To every lipid profile, turn, turn, turn...
When patients in Campinas, Brazil went in for check-ups during the cold months of winter, their cholesterol levels were about 8 percent higher, on average, than those of patients whose blood was tested during the summer.
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LDL, or "bad" cholesterol levels varied significantly -- by an average of 7 mg/dL -- from winter to summer, according to data taken from a quarter of a million individuals who checked in to primary care centers in Campina between 2008 and 2010. In the summer, levels of HDL, or "good" cholesterol, were about 5 mg/dL higher.
That this indication of cardiovascular risk is higher during the winter explains previous findings that mortality from heart disease follows a seasonal trend; the coldest months are commonly believed to be a witching hour for heart attacks.
People who need to closely monitor their cholesterol should be aware that good news received in July might not be an accurate representation of their cardiovascular health, according to Filipe Moura, the study's lead investigator. While 7 mg/dL aren't likely to make a huge difference for most, as there are about 30 points of leeway in the healthy LDL range for people who are not especially at risk for heart disease, getting tested at different times from year to year could make it difficult to track consistency.
The dearth of sun exposure in winter may partially explain the variation, as vitamin D is known to improve the ratio of HDL to LDL. And the cold brings out our hibernatory habits: people tend to eat more and exercise less in the winter. Curiously, triglyceride levels -- which are best kept low and are also affected by diet -- rose in the summer by an average of 12 ml/dL, but they were generally less predictable than cholesterol levels.
While what applies for Campinas' tropical climate might not translate to other regions, Moura believes these fluctuations would be even more pronounced in places where seasonal climate shifts are more dramatic.