The US Department of Agriculture is Exhibit A for foolish, wasteful federal policymaking: It spends billions subsidizing the same unhealthy foods it tells us to avoid.

This year, the USDA will spend more than $1 billion subsidizing the growth of four common food additives: corn syrup, high-fructose corn syrup, corn starch and soy oils (hydrogenated vegetable oil).

These additives “provide a cheap dose of sweetness and fat to a wide variety of junk food products,” warns the US Public Interest Research Group. PIRG found that the USDA spent $18.2 billion from 1995 to 2011 to subsidize junk-food additives.

Yet, while some of your tax dollars pay for these junk-food additives, more pay for public-health education campaigns that strive to convince us to eat less and to eat better.

The USDA lectures and hectors us to avoid anything that’s fatty, salty, sugary or filled with processed ingredients — like the ones our tax dollars subsidize.

Clearly, the Agriculture Department puts the D in double standard: The bureacuracy’s left hand is at war with its right.

Of course, neither bureaucratic hand should really be in the business it’s in, anyway — either playing farmer-in-chief or national health nanny.

Whether they’re targeted at “healthy” or “unhealthy” food, farm subsidies are a bad idea, period. But a couple of hundred million dollars in lobbying seems to buy quite a few bad ideas.

Last year, taxpayers subsidized farmers to the tune of $16 billion, reports the Congressional Budget Office. Among the more idiotic subsidies are $5 billion in so-called “direct payments” to farmers of corn, soybeans and other crops. These are paid simply for owning tillable farm land, even if the farmers don’t plant on it.

On top of costing huge sums that could be better spent (or saved) elsewhere, farm subsidies perversely redistribute wealth to a comparatively few affluent farmers and agribusinesses.

And the subsidies, by financially favoring of certain crops and ingredients over others, grossly distort the agricultural marketplace. This skewers both production levels and prices — harming many smaller farmers in their wallets and many consumers around their waistlines.

It’s hard to count the ways the feds boost obesity. For example, taxpayer-funded subsidies for sugar and beef production maintain an oversupply of these goods and commensurately cheap prices.

Subsidies have proved a catalyst for US agribusiness to produce far more food than the domestic population can eat. This has had the effect of reducing the cost of food. According to anti-obesity campaigners, the cheapness of food has led restaurants to serve larger food portions, adding to America’s weight problem.

Then again, the dietary science that the USDA relies upon to guide its anti-obesity preaching is frequently as junk-filled as the fast food it tells us to boycott. There’s little solid science behind many generic, one-size-fits-all dietary recommendations, such as to eat “five a day” of fruits and vegetables. It’s a matter of record that some guidelines are devised simply to suit the need for a catchy public-health marketing campaign.

The government-funded “healthy versus unhealthy” food war is both misleading and really bad advice. The USDA ignores the scientific reality that there are no good or bad foods — only good or bad diets.

Once you weigh up the junk-food subsidies and the junk science, you may conclude the USDA’s new motto should be, “Do as we say, not as we do.” At least, that government health warning would have the ring of truth.

Patrick Basham directs the Democracy Institute and is a Cato Institute adjunct scholar. He coauthored “Diet Nation: Exposing the Obesity Crusade.”