Vitamin D has been garnering a lot of media attention. Why all the fuss? Research has shed light on the various functions of vitamin D, its role in health maintenance, and the fact that many people may not consume or produce adequate amounts, which results in subpar vitamin D status.
What exactly is vitamin D? It is not a vitamin, but a prohormone that can be made from the skin in the presence of sunlight. Vitamin D, like A, E, and K, are fat-soluble vitamins, meaning that they are typically stored over longer periods in body tissues compared to water-soluble vitamins.
Vitamin D exists in two forms—vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is added to foods, whereas vitamin D3 is synthesized in the skin and occurs naturally in a few animal foods.
What makes vitamin D unique is that we can actually synthesize it with sunlight, which activates the pre-vitamin D3 in our body to the active form (calcitriol). This conversion happens in the liver and kidneys.
To keep vitamin D levels from getting too high, the body has a very efficient “checks and balances” system in place. If calcitriol levels are high, the kidneys don’t activate as much vitamin D. The body can also inactivate what is not needed so that excess vitamin D is excreted . However, this system can be overridden by extremely high doses, so it is important to follow proper dosage recommendations on labels or as directed by a health care professional.
Vitamin D is best known for its role in the maintenance, mineralization, growth and remodeling of bone. Vitamin D deficiency can result in thin, brittle bones. It aids in calcium and phosphorus balance and promotes calcium absorption in the gut. It also influences gene expression. Vitamin D also maintains many other aspects of health, including:*
- Cardiovascular and metabolic health*
- Skeletal muscle function*
- Immune function*
- Memory and cognition*
- Digestive health*
Vitamin D deficiency is an increasingly recognized global public health concern. It is impossible to know whether your levels are low unless you get a blood test. The best marker of vitamin D status is the serum concentration of 25-hydroxy vitamin D, or 25(OH)D, which is also known as calcidiol.
About half of Americans are estimated to have suboptimal levels. According to a 2011 study, 41.6% of adults in the U.S. were deficient in vitamin D. Prevalence is higher among Hispanics (69.2%) and African-Americans (82.1%). The Institutes of Medicine defines a Vitamin D-ficiency as serum calcidiol levels below 20 nmol/L. Insufficiency is defined as < 50 nmol/L. Achieving 80 nmol/L through supplementation may help to maintain healthy levels during the winter months.
Why do people have such low levels of vitamin D?
- Little to no sun exposure
- Other aspects of health, such as glucose levels, obesity, and kidney function can affect vitamin D status
- Aging reduces the skin’s ability to produce Vitamin D
- Darker skin reduces vitamin D synthesis from sunlight
- Impact of genetic factors
- Low fat diets reduce absorption
It is important to have your vitamin D levels measured. We don’t wear a D-ficiency externally.
|Sufficient||≥30 (>40 for bone health)||90-100|
|Toxicity||>150 with hypercalcemia||>374 with hypercalcemia|
So how much vitamin D do we need? The RDA (Recommended Dietary Allowance) is the average daily level to meet the nutrient requirements of healthy people. The Institutes of Medicine suggests 400 IU/day for first year of life, 600 IU/day until age 70 and then 800 IU/day . The Endocrine Society suggests children and adolescents may need up to 1000 IU/day and those over the age of 18 may need 1500-2000 IU/day to achieve serum D levels above 30 nmol/L. So, is it realistic to meet our needs through food? On average, vitamin D intake from foods in males is 204-288 IU/day, and for females it is 144-278 IU/day.
Who is at risk for low vitamin D Levels?
- Those who consume little or no milk or fish
- Individuals who are sensitive to the sun (photosensitivity)
- Those who have difficulty absorbing fat
- Heavy sunblock users (SPF > 8 blocks 98% of vitamin D synthesis)
- Those residing at high latitudes: Locations above 35 degrees N generally do not support vitamin D synthesis during the winter months
- Individuals taking certain medications, such as anticonvulsants, antifungals, corticosteroids, histamine type 2 receptor blockers and theophylline
Supplemental vitamin D may be contraindicated with the following medications, which may increase serum concentrations to a potentially toxic level:
- Thiazide diuretics
- Some statins for cholesterol (Lipitor and Crestor, but not Zocor)
Food is important for fuel, but diet alone cannot correct a D-ficiency. The following table lists food sources and amounts of vitamin D. Not everyone loves cod liver oil, has access to wild salmon or likes mushrooms. If foods on this list do not appeal, and your are D-ficient, a supplement is the way to get your needs met.
Food sources of Vitamin D
|FOOD||AMOUNT||Vitamin D/serving (IU)|
|Cod liver oil||1 TBSP||1360|
|Wild salmon||3.5 oz||980|
|Raw maitake mushrooms, sliced||1 cup||786|
|Swordfish, cooked||3 ounces||706|
|Sockeye salmon, cooked||3 ounces||447|
|Sun dried shiitake mushrooms||1 oz||400-500|
|Canned sardines||3.5 oz||270|
|Farmed salmon||3.5 oz||250|
|Skim milk with D added||8 ounces||120|
|Orange juice with D added||8 ounces||100|
Groups who may want to consider vitamin D supplementation include those who:
- Train/work indoors or outdoors in early morning/evening in the winter
- Live at >35 degrees N or S latitude
- Reside in cloudy, polluted environments
- Wear sunscreen regularly and protective clothing
- Have very dark or very light skin
- Carry excess body fat
- Are seeking support for immune function*
- Have difficulty absorbing fat-soluble nutrients
Individuals with limited sun exposure require at least 1500-2000 IU/day to keep serum vitamin D in the sufficient range (>30 ng/mL). People who have digestive issues, excess body fat, or are taking medications affecting vitamin D metabolism, may require more. Individuals with deficiency may need 50,000 IU/week x 8 weeks to achieve a serum concentration of >30ng/ML followed by maintenance therapy of 1500-2000 IU/day.
Too much D may result in anorexia, weight loss, polyuria, and heart arrhythmia. In addition, vitamin D toxicity can increase blood levels of calcium, resulting in arterial calcification and damage to the heart and kidneys.
For athletes and active individuals, a healthy body is your best bet for optimizing performance. If your Vitamin D levels are less than optimal, you may be weaker, slower, and more prone to injury. In addition, frequent vigorous and/or high intensity exercise or two-a-day training sessions can adversely affect the immune system. An ill athlete cannot train and/or compete at optimal levels. For the athlete who travels, has erratic meal times or is not a regular consumer of fatty fish, dairy and mushrooms, it is extremely challenging to meet Vitamin D through diet. For these reasons, optimizing Vitamin D is a must for athletes and active individuals. Even in Filling the D gap can help athletes capitalize on a healthy supporting structure, and healthy immune system so you can give it 100% every day.*
No matter which supplement or level of D you choose, it is still important to have levels monitored to see the impact of supplementation. More is appropriate in the absence of enough, but is not necessary in the presence of optimal. To get the most out of your D supplement, remember that the presence of fat in a meal enhances absorption.*
Do your due diligence, know your D levels, and consider dietary changes and supplementation to customize and optimize your health.
- Forrest KYZ, Stuhldreher WL. Prevalence and correlates of Vitamin D deficiency in US adults. Nutrition Research. 2011;31(1):48-54.