An exhaustive review of health records of 443,632 veterans nationwide reported by Ross D. Fletcher, MD, of the VA Medical Center in Washington, D.C. revealed that blood pressures are higher in winter. Dr. Fletcher reported his findings at the American Heart Association meetings in Orlando, Florida.
Dr. Fletcher's study involved electronic records of veterans from Anchorage, Alaska, Washington, DC, Philadelphia, Chicago, Minneapolis, Portland, Oregon and 9 other cities. Records were reviewed over a 5-year period. Year-in, year-out, blood pressures appeared to range higher in winter, lower in summer in 60% of veterans studied.
"In all cities, there was a seasonal variation that didn't seem to be related to outside temperature," Fletcher says. The pattern held even in southern climates with sunnier winters, even as far south as San Juan, Puerto Rico.
Although the study was simply a review to identify trends but not to identify causes, Dr. Fletcher speculated that weight gain and sedentary lifestyles in colder weather were to blame. "People gain weight in the winter and lose weight in the summer. People tend to exercise more in the summer and less in the winter," he says.
What's missing from this conversation?
What's the difference between summer and winter? Temperature, of course, and . . .the sun!!
It's sunny in summer. It's also warmer, so people spend more time outdoors and wear less clothing like shorts and short-sleeved shirts. In winter, it's colder (or cooler in southern climates) and people are more likely to wear long-sleeved shirts, jackets, even gloves and hats. They also spend less time outdoors. Days are also shorter and the sun less intense (since it is farther away in winter).
One important result-and one not mentioned in Dr. Fletcher's research or any of the media commentary about the study: far less activation of vitamin D in the skin.
Just how many people are deficient in vitamin D depends to a great degree on how you define deficiency, what part of the country is being studied, what kind of people are being studied (e.g., athletes vs. elderly people living in an assisted living arrangement), race, body size, sex, among other factors. But most studies reveal a startling fact: Even loosely defined, vitamin D deficiency affects at least 50% of all Americans in summer and in winter, though it's clearly more dramatic in winter.
There's even a plausible explanation for this effect. Several important studies by a University of Chicago group have discovered that levels of the blood pressure hormone, renin, are blocked by vitamin D. Renin is a powerful blood pressure-raising hormone, the very same one blocked by drugs like ACE inhibitors lisinopril and angiotensin-receptor blockers like losartan (Cozaar®).
We check blood vitamin D levels (25-OH-vitamin D3) in our patients at least twice a year, in summer and in winter. We check it as routinely as we check blood sugars or cholesterol. Average level for people in winter (in Wisconsin): 17 ng/ml-severe deficiency. Average level in summer: 33 ng/ml-better, though still deficient. (We aim for a blood level of 50 ng/ml year-round.) Everybody in my cardiology practice-did I say everybody?-everybody takes vitamin D supplements, with dose based on their blood level fluctuations (oil-based gelcaps preferred for better absorption).