The benefits of vitamin D supplementation during the Canadian winter.

Numerous epidemiologic studies have suggested exposure to sunlight may improve certain health factors due to an increase in vitamin D levels. Long associated with its role in bone formation, vitamin D is an active prohormone that powerfully influences virtually every cell in the body and builds defences to many chronic health conditions. The question is, are you getting enough?

A Review of Vitamin D Synthesis

Health Implications

Traditionally, we are aware, worldwide, that vitamin D is important for the building and maintenance of strong bones and teeth. For children, vitamin D is important in the prevention of rickets, while adults should be concerned with prevention of osteomalacia (softening of the bones) and osteoporosis (fragile bones). Recommendations for the dietary reference intakes (DRI) in Canada were developed to address prevention of bone disease, and most Canadian research, therefore, is based on DRIs developed with the limited goal of bone disease prevention. Unfortunately, the current research may disregard some other, very important chronic health conditions that may be affected by vitamin D insufficiency.

We know that calcium is an important nutrient for bone health and muscle function. An article cited in the Current Opinions of Endocrinology Diabetes in 2002 notes that humans require vitamin D in sufficient levels to absorb calcium; without vitamin D, the small intestine absorbs no more than ten to fifteen per cent of dietary calcium. In a person with sufficient vitamin D, the small intestine absorbs, on average, thirty per cent of dietary calcium; during growth, lactation, and pregnancy, the efficiency increases to eighty per cent! This alone justifies the importance of sufficient vitamin D levels for children and individuals planning on having children.

Muscle tissues carry specific receptors that are designed to accept vitamin D only, which may suggest it has an important role in muscle function, as reported in the American Journal of Clinical Nutrition in 2006. As calcium is also essential for muscle function and strength, having both nutrients at adequate levels can reduce muscle weakness. Ensuring that active individuals contain sufficient amounts of vitamin D will assist with optimal muscle function and performance. More research is required in this area, but meeting vitamin D requirements certainly maximizes calcium absorption. All Canadians benefit from that, active or not.

Other Health Factors to Considider

Beyond bone and muscle health, current evidence and continuing research indicate an important role for vitamin D and the prevention of or therapy for cancer, obesity, and autoimmune disease (such as multiple sclerosis, rheumatoid arthritis, and Type 1 diabetes). As well, vitamin D may assist in boosting the immune system. One well-documented effect of living at a higher latitude is the increased risk of vitamin D deficiency, which can, therefore, increase the risk of some common diseases. This is important information for all Canadians.

The most intriguing, important, and unappreciated biologic function of vitamin D is its ability to down-regulate hyperproliferative cell growth, according to Feldman et al. in research published in the journal Endocrinology in 2000. Normal and cancerous cells that have vitamin D receptor sites often respond by decreasing their proliferation and enhancing their maturation. This means vitamin D can kill cancer cells.

The strongest evidence for the protective effect of vitamin D against cancer was reported in connection with colorectal cancer, according to Cancer Epidemiology, Biomarkers and Prevention in articles cited in 2004 and 2005. Not only do rates of colorectal cancer rise with increasing distance from the equator (higher latitudes), but higher serum vitamin D levels and higher vitamin D intakes are independently associated with lower risk, according to Grant in the journal Cancer in 2002, research that was confirmed by Jarvinen et al. in the European Journal of Clinical Nutrition (2001).

Both Bell et al. and Wortsman et al. note that obesity is often associated with vitamin D deficiency, as cited in the Journal of Clinical Investigation (1985) and the American Journal of Clinical Nutrition (2000). The vitamin is stored in fat cells very efficiently, therefore leaving little vitamin D bio-available within the human body. Many obese individuals are chronically deficient in this nutrient, leaving many unanswered questions and potentially new research in the field of obesity prevention and therapy.

The link between vitamin D and the boost to the immune system is strong; there is abundant research in the field. Vitamin D was originally "discovered" as the only treatment of rickets, a deforming childhood bone disease; the more sunshine a child received the less risk they had of developing this condition. This is also the case for tuberculosis (TB); Tavera-Mendoza and White, cited in the Scientific American Magazine in 2007, note that in the early twentieth century, the only known treatment for TB was rest and exposure to sunlight.

Research into the area of the flu virus is growing. It is often shown that this contagious virus is highest during the winter months, when sun exposure is decreased. This "seasonal stimulus," as proposed by Hope Simpson in 1981, continues to be a hot topic. Vitamin D dramatically stimulates the expression of potent antimicrobial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract, where they play a major role in protecting the lung from infection, said Cannell et al. in their research in 2006 in Epidemiology and Infection.

The evidence is strong in support of maintaining your vitamin D levels at optimal or sufficient levels. But what does this mean to you?

How to Measure Vitamin D Sufficiency

Holick notes in the American Journal of Clinical Nutrition (2004) that the metabolite best tested for true vitamin D status is the 25-Hydroxyvitamin D [25(OH)D]. This is a simple blood test, non-fasting, having a half-life of approximately two weeks. Keep in mind, your personal serum levels can change seasonally, as they may be higher in the summer months due to increased sun exposure; therefore, getting a test at the end of summer, when you may have been exposed to more sunlight, might not be the best indicator of your entire year's vitamin D levels. Test your vitamin D levels tested in winter (November to January) to truly determine if your stores are adequate and if the food sources or supplements you are consuming are enough to maintain sufficient levels.

As Canadians have limited exposure to the sun during the winter months, it is very common to have low levels of vitamin D, a theory proven within my own practice as we screen all clients on a regular basis. In general, consume a supplement containing minimally 400 IU per day of vitamin D, whether on its own or within a multi-vitamin. Upon a comprehensive nutrition analysis and lifestyle assessment, which includes blood work of your vitamin D levels, this recommendation may change based on unique needs.

What You Can Do

Are You Getting Enough Vitamin D?
Factors to consider for vitamin D sufficiency:
Here is a sample list of staples that includes a good balance of carbs, proteins, and fats to get you started:
• Sun exposure
The most natural source of vitamin D is through direct skin exposure to ultraviolet light (UVB); this allows for photosynthesis and conversion of 7-dehyrocholesterol into a bio-available form of vitamin D. Knowing that sunlight is the best
source of vitamin D, it is important that we still obtain this source; however, we must still factor in skin health and avoid burning or cancer risk. Peak times of photosynthesizing vitamin D occur between the months of April and October,
between 10:00 a.m. and 3:00 p.m.
• Sunscreen use
Holick notes in the American Journal of Clinical Nutrition (2004) that sunscreens efficiently absorb UVB rays and thus markedly diminish the ability of vitamin D absorption. If properly used, a sunscreen with a protection factor of eight reduces cutaneous production of vitamin D by ninety-five per cent, and a sunscreen with a sun protection factor of fifteen reduces capacity by ninety-nine per cent.
• Skin colour
Melanin is a natural sunscreen in the human body that can also compete for UVB photons with vitamin D. Those with darker skin have greater amounts of melanin in their epidermis and are therefore less efficient at producing vitamin D.
• Food sources
Although limited, there are food sources that can increase your levels of vitamin D.

About the Author

Lindy Kennedy, M.Sc., R.D., is the lifestyle director and registered dietitian for Preventous Collaborative Health and owns and operates FitNut Consulting Inc., www.fitnutconsulting.com, www.preventous.com.

"The Sunshine Vitamin" was first published in the November/December 2009 Winter Running & Fitness Issue of IMPACT Magazine.