Should there be a fat tax on soda and junk food?

It's an idea that comes up whenever people bemoan how much sugar Americans consume or how common obesity and diabetes have become.

Discourage excess sugar consumption by taxing sodas, other sugary beverages and junk food. After all, if you have to pay more to consume something, you're probably going to do less of it.

But a new paper from the Tax Policy Center suggests that solution is not as straightforward or as effective as it may sound.

Here are just a few reasons why:

A higher tax may change what Americans eat or drink. But what will they eat or drink in place of the highly taxed items?

"The sugar reduction from taxing sweetened drinks, for example, could be partly offset if consumers switch to juice and beer or eat more bread or cookies," authors of the paper note.

That is, if consumers just substitute one high-sugar item for another, that would undercut the effectiveness of the tax.

All kinds of sugar pose risks. Taxing sugary drinks is usually discussed in the context of beverages that have "added" sugar. But all sugars may contribute to health risks, including the naturally occurring ones in juices and other beverages.

Taxes work best when they target the most direct cause of concern. Cigarettes are a very direct cause of lung cancer and other smoking-related diseases. Treating those diseases is costly. And that cost is born in large part by the public through taxes supporting Medicare and Medicaid.

So taxing cigarettes to the high heavens as New York City does can be effective at dissuading smoking, and reducing costs to society.

"That's a good use of tax policy." noted David Brunori, deputy publisher of Tax Analysts.

But sugar isn't necessarily the only factor affecting the risk for -- or consequences of -- obesity.

Metabolic factors, including genetics, and other health conditions play a role. So do lifestyle choices, like exercise and how moderate (or not) your consumption is of sugary foods and drinks.

"The health effects and medical costs of obesity, moreover, are not uniform; severe obesity is much more harmful than mild obesity," the paper said.

A tax may improve health, but only so much. The paper cites simulations suggesting that obesity rates could be reduced by 1-to-4 percentage points if sugary beverages are taxed moderately.

That's not nothing, but it won't come close to getting obesity under control. Currently about 35% of adults are classified as obese.

A higher tax that was more broadly applied to foods with ingredients that contribute to weight gain "would likely have larger effects."

In either case, a tax can't singlehandedly solve problems with obesity and other health risks, the authors assert.

"[Taxes] are no substitute for efforts to identify and help people at the greater risk from obesity, diabetes and related conditions."

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