Heart disease is the leading cause of death for both men and women. About 610,000 people die of heart disease in the United States every year — that's 1 in every 4 deaths. Obesity has a significant negative impact on cardiovascular disease (CVD), including hypertension, coronary heart disease, heart failure and arrhythmia.

The World Health Organization defines overweight and obesity as abnormal or excessive fat that accumulates and presents a risk to health. Obesity is a critical problem that is increasing in the United States. Nearly 70 percent of adults are classified as overweight or obese, compared with 25 percent 40 years ago.

Even moderate obesity is an important risk factor for heart diseases, directly or indirectly through intervening risk factors, such as hypertension, dyslipidemia and diabetes. Obesity constitutes one of the most important independent CVD risk factors, and positive relationships between CVD mortality and body mass index has been shown in many large-scale studies.

It has been suggested that just because someone is overweight, he or she is not necessarily unhealthy. According to the National Institutes of Health's 1998 report, people can be overweight and considered healthy if their waist size is less than 35 inches for women or 40 inches for men, and if they do not have two or more of the following conditions: high blood pressure, high blood sugar, or high cholesterol.

A 2012 study from the National Cancer Institute found that moderately obese people actually lived about 3.1 years longer than normal-weight women and men. Another study, published in the European Heart Journal, showed that when obese people are metabolically healthy, they are at no greater risk of dying from heart disease or cancer than those who are of normal weight.

However, this fat-but-fit theory may be a myth. A growing body of evidence supports the belief that storing too much body fat is associated with increased blood pressure, blood sugar and cholesterol levels, suggesting that people should maintain a healthy weight in a healthy range.

Researchers conducted a case-cohort analysis in the 520,000-person European Prospective Investigation into Cancer and Nutrition study (EPIC-CVD). During a median follow-up of 12.2 years, they recorded 7,637 incident coronary heart disease (CHD) cases. Participants were categorized as unhealthy if they had three or more of a number of metabolic markers, including high blood pressure, blood glucose or triglyceride levels, low levels of HDL cholesterol, or a waist size of more than 37 inches (94 cm) for men and 31 inches (80 cm) for women.

After adjusting for lifestyle factors such as smoking, diet, exercise and socioeconomic status, the researchers found that compared to the healthy normal weight group, those classified as unhealthy had more than double the risk of CHD, whether they were normal weight, overweight or obese.

But there was another interesting finding: Within the "healthy" group, there was a significant difference in outcomes for people depending on their weight. The research found that compared to those at normal weight, people who were classified as healthy but overweight had an increased CHD risk of 26 percent, while those who were healthy but obese had an increased risk of 28 percent.

According to Dr. Camille Lassale from Imperial's School of Public Health and now based at University College London, this study shows that people with excess weight who might be classified as healthy haven't yet developed an unhealthy metabolic profile. That comes later in the timeline when they have an event, such as a heart attack.