The short answer is ‘NO’ you cannot get too much vitamin D from sun exposure. I occasionally hear this question or incorrect statements to the contrary. Vitamin D has recently become a ‘vitamin superstar’ with research showing it has effects on many different processes in the body. There are many that think most people have low-level vitamin D deficiencies and should supplement. However, I will leave that to you to decide for yourself. In today’s post I want to review a little about vitamin D: what it does, why it is so important, and give more detail about how awesome our bodies are and why you cannot ‘overdose’ on vitamin D from sun exposure.
Vitamin D
First off, vitamin D is a fat-soluble vitamin, along with vitamin A, vitamin E, and vitamin K. This means that it easily dissolves in fat, but not as readily in water. So when it is transported in the blood (water) it needs a carrier protein of some sort, but it can easily pass into cells to be used. Vitamin D is also special because it can be classified as a hormone and acts more like a hormone than a ‘vitamin.’ In fact, vitamin D is made from cholesterol in the body, like other steroid hormones. The fact that the body can produce vitamin D also makes it different from most vitamins as well. There are basically two forms of vitamin D: Vitamin D3 that our bodies make or get from animal sources and Vitamin D2 which is from plant sources.
What Does Vitamin D Do?
Vitamin D has a variety of effects on the body. Receptors for vitamin D have been found on a number of different cells throughout the body and vitamin D can affect cell growth and proliferation as well as muscle strength. However, vitamin D is most famous for its role in calcium absorption and strong bones. In the case of calcium absorption the active form of vitamin D goes to the cells of the intestine and increases the production of a protein named calbindin. Calbindin increases the amount of calcium that is able to be absorbed through the intestines therefore increasing the amount of calcium in the body. This increased calcium absorption is necessary for adequate bone mineralization. A deficiency of vitamin D in children is known as rickets and can lead to bone deformities (because the bones become soft) as well as a number of other health problems.
How Does the Body Make Vitamin D?
Vitamin D in the body is made from a form of cholesterol (7-dehydrocholesterol). The 7-dehydrocholesterol in the skin undergoes a change when exposed to ultraviolet light from the sun to form previtamin D. The previtamin D then undergoes another reaction to become vitamin D but in an inactive form. This vitamin D needs to go to the liver and have a hydroxyl group added to form 25-hydroxycholecalciferol (AKA 25-hydroxyvitamin D or calcifediol). Although the name sounds impressive, we still are not to the active form of vitamin D yet. The 25-hydroxycholecalciferol then would need to go to the kidney and have another hydroxyl group added to form 1, 25-dihydroxycholecalciferol (AKA calcitriol) and this is the active form of vitamin D.
What Happens to Prevent Too Much Vitamin D?
In the skin when 7-dehydrocholesterol is turned into previtamin D and also the inactive form of vitamin D, there is another reaction that will take place to prevent too much vitamin D from being created. If you are exposed to further ultraviolet light the previtamin D will undergo another change to one of two other forms that are inactive: tachysterol or lumisterol. This prevents too much vitamin D all at once. The cool thing is that when levels of vitamin D drop, tachysterol and lumisterol can then be converted back into previtamin D which then could go on to form vitamin D.
Things That Prevent Vitamin D Synthesis in the Skin
There are things that will prevent vitamin D from being made in the skin from sun exposure. Obviously anything that blocks the ultraviolet light will prevent this process from happening. This could include being indoors, being completely covered with clothing, or even sunscreen will prevent this process from taking place. Also those who have darker skin require more exposure than fairer skin individuals to produce the same amount of vitamin D because the pigment in the skin absorbs some of the ultraviolet light. Also in older adults this process does not work as efficiently as well and it takes more exposure to create adequate vitamin D. This is thought to be because of decreased production of the 7-dehydrocholesterol in older individuals. One other thing that I will mention is where you live and time of year. Above or below 40 degrees north or south latitude there may not be enough ultraviolet light during winter months to allow the process to work properly. For us in Colorado, 40 degrees north latitude will cross in the northern part of the greater Denver area. So, north of this area during some of the winter months vitamin D synthesis through the skin will be decreased.
Conclusion
So in short, no it is not possible to ‘overdose’ on vitamin D from sun exposure. Our bodies are amazing and intelligent and will regulate the production of vitamin D so this will not happen. I love learning about the processes of the body and being able to help my patients unlock the healing potential of their own bodies.
