Supplementation of nutrients seems so controversial amongst medical/naturopathic/fitness professionals and enthusiasts. You have probably heard/read things like:
- Supplements are expensive urine.
- They’re not regulated by the FDA.
- This is the best fat burner… proceeds to point at CLA (eye roll)
- You don’t need Vitamin D, just eat a balanced diet and get sunlight!
Well….those statements are predominantly FALSE + Wtf is a balanced diet..? Asparagus spears in one hand and a brownie in the other? Although, the sunlight statement is true-ish, which leads us to the article’s topic: Vitamin D.
What is Vitamin D?
Vitamin D is a fat soluble vitamin (one of the 24 essential micronutrients) that is produced by exposure to sunlight. Vitamin D is actually one of the few vitamins we (humans) can produce depending on our exposure to sunlight. It is estimated that somewhere near 1 billion people are deficient or have some sort of vitamin D insufficiency . One study found that 88.1% of participants were insufficient . For the sake of background knowledge, here is a quick physiology primer on Vitamin D3 synthesis:
- Bare skin comes in contact with ultraviolet B rays and pre-vitamin D is produced .
- Pre-vitamin D attaches to a binding protein that is then transported to the liver .
- Once in the liver, it is converted to calcidiol and is making itself available for tissues that need it (this is what is measured by doctors) .
- When the kidneys or tissues (brain, immune cells, cardiac muscle, etc.) need vitamin d, it is converted to calcitriol (hormonally active form) and is ready to be used .
Functions of Vitamin D
Vitamin D is considered by some to be more a steroid prohormone than it is to be a vitamin. This is because vitamin D as a precursor is a steroid , and in its later forms is involved as an endocrine molecule in calcium absorption and interacts with signals from the parathyroid and kidneys . Knowing this, it is safe to say that vitamin D plays a key role in bone health by shuttling calcium to the bones from the diet. Other functions of vitamin D may include (more studies needed, as always):
- Decreased cardiovascular disease risk 
- Decreased colorectal cancer risk 
- May help increase testosterone 
- Decreased fat mass 
- Increased insulin sensitivity 
Among other things.
How Much to Take and Where to find Vitamin D
Vitamin D is naturally produced when our skin comes in contact with sunlight, as mentioned above, but there are other sources of vitamin D. Keep in mind, there are no GREAT oral sources of vitamin D besides taking a supplement or cod liver oil. NOTE** The following statement is according to my own findings on previous patients, not on clinical research trials** Based on what I have seen in my practice, analyzing thousands of patients labs, most were deficient (<30 ng/mL) or suboptimal (<40-100ng/mL according to the Vitamin D Council). Interestingly enough, even some who were taking a multivitamin (typically containing 400 IU of vitamin D) or vitamin D on its own (between 1,000 - 2,000 IU) were still under 40 ng/mL. Based on these findings, I typically suggest 5,000 IU of vitamin D3 with food, preferably a fat containing food (olive oil, avocado, fish oil, or butter coffee if you’re into that kinky stuff) to help with absorption since vitamin d is fat soluble. Another interesting find via my experience as an R.D. is that everything truly comes down to the individual. Some folks claimed to take 5,000 IU daily and with food and their levels were still <40 ng/mL. These individuals were recommended to take a higher dose (6,000 IU – 10,000 IU) daily. Keep in mind, these patients were having their levels measured on a regular basis to ensure we were not reaching toxic levels (>150 ng/mL). Some doctors may also prescribe a weekly dose of 50,000 IU.
A few things to note on what affects vitamin D absorption:
- Darker skin pigmentation
- People with inflammatory bowel disease 
- Minimal skin exposure or wearing clothes in the sun (ideally we would get sun while naked, but that’s not ideal nor legal in many places, so don’t get arrested and blame it on me or vitamin D)
|Food||IUs per serving*||Percent DV**|
|Cod liver oil, 1 tablespoon||1,360||340|
|Swordfish, cooked, 3 ounces||566||142|
|Salmon (sockeye), cooked, 3 ounces||447||112|
|Tuna fish, canned in water, drained, 3 ounces||154||39|
|Orange juice fortified with vitamin D, 1 cup (check product labels, as amount of added vitamin D varies)||137||34|
|Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup||115-124||29-31|
|Yogurt, fortified with 20% of the DV for vitamin D, 6 ounces (more heavily fortified yogurts provide more of the DV)||80||20|
|Margarine, fortified, 1 tablespoon||60||15|
|Sardines, canned in oil, drained, 2 sardines||46||12|
|Liver, beef, cooked, 3 ounces||42||11|
|Egg, 1 large (vitamin D is found in yolk)||41||10|
|Ready-to-eat cereal, fortified with 10% of the DV for vitamin D, 0.75-1 cup (more heavily fortified cereals might provide more of the DV)||40||10|
|Cheese, Swiss, 1 ounce||6||2|
** DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration to help consumers compare the nutrient contents among products within the context of a total daily diet. The DV for vitamin D is currently set at 400 IU for adults and children age 4 and older. Food labels, however, are not required to list vitamin D content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
The Take Home
Vitamin D is essential for optimal body functions. Based on studies mentioned above and my own experience, many of us are deficient or have suboptimal levels of vitamin D. In my practice, ~90% of the patient labs I looked at had a vitamin D blood level of <40 ng/mL. Be sure to get your levels measured by your health professional and supplement accordingly. It may be affecting your gains.