, vitamin D is a group of hormones produced by a chemical reaction when you eat certain foods or absorb sunlight through your skin. But it's easier to understand if we think of it like any other vitamin.
Vitamin D is fat-soluble, which means that — along with vitamins A, E and K — it is absorbed through the intestines in the presence of fat and can be stored in your system for a long time, explains a February 2016 report in The Clinical Biochemist Reviews.
Your liver and fatty tissues can store fat-soluble vitamins for use in the future. Because vitamin D from your food is fat-soluble, it dissolves in fat droplets, and you need to eat vitamin D-containing foods with a small amount of fat to promote good absorption.
Eating a normal, well-balanced diet shouldn't lead to toxicity of vitamin D3 in most people, but taking large doses of vitamin D supplements, in addition to eating vitamin D-rich foods, as well as sun exposure, can pose risks.
There are two primary types of vitamin D:
- Vitamin D3, or cholecalciferol, is found in foods such as egg yolks and fatty fish. It's also the type your body makes when it's exposed to sunlight.
- Vitamin D2, or ergocalciferol, is present in some plants, such as some types of mushrooms, as well as yeast.
While both types of vitamin D are beneficial, "Vitamin D3 has been proven more effective in improving and sustaining your vitamin D levels," says Melaina Bjorklund, RD, a practicing clinical dietitian at Penrose-St. Francis Health Services in Colorado Springs.
Here's more confirmation: Vitamin D3 was deemed the best for helping us achieve optimal levels of the nutrient, and it should be the preferred form of supplementation, per a June 2012 meta-analysis in the American Journal of Clinical Nutrition.
Normal Vitamin D Levels
Levels greater than or equal to 50 nmol/L (or greater than or equal to 20 ng/mL) are generally considered sufficient for bone and overall health, according to the National Institutes of Health (NIH).
The recommended dose for people between the ages of 1 to 70 is 10 mcg each day. Adults older than 20 need 15 mcg of vitamin D to stay healthy.
Benefits of Vitamin D
1. Vitamin D Can Support Bone Health
Vitamin D is necessary for helping your body build strong and healthy bones, according to the Mayo Clinic. Your body depends on vitamin D to absorb calcium, which is the main component found in your bones.
That's why a lot of breakfast cereals are fortified with vitamin D. Typically, cereal is eaten with milk, which is a good source of calcium, according to the NIH. So, when you eat these two together, you're getting the full bone-health benefit.
2. It Benefits the Immune System
Vitamin D also plays a role in nerve signaling and in regulating the immune system, Bjorklund says.
In other words, the nutrient helps your muscles function properly and aids the body in fighting off invading bacteria and viruses.
Getting adequate vitamin D (more on that below) is also necessary for your muscle health and strength, according to a November 2012 study in Sports Health.
Increasing your vitamin D can help improve your muscle protein synthesis and ATP concentration, a compound that supplies your body with the energy needed for muscle function.
Vitamin D supplementation can also help reduce inflammation or pain, making it helpful for athletes, according to the above-mentioned study. Increasing vitamin D levels is tied to helping improve athletic factors like strength, jump height, jump power, exercise capacity and physical performance.
Especially early in life, vitamin D plays a big role in your overall brain development and function, according to a July 2018 study in Cureus.
Even for adults, vitamin D supports normal brain function and development, according to a November 2017 study in the International Journal of Molecular Sciences.
Unfortunately, despite speculation, there isn't any firm evidence to suggest that vitamin D can help prevent neurodegenerative conditions or diseases.
5. Vitamin D Is Tied to Good Gut Health
The vitamin can also help regulate your gut health, according to an August 2018 study in the Journal of Research in Medical Sciences.
While you shouldn't try any treatments without a doctor's approval, vitamin D supplementation may help regulate gastrointestinal inflammation and IBS and may even help treat colitis (a chronic digestive disease).
Increased vitamin D is also associated with reduced risk for colitis caused by immune checkpoint inhibitors, a type of drug often used in cancer treatment, according to a June 2020 study in the American Cancer Society Journals.
Sunlight As a Key Source
Exposure to sunlight is crucial to help your body make vitamin D. In fact, according to a June 2019 article in StatPearls, 50 to 90 percent of the vitamin D in your body is accrued through sun exposure (specifically, UVB rays).
If your climate has a lot of sunshine, get outside a few times a week to soak it in. Keep in mind that you need to let the sun penetrate a large part of your body — not just your face and hands. The StatPearls paper notes that, on average, 20 minutes of sunshine daily with about 40 percent of your skin exposed is required to prevent vitamin D deficiency.
There's a catch, though: Wearing sunscreen decreases your absorption. But, sunscreen protects you from the sun's harmful ultraviolet rays — and foregoing it increases your risk of developing skin cancer, according to the American Academy of Dermatology (AAD).
For this reason, the AAD suggests that most people can and should get the vitamin D they need from a combination of D-rich foods and supplements, although you should speak with your doctor to determine the best approach for you.
Can You Get Vitamin D Through Windows?
While sunlight can pass through window glass, the UVB wavelengths in sunlight cannot, according to the NIH.
Because you need UVB exposure to jump-start the vitamin D production process, sunlight that passes through a window cannot increase your vitamin D levels, even if it strikes bare skin.
Bottom line: You won't get any vitamin D from absorbing sunlight through windows or glass — and you need direct sunlight for vitamin D.
Vitamin D Foods
There aren't many foods high in vitamin D but the nutrient can be found naturally in beef liver, egg yolks and fatty fish, such as canned and fresh salmon, mackerel, swordfish, halibut and eel, says Colette Raymond, RD, a registered dietitian based in Colorado Springs.
Some processed items are fortified with D, Raymond says. Soy milk, orange juice, cereal and milk are some examples.
