A federal tax on soda is a good idea. Sugary, syrupy beverages are linked to obesity and other maladies, so taxing them would help reduce their consumption and improve health. (If it were only that simple.)

Soda taxes have emerged during this national conversation about health care, after the New England Journal of Medicine published a study in September saying a tax on sweetened beverages would cause a measurable, beneficial effect on public health.

“Much as taxes on tobacco products are routine at both state and federal levels because they generate revenue and they confer a public health benefit with respect to smoking rates,” the study’s authors said. “We believe that taxes on beverages that help drive the obesity epidemic should and will become routine.”

If it were only that simple.

As public health policy, a soda tax is flawless. The costs of obesity — in social and fiscal terms — are well-known, particularly among children. Maine is facing an epidemic of childhood and adult obesity, along with its dangerous corollaries, diabetes and heart disease. If taxes on sweetened beverages could reduce their consumption as much as thought, healthier living would result.

Yet taxes are a two-way street; money is raised, and money is spent. A tax is not just its noble purpose, like improving public health. How the revenue from the levy is distributed, how much and for what, is important. On this latter step, soda tax plans have stumbled.

In Maine, the last attempt to raise soda taxes came along as the solution to funding Dirigo Health, the state’s controversial subsidized public insurance plan, which is oft-criticized for spending way too much and delivering far too little. The taxes were roundly rejected at the polls last November.

Tobacco taxes are a problem too, though they’re usually enacted, anyway. Revenue from tax increases has been used to fund programs whose costs are growing. The recent example is children’s health insurance, which Congress paid for with an increase tobacco taxes earlier this year.

If the tax works, there will be less money. Yet the programs benefiting from these taxes involve health care, which needs an increasing amount of funding. This doesn’t square — health policy cannot be built on funding derived from habits the health policy wants to stop. It won’t work.

Public benefits from a soda tax are not debatable. But if policymakers want to make a compelling argument for it, they must be rational about how the new money is spent, so giving them a small pile of new money now doesn’t lead to needing a bigger pile of new money later.

Again, if it were only that simple.

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