Ever since Americans started plumping to record rates of obesity, doctors have been crusading against fatty drinks and foods to tame an epidemic in the making. But there's something cardiologists seldom talk about that has baffled research doctors for years: In the face of illness, being fatter adds years to people's lives.

On Wednesday, scientists published two reports in the Mayo Clinic Proceedings announcing new clues in the mystery known as the obesity paradox. Being fat can cause all kinds of nasty problems, from heart disease to diabetes. But study after study has found that being overweight is like a shield against death when those diseases set in. As one study author put it, obesity is like a bad friend who gets you sentenced to jail, "but once imprisoned the friend remains loyal and protects you against poor prison conditions and other inmates."

In a first paper, researchers conducted a meta-analysis of 36 studies, searching for instances of patients undergoing surgery to open up blocked arteries. Obesity frequently leads to coronary artery disease, but thin people can be at risk, too. They reviewed "tens of thousands" of cases and discovered obese and severely obese people had higher survival rates post-surgery than normal weight people. (They were around 25 percent less likely to die.)

It wasn't the first major review of the literature. One study published last year analyzed three million sick people around the world. Their results were bewildering: "For people with a medical condition, survival is slightly better for people who are slightly heavier." Yet the cause still eludes scientists.

"At this stage we can only speculate on the reasons for this paradox," said lead author Abhishek Sharma, of the State University of New York Downstate Medical Center in Brooklyn, in a press release. Maybe doctors are more likely to give overweight patients more drugs, he says. Or maybe they have a higher metabolic reserve. Or maybe skinny people have bad genes. Who knows?

The second paper, however, offers more insight. It attempts to chip away at a nagging question among the people studying the obesity paradox: Is body mass index, or BMI, the right way to measure obesity? Many believe other measures, like the ratio of fat to lean mass, may have more to do with the phenomenon than BMI, which is based on height and weight.

"Body composition plays a critical role in the obesity paradox," said Carl Lavie, a cardiologist at the University of Queensland School of Medicine in New Orleans and a co-author of both papers. He and his colleagues examined something called the lean mass index — the proportion of human stuff, like muscle and bone, that isn't body fat — among 48,000 patients.

Their findings yield more evidence for what many have suspected. "At higher BMI, body fat is associated with an increase in mortality," Lavie said. In other words, the fat isn't the elixir; being big is. "Whenever examining a potential protective effect of body fat, lean mass index — which likely represents larger skeletal muscle mass — should be considered," Lavie said.

This is a good lesson. With nearly one in 14 Americans now considered extremely obese, the authors of the two studies were quick to caution that being overweight in the first place is still dangerous. In fact, these papers came on the heels of another that found extreme obesity kills about as effectively as smoking does.

Sources: A. Sharma, A. Vallakati, A. J. Einstein, C. J. Lavie, et al. Relationship of Body Mass Index With Total Mortality, Cardiovascular Mortality, and Myocardial Infarction After Coronary Revascularization: Evidence From a Meta-analysis. Mayo Clinic Proceedings. 2014.

A. De Schutter, C. J. Lavie, et al. Body Composition and Mortality in a Large Cohort With Preserved Ejection Fraction: Untangling the Obesity Paradox. Mayo Clinic Proceedings. 2014.