Health advocates have one tip for Americans trying to eat less fat, sodium and sugar in their diets: Shop the perimeter. In other words, buy food from the produce, dairy and meat aisles and avoid the inner lanes where processed foods and sugary snacks hang out. But it turns out this advice is either badly needed or being badly ignored, as the greatest portion of what we spend on groceries is coming straight from the center aisles.

A series of charts published as part of Planet Money's Graphing America series on National Public Radio's website shows the real reason Americans are getting fatter: Nearly 23 percent of grocery bills is being spent on processed foods and sweets. The next largest chunk of our grocery bills is being spent on meat (21 percent). Compare that to 30 years ago, when meat made up the biggest portion of the grocery bill, fruits and vegetables second and processed foods ranked a distant fifth.

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Credit: Planet Money/National Public Radio

Part of this has to do with shifting costs. The dropping cost of sugar -- down more than 16 percent over the last 30 years -- has made sweets more affordable for manufacturers to make and for consumers to buy. And this category will grow even more -- Kellogg, maker of breakfast cereals, is moving forcefully into the snack category, and PepsiCo is going to start marketing its own line of chips called Stax, the New York Times reported earlier this year.

Meat today is also significantly cheaper than it was in the early Reagan years, which means it takes up less of the total grocery bill. All the major meat groups -- from steak to chicken legs -- have seen significant price cuts over the past three decades. Pork chops have seen the biggest price drops, going from $6.00 per pound back in the day to $3.72 today. Steak has also dropped in price from $7 on average to $4.90 a pound, according to the Planet Money data breakdown. All prices are in 2012 dollars.

Here's the good news -- Americans are overall spending less on groceries, according to Planet Money. In 1982, more than 12 percent of Americans' spending went toward groceries; today that figure is less than 9 percent.

But perhaps this figure only serves to underscore the debate of quantity versus quality. Processed food offers more caloric bang for the buck than non-processed foods on average, thereby providing a significant share of daily calories in one cost-effective -- and time-effective -- serving. The reason that we don't spend as much money on groceries is because we don't have to.

Cheap, processed food has become an increasingly popular staple at dollar stores, where discount ramen noodles, frozen casseroles and other packaged food are finding a home on the shelves, Huffington Post's Alice Hines reported earlier this year.

But public health researchers worry that cheap food has a much higher cost down the road. Much has been made about food deserts, or areas that don't have full-service food markets that include a wide range of foods that one would find in the perimeter of a traditional supermarket, including fruits, vegetables and fresh meat. One researcher, reported Hines, has even linked diet-related deaths with the proximity to non-traditional or "fringe" food sources that include fast-food outlets, dollar stores and convenience stores.