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Vitamin D: More Than Bones, But Treated Like Only Bones Matter

The athletic department I work within recently updated their vitamin D protocol. In reviewing the research, there were a lot of surprises regarding both the uncertainty of what constitutes a normal blood value and the purpose of “optimizing” vitamin D blood levels.

So here’s my nerdy sunshine vitamin rant.

And in case you become confused:

  • International units (IU): Unit of vitamin D in the food supply (or micrograms, so 1 mcg = 40 IU).

  • 25(OH)D: What doctors measure as vitamin D in the blood.

  • ng/mL: The measurement used for blood values of vitamin D (1 ng/mL = 2.5 mmol/L).

Surfers likely make enough vitamin D from sunshine during the summer.

Depending on What You Read, the Deficiency Level and Dosing Levels Differ

Let’s begin with a few federal documents.

The original 2011 report by the Institute of Medicine (now the National Academy of Medicine), who determined vitamin D recommendations, listed that a 25(OH)D value of less than 20 ng/mL is considered deficient—yet this value was solely based on optimizing bone health.

Moving to the current National Institutes of Health Office of Dietary Supplements vitamin D factsheet for professionals (last updated in March 2018), lists a deficiency as less than 30 ng/mL. Values below this increase one’s risk of poor bone health.

But what does the current research say? It’s all a grey zone.

Researchers tend to agree that a very deficient person will have a 25(OH)D of less than 20 ng/mL and an optimal level falls somewhere in the range of 30-50 ng/mL (if not higher).

In between 20-50 ng/mL (i.e., “insufficient”), recommendations for how much supplemental vitamin D to provide an insufficient athlete is all over the place.

But here’s the concern for the 20 ng/mL deficient value: It doesn’t line up with optimizing other concerns. For instance, at least:

Vitamin D is a Hormone and has Multiple Functions

I love vitamin D because it seems that there are always new functions being discovered. Here’s a short list:

  • Skeletal muscle repair and remodeling (3) (5).

  • Enhanced adaptive response to exercise.

  • Inflammatory modulation (5) (6).

  • Improvements in immune functioning, minimized infection risk, and reductions in the duration and severity of sickness (2) (3) (4) (5) (6).

  • Calcium absorption from the intestine. When 25(OH)D is above 30 ng/mL, calcium absorption increases to greater than 30% [compared to 10-15% when 25(OH)D is low] (7) (8).

So is it fair to base vitamin D recommendations solely on bone health? Probably not.

Nutrition Labeling Isn’t Based Off of the Daily Recommendation

This drives me nuts, since educating athletes about food isn’t already a challenge.

One would think that the percent daily value on a food package’s nutrition label would be based off the daily recommendation of 600 IU for men and women ages 19-50 years.

Nope. The percent daily value for vitamin D is based off of 400 IU. Note that once the updated nutrition facts label becomes mandatory, the vitamin D value will increase to 800 IU by January 1, 2021 for all companies.

However, some companies are already using the new label--only adding to the confusion.

For instance, and as of right now, if you’re choosing a 16-ounce bottle of white milk that lists 25% vitamin D per serving you’re only consuming 100 IU. If you do your math wrong, you come up short 50 IU. And for a nutrient that is naturally sparse in our food supply, and arguably should have a higher daily recommendation somewhere between 1,000-2,000 IU, I think elusive labeling is problematic.

The percent daily values comparing the old (left) and new (right) nutrition fact labels. Source: U.S. Food and Drug Administration

The Final Kicker: Obese, Thin, and Dark-skinned Individuals Tilt the Recommendations on Their Head

The process of creating vitamin D in the body begins when skin is exposed to sunshine. Yet vitamin D is fat-soluble, meaning that it can be stored in the body’s adipose (fat) tissue. Research exists to show that obese individuals (i.e., body mass index greater than 30 kg/m^2) require more dietary and supplemental vitamin D as the amount their body makes is somewhat trapped (i.e., unable to be used throughout the body). For obese BMI athletes with deficient and insufficient 25(OH)D levels, they likely need greater supplemental doses to pull them up into the optimal category (8) (9) (10) (11).

On the other hand, very thin athletes have low body fat levels. During the winter months, their vitamin D fat stores may eventually drop and will likely require supplementation.

Lastly, the melanin pigment in a dark-skinned athlete’s skin that provides them with color reduces their ability to create vitamin D when exposed to sunlight. Yet even with low 25(OH)D levels, African Americans tend to have a lower bone fracture risk and a higher bone mineral density when compared to Caucasians (i.e., the complete opposite of current knowledge). The question then becomes, "is 25(OH)D the best way to measure vitamin D?"

Gymnasts are a group of athletes known for their low body fat percentages.

Take-away Message: Apply Flexibility to Your Plan

Since the research tends to hint that individuals respond differently to vitamin D supplementation when deficient or insufficient, it’s best to first create a protocol of how to optimize an athlete’s status, track the results, and adapt the protocol to the athlete as needed.

Also consider:

  • Trends over time: If an athlete has fluctuating 25(OH)D levels, once optimized you may want to keep them on a maintenance dose.

  • Educating athletes about dietary sources of vitamin D: Check out table 3 at this link. For how important vitamin D seems to be throughout the body, it’s sad that Mother Nature somewhat forgot to add it to our food supply…

  • Calcium education for optimizing bone health: The daily recommendation sits at 1,000 mg for men and women ages 19-50 years. Calcium exists throughout the food supply, so this is an easier dietary intervention than vitamin D. Since the Institute of Medicine and National Institutes of Health bases their deficient value on bone health, let’s keep calcium in the care plan. Measuring blood calcium is pretty useless (it's tightly regulated and won't necessarily show a deficiency) if you think an athlete has a calcium deficiency then consider measuring both 25(OH)D and parathyroid hormone.


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