The hot idea in the war on obesity? Taxes.

Denmark's fat tax took effect this weekend. Calculated on the amount of fat used to make a food (not the final food product), the tax rate is 16 Danish kroner per kilogram of saturated fat.

The tax adds about 12 cents to a bag of chips and about 40 cents to the price of a hamburger.

Taxing unhealthy foods? It's not just in the land of Dyrlægens natmad and Leverpostej. France just levied a soda tax; Hungary did the same with fatty and salty foods; the Swedes are considering a tax on calories.

Here in North America, there are growing calls for some type of tax to battle the bulge. In Arizona, policymakers proposed a tax on obese Medicaid patients. Philadelphia Mayor Michael Nutter launched his second attempt to impose a soda tax weeks ago. In February, California Assembly member Bill Monning (D-Carmel) favours that tax and introduced a bill, which was later voted down. And here in Canada, there has been talk of a fast-food tax.

That more and more policymakers are considering some type of fat tax is hardly surprising. As I've noted here before, the world is not only flat, but increasingly fat.

America leads the way, with one in three adults now qualifying as medically obese in some states; nearly two thirds qualify as obese or overweight. And, according to a new report, the trend is crossing the 49th parallel: a full one in four adults in Canada is obese, roughly a doubling of the percentage of the obese population since 1980.

Among these different tax proposals, one common idea emerges: if people are eating too many calories, hit them in the wallet. The idea has its appeal, which is why policymakers are talking up taxes, from Copenhagen to California.

But call me a fat tax skeptic.

Start with a practical issue. By all accounts, even the most favorable studies argue that dietary taxes have to be very high -- in the 18 per cent to 25 per cent range -- before they'll have any real impact on consumers.

Even then it's not clear that people will eat healthier; they may just shift their calories. Danes may put down their potato chips, but swill more beer.

And that leads to the larger issue: a person's diet is more complicated than a single meal choice. A man can eat a healthy breakfast of oatmeal and fruit every day, only to wipe out the benefits over lunch. His wife could be a vegetarian, only she's drowning herself in sugar by drinking two or three caramel-based coffee drinks at work. Their adult son might eat a slice of cake every night, yet he walks five miles to work and back every day to try to make up for it.

Only you can manage your own diet and your own calorie intake. No government, no restaurant, no physician can do it for you. It's this complexity that makes personal health responsibility so important in reversing the obesity epidemic.

Consider the response of one Dane to the new tax.

"(Mathias) Buch Jensen, for one, is not planning to change his eating habits. Asked if he would be giving up butter, he offered a compromise: 'I would fry cabbage in butter, and add a little more butter at the end. That way at least I'm getting my vegetables.'"

I've never met Buch Jensen, of course, but maybe he is the sort of person who can fry cabbage with butter -- a thin, athletic Dane who exercises regularly and generally eats well.

We can look at his countrymen. For as much as Danes seem to love their bacon and butter, the overall obesity rate is far below the European average -- and far, far below the Canadian average. Here's the point: Danes seem to practice moderation.

Fat tax advocates point to the war on tobacco to bolster their argument. And let's be clear: curbing tobacco has been a major public-health accomplishment, literally saving millions of lives here in North America over the past six decades. And, yes, taxes on tobacco have been a major component of the long campaign.

But food isn't tobacco. There is no such thing as a healthy amount of smoking, after all.

That's not to suggest that obesity isn't a major public health problem. Nor is it to suggest that policymakers are wrong to consider options. But like many efforts to address public-health issues, this initiative is simplistic -- in Denmark or here.