MAYOR NUTTER and Health Commissioner Don Schwarz are to be commended for singling out a problem everyone hates to talk about: most of the city is too fat. Sixty-four percent of adults and 57 percent of children here are overweight.

Schwarz makes a compelling case in particular for the damages done by sugary sodas. Among one of his more shocking charts: the trend lines for sweetened beverages versus milk consumption has swapped positions, with children now drinking more soda than milk.

Sugared sodas and drinks are a big culprit. And imposing a tax on those things to discourage consumption isn't a bad idea.

Too bad that's not what the mayor is proposing, despite that the administration is insisting on calling a proposal to raise $77 million a tax on sugary drinks.

Most taxes designed to encourage healthier behavior (often called sin taxes) are levied at the point of sale, like tobacco. That shows consumers they are paying more for an unhealthy product. Nutter's soda tax wouldn't work this way.

Because the city can't impose a directed sales tax without approval from Harrisburg, this would be an excise tax - assessed via the Business Privlege Tax - on retailers who sell sugary drinks. Exactly how that tax is calculated suggests a series of potential headaches that could quickly torpedo the very intent of the proposal.

Retailers will have to calculate exactly how many drinks they sell, how many of those are sugared, report that amount to the city, then get assessed a 2-cents-per-ounce tax on that amount. That means a new set of accounting tasks, which may not be a problem for big retailers like WalMart, but could be for smaller ones. By the time they have to pass this tax onto consumers, retailers could decide it's easier to spread the hike across all products, not just sugared soda.

Headache No. 2: Every vending machine that doesn't have tiered pricing to distinguish between water, soda and diet soda will have to be replaced; a headache easily avoided if prices go up on all products, not just sugared sodas.

As designed, this tax requires too much change of retailing behavior for it to have the kind of impact on consumer behavior that the administration is hoping for. In fact, consumers confronting higher prices on everything, not just their sugary sodas, may be driven to drown their sorrows in another Coke. *