First, kill all the farm subsidies! That should have been President Obama’s mantra if he truly wanted to curb the nation’s child-obesity “epidemic.” Instead, on Monday he signed into law the Healthy, Hunger-Free Kids Act.

That law has plenty of problems. But it’s certainly striking how it ignores such a flagrant contributor to flabby youth — Uncle Sam’s economically illiterate farm program.

The US Department of Agriculture spends billions a year on farm subsidies that indirectly promote obesity.

How? The subsidies maintain an oversupply of certain foods at commensurately cheap prices. Subsidies have proved a catalyst for agribusiness to produce far more food than the population can eat, reducing the price for consumers. Cheap food has led restaurants to serve larger portions, and arguably this has contributed to the obesity problem.

The feds’ farm policies also promote the substance anti-obesity campaigners call “liquid Satan” — high-fructose corn syrup.

A generation ago, the USDA began paying farmers to grow as much corn as possible. Today, subsidies to crops such as corn total $19 billion a year. Corn subsidies total more than $8 billion a year.

Cheap corn enables the corn-processing industry to profitably churn out an abundance of high-fructose corn syrup, selling it cheaply to food and beverage companies. The syrup, a fructose-glucose liquid sweetener, is a major alternative to sucrose (table sugar) first introduced in the 1970s. Meanwhile, the feds restrict the supply of sugar via import quotas on foreign-grown sugar — raising the US price of sugar to two-to-three times the global level. With the syrup as the cheaper sweetener, the food and beverage industry has replaced sugar with corn syrup in thousands of foods.

The syrup is virtually the only sweetener used in soft drinks, for example. And thanks to farm subsidies, the cost of soft drinks containing it has fallen 24 percent since 1985.

High-fructose corn syrup accounts for 81 percent of the calories added to the diet of the average American in recent decades; US consumption of the substance rose 120 percent from 1991 to 2000. More significantly, it rose more than 1,000 percent from 1970 to 1990, far exceeding the changes in intake of any other food or food group. Over the same period, the average American’s caloric intake increased by 600 calories daily.

Other issues that you’d think would matter to the anti-obesity Obama administration: Higher consumption of sweetened beverages is associated with weight gain over time and some research has found differences in the rates of digestion and absorption between fructose and other sweeteners.

And newly published research by Professor Leon Ferder of the Buenos Aires University Medical School and his colleagues found fructose consumption and obesity are linked because fructose consumption does not cause an insulin response. Without an insulin response after eating, there’s no suppression of appetite — which may lead a person to continue eating or overeating as the case may be. Any excess calories are converted and stored as fat.

Of course, corn syrup by itself isn’t responsible for most child-obesity cases — but it certainly appears to be a contributing factor. Above all, the subsidy makes it more likely that a kid will consume higher amounts of “empty calories.”

President Obama has set a bad example on the obesity issue. As a senator and presidential candidate, he consistently backed corn subsidies. Will the new Congress continue to subsidize the corn-based sweeteners suspected of contributing to the obesity problem? Will the president dare to change his tune?

Certainly, without a politically courageous presidential stand, Michelle Obama’s anti-obesity campaign won’t make a fat lot of difference.