Overview
Specific Aims
Importance
Background
Prelim. Studies

Methods
Study Design
Protocol
Data Collection
Analysis

Data
Introduction
Basic Data Sets
Variables Defs.
Codebook

Results
Working Papers
Publications
References

During the past half-century several small longitudinal and larger cross sectional studies have been published suggesting that cholesterol levels are higher in the fall and winter than in the spring and summer (4, 5, 6, 7, 8, 9). The most striking of these studies suggested that in areas of extreme seasonal climatic variation, such as Finland, there may be as much as a 100mg/dl seasonal variation in serum cholesterol levels (10). Recently Råstam and colleagues reported a cross-sectional study of seasonal variation in plasma cholesterol levels drawn as part of the screening process for the Minnesota Heart Health Program (11). Cholesterol levels were found to peak in January, with a statistically significant seasonal variation evident in men. In women there was a non-significant trend towards higher winter levels. Using the NCEP guideline for hyperlipidemia of >240 mg/dl, 25.4% of men were at or above this level in winter, whereas only 13.5% met this cutpoint in summer.

Gordon and colleagues also have pointed out that seasonal variation in cholesterol levels could be responsible for as much as a 30% difference in the number of patients labeled as hypercholesterolemic during the winter season versus the summer (12). However, seasonal variation in blood cholesterol level is not considered in the U.S. National Cholesterol Education Program guidelines, including the recent update to those guidelines (2, 3). It is also rarely taken into account in the normal clinical management of hyperlipidemia, although the summer/winter differences described above in the frequency of patients being labeled as having "high" cholesterol carry extraordinary cost implications, given the recommendation that all adults undergo cholesterol screening. The described seasonal variation would lead to a much lower percentage of patients reaching goal cholesterol levels if treatment were initiated in the summer and remeasurement carried out in the winter, than if the converse were true.

Our knowledge of the seasonal variation of specific lipoprotein cholesterol fractions and of triglyceride levels is inadequate. In addition, as will be described further below, the physiologic basis for the seasonal variation of blood cholesterol levels is essentially unknown, with no study able to attribute more than a minor portion of the variance to known factors. To date, no careful prospective study has been carried out that attempts to systematically collect and analyze all of the data necessary to study this phenomenon, and to lay the groundwork for intervention studies that would be aimed towards preventing the winter rise in cholesterol levels. Our proposal is intended to fill this void.