In "Bodega," a short film posted to YouTube, jocular and heavyset Bronx residents Dallas Penn and Rafi Kam explain (with a dash of profanity) how to buy a meal for a dollar at a typical neighborhood shop. Viewers learn of the importance of the "iced honey bun food group" and that ice cream sandwiches can be bought for 25 cents, and that the ingredients of similarly priced "quarter water" are water, corn syrup, sodium benzoate and red #5. The duo s earlier masterpiece, "Ghetto Big Mac," reveals how to assemble "the finest burger that a little money can buy." Order a double cheeseburger from the McDonald s $1 menu. Pester the counter attendant for upgraded ingredients, like a seeded bun, thick-cut onions and Big Mac sauce. Then layer half of a small order of French fries between the beef patties.

Penn and Kam seem like a Hollywood comedy franchise waiting to happen, but their films hint at a serious political movement that s giving the junk food industry fits. Caloric, nutrient-poor snacks tend to be cheap, and Americans, especially poor ones, have grown alarmingly rotund. Could a tax on Twinkies and chips make consumers prefer broccoli and slim down the nation as a result?

Fat chance, suggests a new study. Three U.S. professors with backgrounds in economics and law distilled the nation s consumer price index to two food indexes, one for fattening foods and another for slimming ones. Prior research has already shown a link between rising obesity and the falling price of food relative to other goods. According to one study, during the 50 years ended 2000, a drop in food prices explained 40% of the rise in "body mass index," a rough measure of weight compared with height. Indeed, first-year economics course books teach that when an ordinary good becomes cheaper, people consume more of it. It stands to reason, then, that just as Americans grew fatter as food became more affordable, they should have swelled especially quickly during periods when the price of junk food dropped quickly relative to healthy food.

That is not what the new study s authors found. "In general, the price effects we do find can account for only a very small amount of the increase in BMI over the period we analyze," they wrote in a working paper. "From a policy standpoint...our results suggest that food-price sensitivity may not be sufficient to make fat taxes, within plausible ranges, a viable tool to lower obesity."

The study methodology raises the difficult question of which foods are bad. The authors explain that peanut butter, despite its healthy protein, ended up in the index of unhealthy foods because of its high fat content. But Walter Willett, a Harvard professor of nutrition, has written that peanut butter s nutrition profile "compares quite favorably with bologna, roast beef, and many other sandwich fixings," and that studies indicate that people who regularly eat nuts and nut butters are less likely to develop heart disease and type 2 diabetes than those who don t. Such sticky points might explain why, despite poll numbers suggesting that slightly more than half of the American public supports a broad junk food tax, Congress has yet to put a bill to vote.

Soda, on the other hand, has few fans among nutritionists, and politicians are keen on turning America s $70 billion yearly soda habit into a rich source of revenue. About half of states currently impose modest taxes on soda (beyond those for other foods). Last year, New York Gov. David Paterson proposed an 18% tax. New York City s health commissioner has called for a penny-per-ounce tax 72 cents per six-pack. But a 2009 analysis of soda drinking among children suggests that such plans would do far more for fiscal than physical fitness. State soda taxes, the study found, led youngsters to down a little less of their fizzy favorites, but to guzzle more juice and whole milk, resulting in a caloric wash. Perhaps that also explains why junk food prices don t seem much more to blame than all food prices for the rise in obesity rates, and why a junk food tax might not work as intended. Tax the Twinkies but leave most calories cheap, and overeaters will find a way to indulge.