True story: As I write this, one table away here in the cafeteria of my local health club a mother has just finished telling her young son why we eat. "We eat because our body needs fuel," she said. "We burn fuel when we exercise. If you sit on your butt and watch TV all day you don’t burn any fuel." At the end of his meal she handed him a cup of soft-serve frozen yogurt. "Now you need to eat less and exercise more."

Michael Pollan, author of "In Defense of Food," recently popularized the term nutritionism for the grim reductionism that leads us to regard all food like the well-meaning mother sitting next to me -- as a carrier of fuel and nutrients, no more, no less. But the researcher who first coined that term knew it applied to our boundless faith in exercise as well. As Australian social scientist Gyorgy Scrinis has written, our "myth of nutritional precision" overestimates the ability of science to describe the complex relationship between food, the body and exercise.

"An example of this myth," Scrinis wrote last year (PDF), "is the claim that the amount of exercise required to burn off a particular food can be accurately measured in calories, according to an energy in/energy out model of the body."

Like nutritionism, the notion that exercise can make people thinner has become ideology as well, if the outburst of defensiveness that greeted a recent Time magazine cover story is any gauge. In "Why Exercise Won’t Make You Thin," John Cloud made the seemingly intuitive point that exercise may not be very effective in making people lose weight, because, well, it makes people hungrier.

His point was hardly new. As Gary Taubes, author of "Good Calories, Bad Calories," put it in New York magazine, back in 1932 Mayo Clinic counseled the obese to rest more, given that the energy they expended through added activity wasn’t likely to compensate for the extra energy they would go on to consume afterwards. Don’t laugh, some researchers knew about the connection between tobacco and lung cancer in the 1930s as well (the problem was they were Nazis).

Following the Time article, and with almost choreographed precision, blogs and health promotion organizations launched a blistering counteroffensive. The weight-loss blogs -- of which there are ever so many -- were the most unsparing. "After reading this you have no choice but give up all kinds of exercise and enjoy that extra crispy cream doughnut as a compensation for being fooled for a few decades and for sweating in vain on that tread mill,” wrote the authors of a blog billed as "your guide to a balanced life."

But supposedly neutral health-beat writers at major newspapers also felt compelled to rush to the defense of exercise as a means to losing weight. The Los Angeles Times said, "Hey Time, What Were You Thinking?" while the Chicago Tribune concluded that readers should "make fitness, not weight loss your goal -- the pounds will slip off."

Maybe. But then again, maybe not. Would it be so terrible if they didn’t? If exercise and not weight loss is the trigger for health benefits -- another complex research question -- then what business is it of health-beat writers to be advocating one body type over the other? McClatchy syndicated that piece across the country -- apparently it was needed to cheer on distraught stairclimbing readers of Time who suddenly had no motivation to climb another stair.

Even the American College of Sports Medicine took off its pocket protector and put on its coach’s whistle, feeding the following talking points to trainers across the country, according to a trainer with very strong arms named Tom Venuto: "Last Friday, an article appeared in Time Magazine making statements that we believe run counter to fact and the public interest. ... Your assistance is needed in getting the right health message out to the public. Also we encourage you to adapt our letter to the editor and submit it to your local news outlets, helping readers and viewers get the best evidence-based facts and information."

The question of whether exercise leads to weight loss requires more space and brainpower than I have here. But maybe not a lesser question: Why the big fuss? Does exercise have to make us skinny to be valuable?

Whatever one makes of the idea that exercise won’t make you thin, an argument that clearly isn’t going to be resolved any time soon, it’s troubling that so many have committed themselves to such a one-dimensional justification for organized physical activity. Under fluorescent lights. While staring at screens. Screens tuned to cable TV -- at just the right volume needed to drown out the sound of stationary equipment.

Freelancer Paul Scott of Rochester writes frequently about health and fitness for various media. Susan Perry is on vacation.