Editor's note: David Frum writes a weekly column for CNN.com. A special assistant to President George W. Bush from 2001 to 2002, he is the author of six books, including "Comeback: Conservatism That Can Win Again," and is the editor of FrumForum.

Washington (CNN) -- "An army travels on its stomach," said Napoleon Bonaparte.

But what happens if the stomach gets too big to drag?

Congress is passionately debating whether open homosexuality is compatible with military service. But even as this particular culture war seems headed to resolution, a new emerging cultural divide is tearing at military efficiency: obesity and overweight.

In 2008, some 634 military personnel were discharged for transgressing "don't ask, don't tell." That same year, 4,555 were discharged for failing to meet military weight standards.

Young adults "too fat to fight"

Military weight standards are not especially demanding. Male recruits younger than 27 must have a body-fat percentage below 26%. That's twice the fat you'd expect in a young man in peak physical condition.

Yet even the relaxed 26% standard is too stringent for modern America. More than 9 million young Americans -- about one in four -- are too overweight to enlist, a recent report found.

So the military has adjusted its expectations.

Otherwise qualified young men with body fat of 30% (the boundary between "overweight" and "obese") can be conditionally recruited if they can perform a basic workout and then commit to reduce their weight within one year.

Serving personnel who exceed military limits are offered counseling, nutritional programs and other weight-control assistance. Discharge is very much a last and unwelcome resort.

By the military's own numbers, some 61% of active-duty personnel were above ideal weight in 2007, up from 50% in 1995.

The U.S. military reflects the society of which it is a part. Americans are gaining weight, and the gain is steepest among the young.

Why? It's no mystery. The typical young person today spends twice as much time watching television or playing video games than engaged in physical activity. Cheap, appealing, high-calorie foods and drinks are constantly at hand. Changes in family patterns have put an end to the expectation that food be consumed at a table with people as a social event.

What to do? Some experiments have suggested helpful changes. Children who attend a school within half a mile of a fast-food restaurant are fatter than children who attend schools farther away. So changes in zoning laws might help.

Physical education is disappearing from schools: Barely 25% of American high-schoolers take gym every day. Another 25% have no physical education at all.

Fewer and fewer students walk or bicycle to school, in part because the high school day has been pushed back so early: from the old norm of 9 a.m. to a new norm of 7:20.

First lady Michelle Obama has adopted healthier eating as a personal cause. In the hysterical partisanship of current American politics, this has led some critics of the Obama administration, including Sarah Palin, to embrace junk food as a sacred cause. Here's Rush Limbaugh on November 9:

"Anyway, Michelle Obama's on this big obesity kick, right? Gotta eat healthy stuff, gotta eat the garbage that she grows in the garden, nothing but fruits and vegetables. ... Michelle Obama wants to spend $400 million to combat food deserts. She's all upset that the only food available to poor urban people are convenience stores, the 7-Elevens. ... So she's complaining about food deserts, and Michelle Obama wants to punish Big Food and Big Retail for not putting quality food stores in poor neighborhoods, right? And that's why there's an obesity epidemic, right?"

But the first lady is right to worry. A White House vegetable garden is only the very beginning of a policy solution, but the first lady has grasped an urgent national problem.

Opinion: Palin's wrong on obesity

Obesity and overweight are risk factors for ailments from cancer to depression. Obesity and overweight devour 10% of our national health spending. Obese and overweight people earn less and are less likely to be hired and promoted. And since 1990, obesity and overweight have emerged as crucial problems for military planners -- so much so that a group of retired officers has branded America's high-calorie school lunch programs as a threat to national security.

"Don't ask, don't tell" may have had its day. If so, it's time to replace it with a new motto. As one of my Twitter followers recently brilliantly quipped: "Don't Eat -- Don't Swell."

The opinions expressed in this commentary are solely those of David Frum.