Vitamins are nutrients your body needs for normal function but cannot make in sufficient amounts on its own. They can fall into one of two categories: water-soluble or fat-soluble. Water-soluble vitamins readily dissolve in water, so your body gets rid of any any excess in your urine. Fat-soluble vitamins, on the other hand, require fat for release, absorption, transport and storage within your body. As a fat-soluble vitamin, D3 primarily gets stored in your liver, fat and muscle tissue -- meaning too much can be toxic to your body.
Vitamin D is actually a group of related compounds that behave like hormones. Two types naturally occur in the human diet: D2 and D3. Vitamin D2 primarily occurs in plants, while animal products are the primary sources of D3. You also eat vitamin D2 in fortified foods such as milk and cereals. Adults need around 600 International Units of vitamin D per day, though the RDA changes to 800 IUs a day once you reach age 70. Because vitamin D is fat-soluble, it has a tolerable upper limit, which is 4,000 IUs daily for everyone over age 9.
Your skin naturally makes D3 when exposed to sunlight; however, a number of factors can affect its ability to do so, including smog, sunscreen use, clouds, clothing and window glass. According to University of Maryland Medical Center data, a fair-skinned person needs about 45 minutes of weekly sun exposure for an adequate vitamin D supply, while darker-skinned people may need four times as much time. According to UMMC, you can't overdose on vitamin D from sunlight, and it's difficult to do so from dietary sources -- rather, watch your vitamin D supplement, to ensure you're not taking in too much.
Vitamin D helps regulate the blood levels of calcium and phosphorus within your body. Phosphorus helps with energy production and joins with calcium to give structure to your bones, making vitamin D important both to build and maintain bone strength. Vitamin D also helps regulate your immune system, which is your body's disease-fighting machinery.
According to University of Maryland Medical Center, supplying your body with enough D3 can help prevent potentially serious health conditions, such as bone loss, falls in seniors, high blood pressure, diabetes, heart disease, obesity, multiple sclerosis and certain types of cancer. However, because excess vitamin D gets stored in your body's fatty tissues, you should pay attention to nutrition labels, particularly if you're supplementing. Excessive doses of vitamin D can lead to appetite loss, vomiting, diarrhea, weakness and mental changes, says the University of Maryland Medical Center.