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Vitamin D: A Key To Endurance Sports Performance

Low levels are likely limiting your performance, and may compromise your health.

Low Vitamin D levels are very likely limiting your running performance, and may compromise your health.

As runners, we want to do everything possible to perform well and have a great season. We are meticulous about our training schedules, hydration, sleeping habits, etc. Yet many times we fall short when it comes to nutrition.

My Story

In the summer of 2008, I started to feel rather lethargic during workouts and I struggled to maintain my normal training paces. My recovery from my harder efforts and long runs took longer than usual. I immediately and incorrectly assumed I had low serum ferritin levels indicating iron deficiency anemia.

One colleague to whom I lamented about my chronic fatigue asked me if I had checked my Vitamin D levels. “Vitamin D levels?” I exclaimed. I live in California, it is summer, and I am outside way more than the recommended 20 to 30 minutes a day. There is no way I could be Vitamin D deficient, especially since in addition to being exposed to sun most days, I eat a very balanced diet that includes a daily multivitamin supplement. However, a visit to my doctor confirmed that my serum 25–hydroxy-Vitamin D (25(OH)D) value was dangerously low (18 ng/ml). Normal levels are between 40-70 ng/ml. And for those with chronic diseases such as cancer, heart disease, diabetes, and multiple sclerosis, normal blood levels should be between 55-70 ng/ml. I was shocked.

I immediately started daily supplementation with 1,000 IU of vitamin D (Vitamin D3). After eight weeks, my 25 (OH)D level had improved somewhat (28 ng/ml). I did feel slightly better, but I wanted to recover completely, so I upped my dosage to 2,000 IU daily. A few weeks later my levels were within the normal range (56 ng/ml). I felt noticeably stronger and was able to hit my usual paces during training. And in October 2008, I set a half-marathon personal record.

RELATED: Vitamin D Supplements

Vitamin D Deficiency

This was my very first experience with Vitamin D deficiency and I have since learned that Vitamin D deficiency is becoming an epidemic worldwide, not only in geographic regions where sun exposure is limited. And my discussions with fellow dietitians working with college runners and professional athletes in generally sunny states (Texas and Florida) confirmed the alarming prevalence of Vitamin D deficiency across ethnicity and gender.

Athletes who live in northern latitudes (north of 35 degrees), or use sunscreen consistently, perform their sport indoors, or keep their skin covered are at the greatest risk. Melanin affects the production of Vitamin D. So those with more melanin, or darker skin, produce less Vitamin D.

Since Vitamin D is a fat-soluble vitamin, athletes with fat malabsorption problems such as cystic fibrosis, Crohn’s disease, and celiac disease are at risk for deficiency. Those who have normal levels typically (around 50 ng/ml) live in sub-equatorial Africa and work outdoors for most of the summer.

Once thought of as being primarily involved in bone development, activated Vitamin D (calcitriol), a steroid hormone, is responsible for regulating more than 1,000 human genes. Almost every cell in the human body has receptors for Vitamin D. Recent research shows that Vitamin D deficiency increases the risk of different types of cancer (such as breast cancer and prostate cancer), as well as heart disease, diabetes, depression, autoimmune diseases, hypertension, obesity, gum disease, chronic pain, muscle wasting, inflammation, birth defects, osteoporosis, influenza, colds, etc.

Importance For Endurance Athletes

We are only just beginning to understand the complexity and importance of Vitamin D in relation to health. Of importance to athletes is the function of Vitamin D as it relates to overall health, bone density, innate immunity, muscle wasting, and exercise-related inflammation and immunity. To train and race optimally, an athlete should not have any nutrient deficiencies.

Bone Health
Olympic Marathoner Deena Kastor broke her foot in Beijing during the 2008 Olympic Marathon. It was discovered that her calcium levels were normal, but her 25 (OH)D levels were reported to be around 15 ng/ml. And Kastor lives in sunny California. Because of an early scare with skin cancer, she is known to apply sunscreen for all of her outdoor runs, thus limiting her ability to manufacture Vitamin D from sun exposure.

Even with the extensive research to show Vitamin D and calcium’s role in preventing osteoporosis, elite, college, and high school athletes continue to be deficient in one or both nutrients. Stress fractures are quite prevalent in runners and yet so preventable.

RELATED: Are You Consuming Enough Minerals?

Increased VO2 max
German research studies dating back to the 1950s show that athletes exposed to Vitamin D-producing ultraviolet light had improved athletic performance. Other studies showed that athletic performance peaked at the end of the summer. Peak performance was also associated with 25 (OH)D levels around 50 ng/ml.

In addition, maximal oxygen uptake was found to drop when less ultraviolet rays reached the earth, for example, in the late fall. This is particularly a problem for marathoners training through the summer for fall marathons.

Reduced Inflammation
After intense exercise, endurance athletes experience inflammation due to elevated levels of pro-inflammatory cytokines. Vitamin D reduces the production of these cytokines while increasing the production of anti-inflammatory cytokines, thereby speeding the recovery process between hard workouts.

Improved Immunity
In a February 2009 study published in the Archives of Internal Medicine, Vitamin D3 levels were tested in 19,000 Americans. Those with low levels of Vitamin D had the highest incidence of colds and influenza. This is important information for endurance athletes who strive to balance heavy training loads and staying healthy.

What You Can Do

– Check 25(OH)D levels regularly and supplement as needed.
– Check for total 25(OH)D and not 1, 25(OH)D, which will tell you nothing about your blood stores. Total 25(OH)D reflects all sources of Vitamin D – from food, UV energy (photo-production), and supplementation.
– Deficient athletes measuring less than 30 ng/ml should supplement with 20,000 IU to 50, 000 IU of vitamin D3 per week for eight weeks andrecheck serum 25(OH)D until normal values are attained.
– Get regular, safe, twice-daily (5-30 minutes) exposure to sun between the hours of 10 am and 3 pm. Note that sunscreen and glass (being indoors) reduce or block UV energy.
– Supplement with 1000 IU to 2000 IU of Vitamin D3 to maintain normal levels.
– For those living or competing in northern latitudes (north of Atlanta) little to no vitamin D production will occur, so consumption of fortified foods and supplements is a necessity.

Food Sources of vitamin D

There aren’t many naturally occurring foods that contain vitamin D. Most of the foods containing Vitamin D have been fortified, for example milk and certain juices.

RELATED: The Protein All Runners Need

Table 1. Food Sources of Vitamin D

FOOD Serving IU per serving *
Fish liver oils (cod liver oil) 1TBSP 1360
Herring, cooked 3 oz 1383
Wild salmon, cooked 3.5 oz 981
Farm salmon, cooked 3.5 oz 249
Tuna, canned in oil 3.5 oz 200
Milk, non fat, reduced fat, whole (fortified) 8 oz 100
Margarine, fortified 1 TBSP 60
Egg 1 whole 18
Beef, liver 3.5 oz 15
Cheese 1 oz 12
*IU = International Units

Dietary Supplement Fact Sheet. National Institute of Health (2007)


High intakes of Vitamin D can cause nausea, vomiting, poor appetite, weakness, and constipation. Current safe upper limits are set at 2,000 IU by the National Institute of Health, but there are newer data supporting upper limits as high as 10,000 IU per day.


About The Author:

Reyana Ewing, MPH, RD, CLE is a registered dietitian, sports dietitian and runner based in Santa Rosa, Calif. Find our more about her at