These days sports superstars like LeBron James, Peyton Manning and Serena Williams have become nearly as prominent in advertisements as they are on the playing field.

Whether it's Gatorade, McDonald's, Coca-Cola or Oreos, Americans (and especially teenage Americans) have gotten used to seeing sports superstars pitching their favorite products.

And according to a recently released survey, that may not be a good thing.

In a study that will be printed in the November issue of the journal Pediatrics, researchers at Yale's Rudd Center for Food Policy and Obesity determined that unhealthy food and drink products are often marketed to teenagers through star athletes.

"What stood out to us was the striking irony of the practice of having the world's most physically fit athletes endorsing these products," said lead researcher Marie Bragg, a doctoral student in clinical psychology at Yale.

Using 2010 data from the advertisement database AdScope, researchers identified 100 top athletes and charted their 512 endorsements. They narrowed those down to 62 food products, 49 of which were high in in calories and low in nutritional value.

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The athletes also endorsed 46 sports drinks, sodas and other beverages. In 43 of those products, all the calories came from added sugar.

Emma Boyland, who studies marketing and children's food choices at the University of Liverpool but was not involved in the research, said there's a certain disconnect between the athletes themselves and the products they're endorsing.

"We see these people - they've obviously (reached the top) of sports achievement, they're obviously living a healthy lifestyle - and they're endorsing these foods," Boyland said. "And that kind of lends an aura of healthfulness to these foods and beverages that they don't deserve."

Perhaps most troubling is the intended audience of these advertisements. After studying the Nielsen data, the Yale researchers noted that children aged 12-17 saw more advertisements for athlete-endorsed food and drink products than any other age group. Teenagers in this age bracket often idolize sports stars and are not as concerned with nutrition as their parents.

The worst offenders, according to the study, are Denver Broncos quarterback Peyton Manning, Miami Heat forward LeBron James and top-ranked tennis star Serena Williams.

“When taking into account the nutrient quality of the products endorsed and the amount of advertising for each product, Peyton Manning, LeBron James, and Serena Williams are the highest contributors to the marketing of unhealthy foods,” the authors wrote.

The authors understand that athletes like Manning, James and Williams earn tens of millions of dollars from advertising, and they don't expect these superstars to shun ads altogether. Rather, they recommend a different approach.

“Our ultimate hope would be that athletes reject the unhealthy endorsements or, at the very least, promote healthy foods,” Bragg said. “These athletes have an opportunity to work with parents. Instead, they’re promoting really unhealthy foods.”

To read the full study, see here.