Vitamin D is a fat-soluble vitamin that serves a number critical functions in the body. However, there’s a good chance you’re not getting enough of it.

Vitamin D plays an important role in immune function. Not only does vitamin D enhance our immune systems, it also prevents our immune systems from becoming dangerously overactive. An overactive immune system leads to many autoimmune disorders in which your body can’t tell the difference between your healthy, normal cells and invaders.

Additionally, a new study by researchers at University of South Australia finds that people with vitamin D deficiency are more likely to suffer from heart disease and higher blood pressure than those with normal levels of vitamin D. For study participants with the lowest concentrations of vitamin D, the risk of heart disease was more than double that seen for those with sufficient concentrations.

Vitamin D is also very important for people with joint pain, and research shows that people with low levels of vitamin D may indeed suffer more joint pain. In fact, vitamin D deficiency can cause subtle widespread pain that may be misdiagnosed as fibromyalgia and/or chronic fatigue syndrome, including symptoms like muscle and bone aching, fatigue and weakness, a lower pain threshold, and more acute soreness after exercise that is slower to resolve.

Though vitamin D is important for everyone, it is particularly vital for those who exercise vigorously and regularly. Vitamin D dramatically reduces muscle fatigue and studies show it decreases muscle recovery time by 20%. It can also reduce inflammation and increase muscle protein.

High levels of vitamin D in the blood are also linked with better fitness, according to research from the Virginia Commonwealth University School of Medicine. In the study, published in the European Journal of Preventive Cardiology, 20- to 49-year-olds with better vitamin D status also tended to have greater cardiorespiratory fitness, a measure of aerobic fitness often determined by measuring maximal oxygen consumption (VO2max) during exertion.

Lastly, vitamin D and calcium work together to protect your bones; calcium helps build and maintain bones, while vitamin D helps your body effectively absorb calcium. So even if you’re taking in enough calcium, it could be going to waste if you’re deficient in vitamin D. Ultimately, vitamin D is necessary for strong bones and it reduces the risk of fractures. If you don’t get enough vitamin D, you’re more likely to break bones as you age.

Though your body needs vitamin D to absorb calcium, you don’t need to take vitamin D at the same time as a calcium supplement.

But don’t forget magnesium! Vitamin D can’t be metabolized without sufficient magnesium levels, meaning vitamin D remains stored and inactive for as many as 50 percent of Americans. In addition, vitamin D supplements can increase a person’s calcium and phosphate levels even while that person remains vitamin D deficient.

Vitamin D deficiency is more common than once suspected. Nearly 1 in 4 people in the US have vitamin D blood levels that are too low or inadequate for bone and overall health, according to the National Institutes for Health.

The daily recommended amount of vitamin D for adults is 600 IU, but the upper limit is 4,000 IU. Some studies suggest that a higher daily intake of 1000–4000 IU is needed to maintain optimal blood levels. Going above 4000 IU may be toxic and should only be done under the supervision of a doctor or Registered Dietician.

There are two major types of vitamin D:

• Vitamin D2 (ergocalciferol) – which is synthesized by plants and is not produced by the human body

• Vitamin D3 (cholecalciferol) – which is made in large quantities in the skin when sunlight strikes bare skin. It can also be ingested from animal sources

Evidence suggests that vitamin D3 is approximately three times more effective at maintaining serum concentrations because the binding protein has a higher affinity to vitamin D3 than vitamin D2. This allows vitamin D3 to reside in the circulatory system longer and increase the concentration to sufficient levels more quickly.

Most over-the-counter vitamin/multivitamin preparations use vitamin D3.

Since vitamin D is fat-soluble, it is better to choose oil-based supplements or take it with food that contains some fat.

Good dietary sources include fatty fish, fish oils, egg yolk, butter and liver. All of these contain fat.

Vitamin D is special: It’s the only vitamin that we can make ourselves—our bodies create vitamin D in the form of a hormone when we process sunlight. When your skin is exposed to sunlight, it makes vitamin D from cholesterol. That’s why getting enough sunlight is very important for maintaining optimal vitamin D levels.

Factors that impact the ability of the body to synthesize vitamin D through the skin include: time of year, time of day, the presence of clouds and/or smog, skin melanin content, the application of sunscreen, and geographic latitude. For example, residents at 42° N latitude or higher are unable to synthesize vitamin D via the skin during the winter months (from November through February).

Assuming your diet is decent, if there’s only one supplement you should take for your health, it’s probably vitamin D.