BACKGROUND:

Significant seasonal variation of blood cholesterol levels has been shown in a number of cross-sectional studies, with important implications for national screening and therapeutic guidelines. However, the phenomenon has not been well studied: little is known about etiology or about seasonal variation of lipid subfractions.

DESIGN NARRATIVE:

The investigators assessed the magnitude and timing of the seasonal effect for total cholesterol and the important lipoprotein subfractions (low-density lipoprotein, high-density lipoprotein, triglycerides, apolipoproteins AI and B, and lipoprotein (a)) in both sexes and at different ages.

They also identified and quantified the effects of the main factors determining the variation and assessed the seasonal variation of other blood elements thought to play a role in the development of coronary heart disease, including hemostatic factors and antioxidants Finally, they explored the implications of the phenomenon for public health policy.

Baseline and serial measures were repeated quarterly for one year. These included measures of: serum lipids and lipoproteins; antioxidant vitamins; platelets and hemostatic factors; demographic factors; weight, waist-hip ratio, height, body mass index (BMI) and blood pressure; dietary intake; physical activity; light exposure; and psychologic factors. Meteorologic data were also collected. Data were collected by written questionnaire and by the use of computer-assisted evening telephone interviews (CATI), which were used for the collection of dietary, physical activity and individual light-exposure and other lifestyle data. Activity and light data were verified in a subset of 80 patients by the use of a monitoring device. Analyses included methods to describe the seasonal variation in dietary, physical activity, psychologic, and light exposure data and repeated measures in linear regression models to determine the roles of specific dietary, physical activity, psychologic, and light-exposure related covariates in explaining variability in blood lipids, antioxidant vitamins, and hemostatic factors.