Seasonal variations of selected cardiovascular risk factors.

This article reviews research on selected biomarkers of cardiovascular risk - cholesterol and other lipids, C-reactive protein (CRP), fibrinogen, homocysteine - in the attempt to determine the existence of a predictable seasonal chronobiological pattern of variation. Studies dating as far back as the 1930s have reported seasonal variations in cholesterol levels. Statistically significant seasonal changes in lipid levels have been found in individuals irrespective of the country where the research has been conducted, and irrespective of the age, sex, ethnicity, and baseline lipid levels of the study subjects. While not all studies have been in complete agreement on either the amplitude (degree of seasonal change) or month/s of highest lipid levels, a strong winter/summer difference has been found in most studies. Existing evidence for an independent effect of season in variation of CRP is weak. Studies have consistently reported significant seasonal variations in fibrinogen levels. While other biological factors clearly interact to affect fibrinogen variability, seasonality appears to be an independent source of variability. Evidence from several studies points to a lack of seasonal variability in homocysteine levels. Although seasonal variability is just one source of periodicity influencing biological function and assessments in clinical practice, for some biomarkers, including lipids and fibrinogen, it is a source of variability that warrants consideration prior to a decision to treat and in assessing response to interventions.