Vitamin D: 5 Key Questions Answered
October 23, 2012
Culinary Nutritionist Andrea Canada answers everything you wanted to know about Vitamin D (the sunshine vitamin) – and more!
Fall is here in all its apple picking, pumpkin carving and squash soup glory. But with this change in seasons also comes shorter days, more time spent indoors, a change in the angle of the sun and, depending on where you live, a halt to your body’s production of vitamin D.
Vitamin D has been getting quite a bit of attention in the past few years, becoming a darling of the supplement world. According to the Nutrition Business Journal, vitamin D supplement sales jumped from $42 million to $605 million from 2002 to 2011.
So what’s all the fuss about and how can you know if you are making enough vitamin D?
What is vitamin D?
Vitamin D is a fat soluble vitamin (and technically a hormone) that has long been associated with calcium homeostasis, bone health and the prevention of rickets. While naturally present in few foods, humans can synthesize vitamin D from sun exposure on the skin. The richest food sources of vitamin D include fatty fish (salmon, mackerel, sardines, canned tuna in water, cod liver oil), and mushrooms exposed to UV light. A small amount can be found in beef or calf liver, eggs and cheese. Many foods today are fortified with vitamin D, including milk, yogurt, many brands of orange juice and breakfast cereals (which typically contain 100 IU per serving). Once vitamin D is synthesized or ingested, it must be converted by the liver and then the kidney to its final activated form.
How much vitamin D is recommended? The Daily Reference Intake for Vitamin D for most adults is 600 IU per day and has been based on the maintenance of bone health and calcium absorption. However, vitamin D receptors have been found on virtually every cell in our body and many studies have shown its association with the prevention of dementia, diabetes, cancers, and autoimmune diseases, so there is some debate whether this is sufficient.
Do our bodies make enough vitamin D?
With less time spent outdoors and an increased use of sunscreen for prevention of skin cancer, our odds of making vitamin D from the sun have decreased. In order for our bodies to make the recommended amount of vitamin D, we need to sit in the sun with our arms and legs exposed for at least 30 minutes twice a week between the hours of 10am to 2pm. That’s without sunscreen, because an SPF as low as 8 can prevent 95% of vitamin D production. Also factor in the fact that glass, pollution, and clouds reflect UVB rays back into the atmosphere, which further decrease our ability to synthesize vitamin D.
Our body’s ability to make vitamin D also decreases between November - March for those of us living in the Northern Hemisphere (in the US, this includes the area above the state lines of California, Nevada, Utah, Colorado, Nebraska, Kansas, Missouri, Illinois, Indiana, Ohio, West Virginia, Pennsylvania and New Jersey). A general reference is to look at your shadow between the hours of 10am to 2pm. If your shadow is longer than you are tall, you are not making vitamin D.
A simple blood test of your serum levels of 25-hydroxyvitamin D from your doctor can help determine if you have enough of this “sunshine vitamin”. According to the US National Institutes of Health (NIH), the normal range is 30.0 - 74.0 ng/mL. Keep in mind the results of this test could vary with the time of year as your levels are likely much higher at the end of the summer versus the end of the winter.
Are some people at greater risk for vitamin D deficiency than others?
Groups at higher risk include dark-skinned Americans because of the higher concentration of melanin, which is responsible for skin pigmentation. Dark skin acts as a natural sunscreen, blocking the UVB rays that trigger vitamin D synthesis.
Age also plays a role in vitamin D deficiency. As we age, our ability to synthesize vitamin D decreases. It’s further reduced when you consider the majority of the older population that is homebound. On the other side of the spectrum, breastfed infants should receive a supplement of up to 400 IU daily because breast milk alone does not supply enough vitamin D. The amount also depends on the mother’s vitamin D status.
The obese (BMI > 30) are also at risk for vitamin D deficiency. Being a fat soluble vitamin, it is stored in fat cells, which decreases the concentration circulating in the blood.
Moreover, people with malabsorption diseases (especially fat malabsorption) are at risk for vitamin D deficiency. This includes those with Crohn’s, celiac, irritable bowel disease, post gastric bypass surgery and liver and kidney disorders.
Also keep in mind that vitamin D absorption decreases in the presence of antacids, calcium channel blockers, some cholesterol lowering medications, steroids, anticonvulsant medications and mineral oil.
What are the symptoms of vitamin D deficiency? Can I have too much vitamin D?
Vitamin D deficiency is commonly associated with the softening of bones, also known as rickets in children and osteomalacia in adults. Symptoms related to the softening of bones include muscle weakness and bone pain. If not addressed, it may lead to osteoporosis as you get older.
Of equal importance is vitamin D toxicity, where symptoms include weight loss (due to decreased appetite), abnormal heart rhythms, kidney and tissue damage (due to calcification) and mood disorders. The upper limit as recommended by the Institute of Medicine (IOM) is 4000 IU daily.
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