You might already know that the body uses sunlight to produce vitamin D, but not fully understand how this process works besides...science. In a chemical reaction similar to photosynthesis in plants, your body’s skin produces vitamin D when it is exposed to ultraviolet-B (UVB) light from the sun. UVB light has a wavelength of 290–320 nanometers and is converted to previtamin D3 by uncovered skin.
Vitamin D is actually a hormone with many critical functions in the body. According to the National Institute of Health, vitamin D promotes calcium absorption and bone health and supports healthy muscular and immune function. At a cellular level, many genes that modulate healthy cell growth are dependent upon the presence of vitamin D to encode essential proteins.
Once you appreciate the importance of vitamin D to your overall health, there are simple steps you should take to ensure that your body’s needs are met. If your diet and lifestyle don't yet provide sufficient levels of vitamin D, consider taking a daily supplement.
A study published in 2010 found an estimated 41% of Americans are deficient in vitamin D. For those living in cooler climates, there is a possibility you may be deficient in vitamin D, if you don't already supplement or pay careful attention to your diet.
According to the Vitamin D Council, the best method to know if you have sufficient levels of vitamin D is through a blood test called the 25(OH)D test. The test can be administered by a doctor. The results of this test will provide an accurate reading of the vitamin D levels in your blood.
While previous recommendations for vitamin D blood levels were as low as 30ng/ml, more recent data suggests you should aim to fall between 40 and 50ng/ml. Of course, the way you achieve this optimal level will be unique to your specific diet, lifestyle, and whether you choose to supplement.
Even if you do not live in a stereotypically-sunny climate, such as southern California, you still probably receive some of your vitamin D needs through sun exposure.
Somewhat surprisingly, your geographic latitude is not a reliable predictor of the amount of vitamin D present in your blood. This is because there are many factors that impact your body's ability to naturally produce vitamin D3, including the season, cloudiness, smog levels, your skin’s melanin content, and your use of sunscreen.
Of course, the further you live from the equator, the bigger your disadvantage in the quest to soak up sun. However, even in colder climates, the spring, summer and fall months can offer the opportunity to synthesize vitamin D that can be stored as reserves in the body’s fat and liver tissue.
Researchers have suggested that approximately 5–30 minutes of sun exposure in the middle of the day (10 A.M. to 3 P.M.), at least twice a week, is needed for sufficient synthesis of vitamin D. However, it is important to note that a substantial portion of the body’s skin (either face, arms, legs, or back) must be exposed. The presence of glass, or any sunscreen with an SPF greater than 8 will block the necessary UVB rays.
Dietary sources of vitamin D3 — which is believed to be more potent than D2 when consumed at high doses — include fatty fish, beef liver, and cheese. Otherwise, vitamin D is typically found in fortified dairy, cereals, and grains. Nowadays, you’ll find vitamin D added to just about everything, from tofu to orange juice to Spaghettios.
This is largely a result of the societal shift that occurred at the turn of the 20th century. With the dawn of the Industrial Revolution, there was a massive transition from outdoor to indoor jobs, accompanied by a change in the foods people consumed most. This contributed to the rise in diseases like rickets, a bone condition that results from vitamin D deficiency. In response, many producers began fortifying foods, like milk and cereal, with vitamin D.
However, even highly-fortified foods fail to offer the Food and Nutrition Board's Recommended Dietary Allowance of 600 IU of vitamin D per day, which means multiple servings would be necessary. The National Institute of Health has set the Upper Limit for vitamin D intake at 4,000 IU per day for healthy adults. This represents a wide acceptable range for those who believe they need to supplement with vitamin D.
One important distinction to be made is that the body naturally limit its production of vitamin D from sunlight, while supplements should be used carefully to avoid side effects.
Ultimately, your vitamin D levels will reflect the combination of your lifestyle and diet. Unfortunately, many Americans do not meet sufficient levels of vitamin D. The encouraging news is that supplementing with vitamin D is easy and generally safe.
If you are still wondering if you are at risk for vitamin D deficiency, consult with a physician or take a detailed online assessment of your diet and lifestyle.