If there is one population subgroup that doesn't need to worry about weight, it's endurance athletes, right?

These dedicated cyclists and runners, swimmers and rowers burn calories like human furnaces. Carbohydrates fuel their fire and, if they worry at all, it's about running on empty during training. It's become a cliche for a marathoner to answer the inevitable why-do-you-run question with a flippant, "So I can eat what I want."

But here's the reality: Endurance athletes do fret about weight. They count calories. They chart body fat percentage. They're on a quest for leanness.

They don't talk about it much, for fear others may suspect them of having eating disorders, which truly is a problem for some (especially female) athletes. But it's there. A 2009 survey of endurance athletes, published in the Annals of Behavioral Medicine, reported that 74 percent were "concerned" about weight, 54 percent "dissatisfied."

A new book, "Racing Weight: How To Get Lean for Peak Performance" (Velo Press, $18.95, 224 pages) by certified sports nutritionist and runner Matt Fitzgerald, has brought the issue of body weight among even the most fit into the open.

When Fitzgerald, the author of 17 fitness books and a senior editor at Triathlete magazine, made his pitch to publishers for what he jokingly calls "a diet book for skinny people," he encountered much head-scratching, even skittishness.

Eventually, Velo Press bought the rights. In December, when "Racing Weight" was first published, it cracked Amazon.com's top 100 best-sellers.

The book's thesis is simple: "Excess body fat is the enemy of performance. ... As body-fat levels go down, aerobic capacity goes up, because muscle has less competition from fat tissue for oxygen and fuel."

Studies support that notion. University of Georgia researchers found that a 5 percent weight gain decreased performance by 5 percent in 12-minute test runs. In another study, it was determined that a 160-pound runner needs to exert 6.5 percent more effort to run the same pace as a 150-pound runner.

"Let's face it," Fitzgerald said in a recent phone interview, "even a few pounds can make a difference."

The concern among nutritionists and some athletes is that people will interpret the findings as a dangerous "can never be too thin" prescription.

But Fitzgerald is not advocating anything as severe as calorie restriction. He stresses "body composition" (percentage of body fat) more than weight and acknowledges that the "lighter is better" mind-set is simplistic and sometimes hurts performance.

Rather, he advocates boosting the nutrition-per-calorie ratio. In other words, eating higher-quality foods: grains, legumes, lean meat, vegetables and fruit. He says the accepted nutrient ratio - 60 percent carbs, 20 percent protein, 20 percent fat – can be tweaked depending on training load.

In fact, Fitzgerald says, endurance athletes can consume more fat - provided it is made up of "essential fats" such as omega-6 and omega-3 fatty acids - and less protein than the 20 percent commonly recommended.

"There has been some research where, when you bump protein intake way up, it can harm performance," he says. "An analysis of elite Kenyan runners shows they're right at 10 percent (protein intake), and they are the best runners in the world. So there you go."

Fitzgerald acknowledges that an athlete's leanness can reach a point of diminishing returns - but few ever reach that point. He says athletes should aim for "ideal weight" for big competitions and "limit your off-season weight gain to no more than 8 percent of your optimal performance weight."

He uses the example of Olympic distance runner Ryan Hall, who routinely gains 5 to 7 pounds during recovery periods after marathons. Another test case is cyclist Lance Armstrong, who after battling cancer in the mid-1990s transformed his "linebacker genes" body to one featuring leaner muscle mass and lower body fat to become a better climber. Armstrong still has a bigger frame than most elite cyclists but compensated for it with a strict diet regimen.

"Your genes place limits on how much more muscle and fat you can gain or lose, but within these limits, there is a fair degree of adaptive potential," Fitzgerald writes, citing a Wake Forest University study showing that body fat percentage is 64 percent inherited but that lifestyle controls the rest.

"I'm 6-foot-1, so I'm never going to be as light as a 5-6 runner," Fitzgerald adds. "But leanness should be a universal ideal across all endurance sports, whether you're young or old, male or female."

Still, Olympic Trials marathoner Mary Coordt, who holds a master's in nutrition from the University of California, Davis, worries that an emphasis on leanness could be detrimental.

"I'm always cautious telling people they need to lose weight to run faster," Coordt says. "There's a category of runners who become 'fataphobic' in terms of fat choices. Show them olive oil, and they start running away. At that point, you need to worry about their nutrient intake."

Coordt says she "doesn't really even own a scale" and can tell by feel whether her weight is right during training.

"I do several running camps with (former elite marathoner) Dick Beardsley, and runners tend to be the ones already eating well," she says. "I mean, they know what a whole grain is. It's the general population that doesn't."