Doctor advises. Nutritionist or medical worker holds a VITAMIN D sign, healthy lifestyle concept.

What You Need to Know About Vitamin D and Cancer

Vitamin D deficiency or insufficiency is widespread and may contribute to falls, bone fractures, or bone loss in older adults. Nevertheless, it is not standard to routinely check blood levels of vitamin D for most individuals.

If you have a low blood level of vitamin D, you may be at a higher risk for several cancer types. For example, there appears to be a link between vitamin D deficiency and breast cancer. A 2017 review found that most studies demonstrate an inverse relationship between vitamin D levels and breast cancer risk: Vitamin D deficiency is associated with a higher risk of breast cancer.

It’s not just breast cancer. A 2014 study revealed a link between low blood levels of vitamin D and aggressive prostate cancer among African American and European American men. The connection between vitamin D and prostate appears to be especially strong in African-American males.

Furthermore, a report from an international group of researchers demonstrated higher vitamin D levels in the blood to be associated with a lower risk for getting colon or rectal cancer.

Here’s the problem with the studies I just cited. These findings are observational — that is, the research doesn’t prove that low vitamin D levels cause breast, colorectal, or prostate cancer. Instead, low levels of the vitamin are associated with these cancers.

Photo by Michele Blackwell on Unsplash

Now that we have established an association between low vitamin D levels and some cancers, should we all be taking vitamin D supplements? There are biological reasons why vitamin D may protect against cancer, but we don’t have high-level evidence for supplementation. Expert groups do not recommend vitamin D supplements for the sole purpose of cancer prevention.

The VITAL trial is a large randomized controlled trial of vitamin D3 (2,000 international units daily) that enrolled 25,000 men and women. The study did not show a reduction in cancer diagnoses after a median follow-up of 5.3 years. But hold on, there’s an exciting finding that offers promise:

While vitamin D did not significantly reduce the primary endpoint of invasive cancer incidence, it showed a good signal for reducing total cancer mortality. Supplement users had a one-fifth lower chance of having an advanced or deadly cancer than those on a placebo.

We need more research to determine who may most likely derive a net benefit from vitamin D supplementation. In addition, I highly doubt five years of follow-up is sufficiently long to see the full potential benefit from taking extra vitamin D.

There is an important caveat, however. The risk reduction appeared only among those with average body weight. Vitamin D supplementation appeared associated with a 38 percent relative drop in risk for those with an average weight. Vitamin D offered no cancer risk-reduction among the overweight or obese.

My readers often rightly remind me that relative decreases often don’t tell the whole story. Thirty-eight percent reduction from one percent or 50 percent? Let’s take a glance at the absolute numbers. Among the study subjects, about 1,600 received a diagnosis of cancer. Only some of these cancers became advanced.

Of the 12,927 in the supplement group, 226 (1.7 percent) developed cancer that spread or was fatal, compared with 274 (2.1 percent) in the placebo cohort. A slightly more significant benefit to vitamin D supplementation appeared among those with an average weight. Among the 7,800 normal-weight subjects, 58 (0.7 percent) developed metastatic or fatal cancer in the vitamin D group, compared with 96 (1.2 percent) in the placebo group.

What You Need to Know

The bottom line? The current evidence is insufficient to recommend large-dose vitamin D supplementation to prevent cancer. However, the majority of vitamin D intervention trials have with adults without vitamin D deficiency. If you have vitamin D deficiency (say a blood 25[OH]D level of under 20 ng/mL), you should talk to a valued health professional about supplementation.

If you consider vitamin D supplementation, you may ask how much vitamin D supplementation is too much for the average person? The level that induces toxicity is unclear, but the US Institute of Medicine (IOM) has defined the upper limit for vitamin D as 4,000 international units (IU) daily for healthy adults.

In summary, we have not shown a causal association between poor vitamin D status and cancer. We have some signals that vitamin D supplementation may (slightly) reduce the risk of fatal cancer among those with an average weight.

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