“Fed Up” is now playing at Point Cinemas. PG, 1:36, three stars out of four.

It’s a common saying that parents want to give their children more than they had. Usually it refers to material goods or opportunities. As parents, we never thought we’d have to work to give our children longer lives than we had.

But one of the many, many sobering statistics in Stephanie Soechtig’s documentary “Fed Up” is that the current generation of American children is expected to have a shorter lifespan than their parents. The reason is an explosion of disease and malnutrition brought on by childhood obesity in the last 30 years — Type 2 diabetes has become a childhood disease.

Soechtig’s documentary, narrated and produced by Katie Couric, sticks very closely to the traditional template of advocacy documentaries from “Bully to “A Place at the Table” — talking-head interviews and lively graphics mixed with a personal narrative following several subjects, in this case overweight ‘tweens struggling (and often failing) to get their lives under control. But the subject matter is so compelling, and the facts presented so surprising, that it works.

First of all, it’s not our fault. The knock on obese America is that we’re too lazy and sedentary, that if we just took the stairs instead of the elevator at work and ate “healthier options” we wouldn’t get so fat. Sure, exercise is great and necessary, but the real culprit in America’s obesity epidemic is what’s in our processed foods. This includes — and in particular includes — the so-called “healthier options” in our supermarkets like low-calorie yogurt and reduced-fat Oreos. (Here in Madison, with our abundance of farmers’ markets and CSAs, we can scoff at this. But remember than in many rural and urban areas of the country, fresh fruits and vegetables aren’t so easy to come by.)

While food companies have been creating these options in response to consumer demand, they often compensate by jacking up the sugar content. And it’s the sugar, an addictive substance often in foods we don’t even think about, that’s a chief culprit in making us overweight.

And the sugar lobby has a stranglehold on the political process in America to make sure it stays that way, and that Americans don’t find out how much sugar they’re really ingesting. Here’s an experiment — right now, go to your kitchen cupboard, pull out an item and read the “Nutrition Facts” label. Notice how most ingredients include both the amount AND the percentage value for a 2,ooo-calorie diet? Notice how sugar is almost the only one that DOESN’T include a percentage? That’s not by accident.

This is a bipartisan problem, and Soechtig to her credit isn’t afraid to ding all sides of the political spectrum for their complicity. Sure, she shows Sarah Palin sipping from a Big Gulp and equating super-size portions with freedom. But she also is critical of Michelle Obama’s Get Moving program, a well-intentioned plan to get kids active, but that largely let the big food companies off the hook.

Some advocacy documentaries end on a triumphant “We can change this!’ up note; I didn’t really feel that surge out of “Fed Up,” because Soechtig shows what an intractable, decades-in-the-making problem this is, and how it will require tackling entrenched interests in both politics and big business to stop. At the very least, though, “Fed Up” will persuade the viewer to make smarter individual choices (the movie’s website lets visitors sign up for a 10-day low-sugar challenge). And I’ll bet when employees clean up the theater after a screening, they find lots of half-drunk Cokes and half-eaten popcorn boxes.