Mushrooms, particularly cremini and portabella varieties that have been exposed to UV light, as well as maitake mushrooms, also have a good amount of vitamin D2, according to the USDA.
How to Better Absorb Vitamin D
Vitamin D is a fat-soluble vitamin and dissolves in the fat within your digestive tract. When your body absorbs this fat, it also absorbs the dissolved vitamin D.
Foods high in healthy fat, like fatty fish, avocado and olive oil, can help you absorb vitamin D.
If you have a digestive issue that hinders your ability to absorb fat, you also face an increased risk of a vitamin D deficiency.
How Much Should You Really Be Getting?
The NIH suggests adults get 600 IU of vitamin D daily, while adults over the age of 70 should get 800 IU per day.
Some other organizations recommend higher doses, but according to the Mayo Clinic, taking in more than 4,000 IU a day is dangerous, and could result in side effects like nausea and vomiting, heart rhythm problems and kidney damage.
Yet, about 1 billion people worldwide are deficient in vitamin D, according to the June 2019 article in StatPearls. And half of the global population has insufficient, but not medically deficient, levels of the nutrient.
Common Vitamin D Deficiency Symptoms
Low vitamin D can cause leg cramps. Muscle spasms (also known as tetany) can occur in people with severe vitamin D deficiency, per Merck Manual.
Vitamin D deficiency may lead to muscle twitching by causing hypocalcemia, or low calcium levels. This leads to irritability of your muscle cells and involuntary contractions, aka twitching or spasms.
Muscle twitching related to vitamin D deficiency most commonly occurs in your hands, feet and face. In addition to twitching, you may experience muscle aches and progressive weakness. Muscle weakness and pain caused by vitamin D deficiency is called osteomalacic myopathy.
On the other hand, too much vitamin D can cause muscle pain because high levels of D can cause extra calcium absorption, according to the Cleveland Clinic.
Other common vitamin D deficiency symptoms include:
- Frequent illness
- Bone or back pain
- Muscle pain
- Hair loss
Not everyone with a deficiency has obvious symptoms, though. You may not know you're running low on D until bone loss has occurred, says Bjorklund.
Can Low Vitamin D Cause Fatigue?
A vitamin D deficiency may cause a lack of energy, including depression and fatigue, according to the Cleveland Clinic. Having vitamin D deficiency means your D levels fall below 50 nmol/L.
Seasonal affective disorder (SAD) occurs during certain seasons, specifically the fall and winter, and is marked by feelings of depression, fatigue and sleepiness. It's believed that a lack of sunlight and vitamin D deficiency may be the cause of this disorder. When the winter months lose sunlight, you may become low in vitamin D, which increases melatonin and fatigue.
Long-Term Health Implications
1. Vitamin D and Bone and Muscle Health
Low D levels may contribute to poor bone health — including osteopenia and osteoporosis — and serious injury from falls, especially in older people, according to comprehensive research published in August 2015 in the Singapore Medical Journal.
But the fix isn't quite as simple as just taking more vitamin D: Daily doses of 10,000 IU did not improve bone health compared to 4,000 or 400 IU in a three-year study of 311 older adults published in JAMA in August 2019.
What's more, researchers found an association between low vitamin D levels and limited mobility in older adults who recently underwent hip fracture surgery, according to a February 2020 study in The American Journal of Clinical Nutrition. The study also suggests a link between higher D levels and better walking rates and mobility.
Older adults with low D levels are also more likely to have impaired muscle strength, according to an October 2019 study in Clinical Interventions in Aging. Note that the researchers didn't find an association between D levels and falls, but they did note that those who did moderate physical activity on a regular basis had significantly lower odds of impaired muscle function.
2. Vitamin D and Cancer
A chronic deficiency in vitamin D is associated with increased risk of cancer, type 2 diabetes and other immune system disorders, notes the February 2016 report in The Clinical Biochemist Reviews.
And vitamin D may play a role in the prevention and treatment of some other chronic disorders, including hypertension and diabetes.
And on a brighter note, taking vitamin D supplements has been shown to help people with cancer. June 2019 research from Michigan State University determined that supplemental vitamin D, taken in adequate amounts for at least three years, significantly lowered the risk of death among those with cancer compared to those that took a placebo. The study was based on information from more than 79,000 people.
3. Vitamin D and Blood Sugar
Observational studies suggest an inverse relationship between vitamin D and the risk of type 1 and type 2 diabetes, according to Oregon State University.
Getting five to 10 times the recommended dose of vitamin D for six months may help slow the progression of type 2 diabetes in newly diagnosed people and in those with prediabetes, a July 2019 study in the European Journal of Endocrinology found.
A May 2004 study in the American Journal of Clinical Nutrition found that people with a vitamin D deficiency are at higher risk of glucose intolerance and metabolic syndrome.
Who's Most at Risk for Vitamin D Deficiency?
Vitamin D deficiency more often shows up in older adults and people with darker skin pigmentation, according to the NIH.
If you work indoors or habitually avoid the sun by staying in the shade or wearing protective clothing, you're also at risk.
Low calcium intake is also linked to low vitamin D levels because high-calcium foods like milk and yogurt are typically fortified with the vitamin. This means that people who are lactose intolerant, have a milk allergy or follow a vegan or ovo-vegetarian diet may not get enough vitamin D from the foods they eat.
Those with nutrient absorption issues, such as Crohn's disease or inflammatory bowel disease, might be deficient as well.
People who live in northern climates with less exposure to UVB sunlight often don't get enough vitamin D, either.
If you believe you're not getting enough vitamin D, ask your doctor for a blood test to evaluate your levels. If you're deficient, you may benefit from taking a supplement.