When it comes to vitamin D, it’s possible to get too much of a good thing, say the authors of a new study that looked at the stalwart vitamin’s connection to heart health. Published in the American Journal of Cardiology in January, the study of 15,000 healthy adults age 18 to 85 found that, while increasing levels of D in the blood are associated with decreased cardiovascular inflammation to a point, once D levels go beyond that point, inflammatory markers actually begin to rise.

This indicates a growing risk of stiffening blood vessels and other cardiovascular problems. This latest research is among several new studies that suggest, as Virginia Moyer, MD, chair of the U.S. Preventive Services Task Force, recently put it: “The nutrient falls into the category of something that both benefits and harms.”

We asked the American Journal of Cardiology study’s lead author—Muhammad Amer, MD, an assistant professor of internal medicine at Johns Hopkins University School of Medicine—to explain the findings.

Natural Foods Merchandiser: What prompted you to do this study?

Muhammad Amer, MD: We know that vitamin D plays a role as an anti-inflammatory agent, as measured by markers such as C-reactive protein (CRP). But we do not know if this benefit remains in the long run. If you look at the literature on vitamin D, cardiovascular disease and mortality, it is pretty conflicting and controversial—with some trials suggesting a decrease and others showing no effect on inflammation. We wanted to know why.

NFM: How did you conduct this research?

MA: We examined the data from 15,000 healthy adults from the National Health and Nutrition Examination Survey. After adjusting for variables such as body mass index, cholesterol levels, serum glucose and hypertension, we looked at vitamin D and CRP levels in the blood.

NFM: What did you find?

MA: We found an inverse relationship between vitamin D and CRP in adults with low vitamin D levels. However, the relation reversed once the vitamin D level increased above 21 ng/ml—after which any additional increase in vitamin D was not associated with a decrease but rather an increase in CRP levels.

NFM: How physiologically might excess D boost inflammation?

MA: That has yet to be explored.

NFM: What do this study’s findings mean for consumers and retailers of vitamin D supplements?

MA: It means that people should have their D levels tested before taking vitamin D supplements and tested again a few times a year if they stay on them. They should not be ignoring the fact that D is a steroid-like hormone and may be harmful at some level.

NFM: Is there a safe upper limit that people can take daily without having their levels checked?

MA: I have no clear answer to this because different people metabolize vitamin D differently.

NFM: Is there any good news about vitamin D to come out of this study?

MA: Certainly. Vitamin D is beneficial for your cardiovascular health because it curbs inflammation, which is an underlying reason for atherosclerosis (hardening of the arteries). You should be on it if you are a candidate (because your D levels are low), but you should not keep on taking it indefinitely without keeping track of your levels. Again, at certain levels in the blood, vitamin D may become pro-inflammatory. If you can be on vitamin D rather than being on expensive statin drugs that compromise your kidney, liver and muscle, why not? It can definitely benefit you—it just has to be used judiciously.

Want another perspective? The Council for Responsible Nutrition speaks up about vitamin D.