Researchers have discovered that the risk of heart disease is highest during the winter months but lowest in summer, according to a study from the European Society of Cardiology.

Heart disease is the leading cause of death in the US, causing an average of 193.6 deaths per every 100,000 people, according to the Centers for Disease Control and Prevention (CDC).

As deaths from cardiovascular disease tend to be higher in winter, researchers from Switzerland decided to conduct a large-scale study to determine whether the risk of heart disease has a seasonal pattern.

The study was conducted using cross-sectional data from 10 population-based studies over seven countries. The studies included 107,090 participants between the ages of 35 and 80.

The researchers took the participants' measurements for:

The measurements were compared according to season and were adjusted for age, gender and whether a participant smoked.

Findings of the analysis revealed that many heart disease risk factors, such as blood pressure, total cholesterol and waist circumference, were higher in January and February but lower in July and August, compared with the annual average.

Waist circumference was around 1 cm smaller in summer compared with winter, while total cholesterol was on average 0.24 mmol/L lower in summer than in winter.

The results also showed that systolic blood pressure levels were 3.5 mmHg lower in summer compared with the winter months.

Dr. Pedro Marques-Vidal of the Institute of Social and Preventive Medicine at the University of Lausanne, Switzerland, says:

"Although this difference is almost irrelevant for an individual, it is considerable for a whole population because the whole blood pressure distribution is shifted to higher values, increasing cardiovascular risk.

Indeed, the impact of season on blood pressure levels might have as great an impact on cardiovascular risk as genetic markers for blood pressure. This is because the joint effect of genetic markers on blood pressure is modest, between 2 and 3 mmHg."

Dr. Marques-Vidal adds that although there was a seasonal variation in waist circumference, the average BMI did not change throughout the year. There was also no variation in glucose levels.

"We have no clear explanation for this finding. Total cholesterol may increase during the winter because of changes in eating habits," he says.

"There was no seasonal variation in glucose, probably because several cohorts did not collect blood samples in the fasting state."

Dr. Marques-Vidal adds that further research has already begun, which will investigate the seasonality of food intake and may help to explain these findings.

He says there are also plans to conduct another study to determine whether there are seasonal patterns in heart disease risk factors in the Southern Hemisphere, as seasons are inverted relative to the Northern Hemisphere.

He emphasizes that as the findings of this current study show that heart disease risk is heightened in the winter, people need to make an extra effort to exercise and eat healthily during those months to protect their health.