“Man Bites Dog” is always a more intriguing headline than the other way around—so, in flipping through the abstracts from last month’s American College of Sports Medicine conference, it was inevitable that my interest would be piqued by the words “In contrast to our hypothesis…” As a journalist, I like surprises.
The abstract in question was from a team at the University of Connecticut and Hartford Hospital (along with collaborators from the University of Massachusetts). The researchers were investigating the role of vitamin D in blood pressure, both at rest and during maximal exercise—and to their surprise, they found that their subjects with the highest vitamin D levels also had the highest blood pressure.
Despite vitamin D’s current reputation as a super-panacea, there has been some evidence in recent years that more isn’t always better. Could blood pressure be the source of these apparent negative effects? I contacted Amanda Zaleski, the study’s lead author, to find out.
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The study’s main goal, she explained, was to explore the links between vitamin D and blood pressure during all-out exercise. Typically, your systolic blood pressure (the bigger number) rises during exercise, thanks to the increased volume of blood being pumped by your heart, while your diastolic blood pressure (the smaller number) stays roughly constant. But some people have a larger-than-normal increase in blood pressure during exercise, and these people are two to four times more likely to subsequently develop hypertension (i.e., high blood pressure) and cardiovascular disease, so it serves as a potential early warning system in people whose resting blood pressure is still normal.
Separately, there’s been a lot of interest in the links between vitamin D and blood pressure, with some evidence that vitamin D deficiency is linked to hypertension. A bunch of studies were launched to see if taking vitamin D supplements could lower blood pressure, and while a few produced encouraging results, Zaleski says, the majority didn’t. She was involved in one of the most rigorous randomized studies, the Daylight trial, whose results, published in 2015, failed to show any resting blood pressure benefit from vitamin D supplementation in people with high blood pressure and low vitamin D levels.
But if vitamin D doesn’t help resting blood pressure, maybe it still helps more subtle traits like blood pressure during exercise, which would still be an important finding for helping people who are currently healthy avoid future high blood pressure. That’s what the new study investigated.
The study involved 417 adults whose vitamin D levels were measured, along with their blood pressure at rest and in response to a graded exercise test to exhaustion. The results? No apparent link. The people with “clinically sub-optimal” levels of vitamin D (defined in this study as less than 32 ng/mL, or 80 nmol/L) were no more or less likely to have exaggerated blood pressure responses to exercise than those with normal levels. In fact, baseline vitamin D levels could explain only about 1 percent of the individual variation in response.
Still, there was the man-bites-dog detail. Subjects with optimal vitamin D levels had average blood pressure readings of 120.6 over 76.0 mmHg; subjects with lower-than-optimal vitamin D levels had healthier average readings of 116.4 over 74.1 mmHg. For both systolic and diastolic values, the differences were statistically significant.
Is it really possible that having low vitamin D might be good for your blood pressure?
“It is always important to entertain all possibilities,” Zaleski says diplomatically. There are vitamin D receptors in your blood vessels, and it’s theoretically possible that having too much vitamin D might somehow disrupt and harden the vessel linings.
But given that this result contradicts the results of so many other trials—like this one, with over 100,000 subjects, in which those with vitamin D levels below 15 ng/mL were three times more likely to develop high blood pressure than those with vitamin D levels above 30 ng/mL—Zaleski’s strong suspicion is that her result was essentially a fluke, resulting from the particular characteristics and genetic susceptibilities of her subjects.
The key significance of the study is that it adds further evidence to the (somewhat disappointing) notion that links between vitamin D and blood pressure probably aren’t causal after all. Yes, people with low vitamin D tend to be more likely to develop high blood pressure; but no, taking extra vitamin D doesn’t seem to eradicate the risk, which means there’s probably some underlying variable—not getting enough exercise, to pick a possible example at random—that explains the link.
And for me, the study is also a good reminder about the usual nature of surprising and unexpected findings. It’s fun to read the occasional story about a man biting a dog, but it’s a mistake to then conclude from it that men always bite dogs.
