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This point has been argued extensively over the last decade, and recommendations vary greatly. A recent article published in JAMA (The Journal of the American Medical Association) helps to answer this important question.

Vitamin D is a fat-soluble vitamin, and once it’s converted to its active form, it actually acts like a hormone. Nearly every cell in your body has a vitamin D receptor, which modifies how your cells function.

For the last 100,000 years, humans have obtained vitamin D from sunlight. We basically ran around half naked hunting and gathering in direct sunlight. Today, most people wear clothing and are indoors most of the time. And when we are outside, we wear sun screen, which blocks the skin from making vitamin D when we are in the sun.

Most of us would need 1 hour of sun daily to meet our vitamin D needs from sunlight, like hanging out at the pool in a bathing suit, getting sun damage.

Don’t count on getting your vitamin D from food. Research from my clinic shows that most people need at least 2000 IU daily to reach a normal vitamin D blood level. If an egg yolk has 25 IU per egg, that would require you to eat 40 eggs per day. A glass of milk has 100 IU per cup—only 20 cups daily and your there. The reality is that you are not going to get adequate vitamin D from food.

Past studies have shown that if you have really low vitamin D levels, adding just 1000 IU of vitamin D daily with a bit of calcium lowers your risk for getting cancer by a whooping 40%. That is a pretty amazing reduction.

Other studies have shown that vitamin D helps prevent bone loss, auto-immune disease (like multiple sclerosis), heart attacks and strokes, and even memory loss.

The latest study that compares vitamin D supplements with cancer risk (published in JAMA)* was conducted in 31 rural counties in the US. It was a 4-year study that randomized 2303 post-menopausal women to vitamin D and calcium to see if this would decrease the risk for developing cancer.

The team randomized the women to receive either 2000 IU of vitamin D daily plus 500 mg of calcium three times per day (1500 mg of calcium daily), or a placebo with only 800 IU of vitamin D daily and no calcium for four years. During the study, 45 cases of cancer appeared in the vitamin D and calcium group, and 64 cases in the placebo group (a 29% reduction). When they calculated the difference in cancer rates, using advanced statistical methods, they noted that there was a 6% chance that this difference could be due to chance, and they needed less than a 5% chance to conclude that this intervention was effective. Their brief conclusion was that this intervention was not effective in reducing the risk of cancer significantly.

The results and the longer discussion at the end of the article was far more interesting and revealed much more valuable information. My bias is that this would have been a better study if they had not included calcium with the vitamin D and would have been better served to have looked at vitamin D alone.

Commonly during cancer studies, you don’t count cancers that occur during the first year. (After all, taking vitamin D for a few months isn’t long enough to prevent cancer.) If researchers detect cancer during the first year, the women may have had it before the study started.) Once the researchers excluded first year cancer occurrence, and looked at years 2-4 of the study, they did see a significant 34% reduction in cancer risk from taking 2000 IU of vitamin D (plus calcium), compared with taking 800 IU daily.

Further, the researchers noted that the vitamin D blood level achieved was a better predictor of preventing cancer than how much of a vitamin D dosage a woman took. If vitamin D levels increased from 30 ng/mL to 55 ng/mL with 2,000 IU daily there was a 35% reduction in cancer rates, again highly significant.

More studies are planned to assess the optimal dosage and blood level for vitamin D.

In the meantime, here is what I recommend:

  1. Take 2000 IU daily
  2. At some point, after you have been taking vitamin D consistently for at least three months, you should have your blood levels of vitamin D checked. With my patients, I aim for a level somewhere between 40-60 ng/mL. If your initial level is below this target, then increase your dosage to between 3000 and 5000 IU daily, and recheck in 3-4 months. If you need to take more than 3000 IU daily, I always suggest you check with your doctor to see if some other health problem is present.
  3. If you take more than 3,000 IU daily, make sure that your level does not exceed 70 ng/mL without talking to your doctor. Vitamin D levels greater than 100 are associated with an increased risk for atrial fibrillation, a serious heart condition. More is not always better.
  4. Of interest, if you are taking 2000 IU of vitamin D daily and your level doesn’t exceed 40 ng/mL, you should consider that you may have some form of gut malabsorption. In our clinic, the most common cause for poor vitamin D absorption happens to be gluten sensitivity.

* Lappe J, et al. Effect of vitamin D and calcium supplementation on cancer incidence in older women. JAMA 2017;317:1234-43.

I wish you the best of health!

Steven Masley, MD, FAHA, FACN, FAAFP, CNS