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Vitamin D supplements don't prevent cancer or heart disease

For years people have been taking Vitamin D supplements in conjunction with calcium supplements as a way to prevent bone loss. More recently some research has shown higher incidences of cancer and heart disease in people whose blood levels of Vitamin D were considered low. This is quite serious, as many, if not most, people don't get enough Vitamin D: few foods provide Vitamin D in significant amounts, and for most people their primary source of Vitamin D is sunlight. Yet with a greater awareness of the risk of skin cancer, people are avoiding the sun and wearing sunscreen more often, leaving them at higher risk of Vitamin D deficiency.

That said, seeing a relationship between low Vitamin D levels and cancer or low Vitamin D levels and heart disease is not the same as establishing that low Vitamin D causes cancer or heart disease: the only way to establish that is a randomized, controlled trial.

Funded by grants from the National Institutes of Health and other organizations, a team of Harvard-affiliated researchers designed the Vitamin D and Omega-3 Trial (VITAL) to assess whether taking oral supplements of Vitamin K or omega-3 fatty acids would prevent cancers or heart disease (NEJM DOI: 10.1056/NEJMoa1809944).

They recruited over 25,000 men and women from all over the United States who were at least 50 years of age (55 for women). They made sure to recruit an ethnically diverse population that included at least 5,000 black persons, as they are particularly prone to lower levels of Vitamin D as their rate of synthesizing Vitamin D from sunlight is lower than that of other ethnic groups. The participants had no history of cancer (other than skin cancer) or heart disease and agreed to limit their intake of Vitamin D supplements to less than 800 IU per day.

At the start of the study the participants responded to a demographic and health history questionnaire as well as a dietary questionnaire. About 2/3 of the participants also underwent blood tests that included an assessment of their baseline Vitamin D levels. Of those, 12.7% had levels that would be considered inadequate.

The authors randomly assigned the participants to one of four groups:

1. Vitamin D supplement (2000 IU/day) and omega-3 supplement (1 gram/day)
2. Vitamin D supplement and placebo
3. Placebo and omega-3 supplement
4. Placebo and placebo

For an average of five years the participants were provided their assigned supplements on a monthly basis. On a yearly basis the participants updated their health information, and when a participant reported a relevant health-related issue such as a heart attack, stroke, or cancer diagnosis, the authors requested access to their relevant medical records to verify the diagnosis.

At the close of the study, the authors looked at how many participants of each of the groups experienced a heart attack, stroke, death from other heart-related causes, or invasive cancer of any kind. Unfortunately (or is that fortunately?) the rates of heart disease-related illnesses or cancer did not significantly differ among those assigned to receive either a Vitamin D supplement or a placebo replacement. This was regardless of whether the participants received omega-3 supplements - that study will be featured in next week's Health and Nutrition Bite.

In an effort to tease out any clinically significant outcomes, the authors broke out the participants by various characteristics, including gender, race, Body Mass Index (BMI), and baseline Vitamin D levels. While the baseline level of Vitamin D levels didn't seem to have an effect on outcomes, it did seem that Vitamin D supplementation reduced the risk of invasive cancers for black participants - to the tune of reducing cancer risk by about 23%. The risk of heart disease, however, was not affected to a degree the authors considered clinically significant (less than 10%).

What this means for you

It's important to remember that what the authors were investigating was the use of supplements - NOT an increased intake of Vitamin D from food sources. What this study means is that taking Vitamin D supplements to prevent heart disease or cancer is largely pointless unless you are black, in which case it may be helpful in preventing invasive cancers.

We know from other research that vitamin and antioxidant supplements may in fact be harmful, while adequate intake of the same substances from food is helpful. This may also be true with Vitamin D; we don't yet know. So make sure you're getting enough Vitamin D by getting out in the sun (just a few minutes a day is enough, and sunlight through a window doesn't count) or look for foods that have been fortified with Vitamin D (these commonly include orange juice, breakfast cereals, and milk).

Prefer other sources of Vitamin D? Three ounces of salmon contains an estimated 112% of the recommended daily intake for adults. Here are some great salmon recipes:
Salmon Mac and Cheese
Roasted Salmon with Red Thai Curry
British Fish Cakes
Salmon Burgers
Salmon Cheesecake
Salmon with Poblano Sauce

First posted: November 14, 2018