Vitamin D: The Nutrient That’s Not a Given

Fri, 24 May 2013 - 9:59 p.m. MT
Credit: ARA Staff - American Running Association

The National Center for Health Data, which compiles and dispenses statistics on the nation’s health, reports this year that nearly one in three Americans has vitamin D blood levels below the threshold that the Institute of Medicine says is needed for good bone health.

Vitamin D is a tricky nutrient to obtain through diet alone—few foods contain it naturally. Many doctors recommend taking 800 IU of vitamin D3 a day. But it’s also produced by the body in a complex process that starts when rays of ultraviolet B (UVB) are absorbed by the skin. The liver and kidneys are then involved in making available the form of vitamin D that the body can use. Now, a Swedish medical journal reports other factors that influence vitamin D level. Given that so many people are low in vitamin D, it’s worth looking closely at these factors. 

Living points north. It may not be a coincidence that the country where the journal is published is in an area of the world that receives little daylight in the winter months. At higher latitudes, the amount of UVB light reaching the earth’s surface goes down in winter. This is because of the low angle of the sun. Short days combined with cold-weather clothing covering legs and arms make UVB exposure limited at best from November through February, which in turn results in low vitamin D levels.

The air you breathe. Burning fossil fuels and wood scatters UVB rays, as well as absorbs them, by producing carbon. This reduces the amount of vitamin D you manufacture. Interestingly, however, the ozone absorbs UVB, so holes in the ozone layer caused by pollution may enhance your vitamin D levels by letting more UVB through.

Sunscreen…in theory. Though it prevents UVB from penetrating your skin, studies of sunscreen’s effects in practice have shown that its adverse influence on vitamin D are minimal. Most people do not apply enough sunscreen to block all UVB, or they simply use it in an irregular way that allows some UVB exposure. An often-cited Australian study showed no difference in vitamin D between adults randomly assigned to use sunscreen and those given a placebo cream.

Skin color. Melanin is the substance in skin that makes it dark. It “competes” for UVB with the substance in the skin that kick starts the body’s vitamin D production. As a result, dark-skinned people tend to require more UVB exposure than light-skinned people to generate the same amount of vitamin D.

Skin temperature. Warm skin is a more efficient producer of vitamin D than cool skin. This doubles down on summer’s effect on vitamin D production, since the sun’s rays are already more powerful and longer lasting during summer’s long days. The added heat facilitates the production of more vitamin D than temperatures on a cool day. 

Body fat. One frequently overlooked factor is that fat tissue soaks up and stores vitamin D, making it a kind of rainy-day fund for the substance. But other research has shown that being obese is correlated with low vitamin D levels, and that being overweight may affect the bioavailability of vitamin D. It’s unclear how these factors ultimately play out in the balance of vitamin D production, but, much like sun block’s protection from melanoma, it goes without saying that there are myriad reasons beyond effect on vitamin D that merit keeping a healthy body weight.

Age. Older people have lower levels of the skin’s conversion substance for making vitamin D than do those under 40. There’s also experimental evidence that older people are less efficient vitamin D producers than younger people. Still, the National Center for Health Statistics data on vitamin D levels does little to buttress the conventional wisdom that older folks are any more vitamin D deficient than younger people. There just isn’t a statistical drop-off after 50 in the way you’d expect if there were a significant inadequacy by age.

Gut health. The area of the small intestine just below the stomach is the one responsible for supplemental vitamin D absorption. Pancreatic and stomach secretions, liver bile, and intestinal wall integrity all influence how much D gets absorbed. Therefore, conditions that affect the gut and digestion--celiac disease, chronic pancreatitis, Crohn’s disease, even cystic fibrosis--can reduce vitamin D absorption.

Liver health. Some types of liver disease can reduce absorption of vitamin D because the ailing liver isn’t producing normal amounts of bile. With other types, steps essential to vitamin D metabolism occur incompletely or not at all. Kidney health matters too. Levels of the bioactive form of vitamin D tend to track with the health of the kidneys, so in someone with kidney disease, bioactive vitamin D levels decrease as the disease gets worse, and in end-stage kidney disease, the level is undetectable.

Knowing these factors—some of which were not previously considered as vital to vitamin D production—can help you make informed choices about whether a D supplement is necessary. Consumption of fortified dairy products like milk, which has vitamin D added to it in part because of D’s role in calcium absorption, falls off after childhood. Given this fact and the new data on how chronically undernourished we are in vitamin D, a conversation with your doctor about whether supplementing vitamin D is appropriate is certainly one worth having. 

 

Harvard Health Beat, Aug. 2011, http://tinyurl.com/4239l62

 

CDC&P, http://www.cdc.gov/nchs/

 

 

(RUNNING & FITNEWS® September / October 2011 • Volume 29, Number 5)





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