Prisoners who've had the loaf hate it. Johnnie Walton had to eat it in the Tamms Supermax in Illinois. He describes it as "bland, like cardboard." Aaron Fraser got the loaf while he was serving time from 2004 to 2007 in several different institutions for a counterfeit-check scheme. He loathed it.

"They take a bunch of guck, like whatever they have available, and they put it in some machine," Fraser says. "I would have to be on the point of dizziness when I know I have no choice [to eat it]."

No one knows exactly how many institutions use it, but Benson Li, the former president of the Association of Correctional Food Service Affiliates, estimates that the number is over 100. At least 12 states — including California, Texas and New York — serve it in state-run institutions, as do dozens of municipal and county jails across the country.

In Pennsylvania state prisons, "food loaf" is made with milk, rice, potatoes, carrots, cabbage, oatmeal, beans and margarine. The Clark County Jail in Washington state serves a version with most of those ingredients, plus ground beef or chicken, apples and tomatoes.

Law enforcement says the loaf isn't so bad. "It's a food source; it contains all the vitamins and nutrients and minerals that a human being needs," says Milwaukee County Sheriff David Clarke, who has used the loaf in his jail for five years. "It's been approved by the courts. I've had it myself — it's like eating meatloaf. "

But prisoners who misbehave don't just get it once. They have to eat it at every meal, for days or weeks at a time. That's why it works as a deterrent, says Sheriff Clarke.

"If you're up on a first-degree murder charge, or some serious sexual assault of a child, you don't have much to lose in jail," says Clarke.

"But when we started to use this in the disciplinary pods, all of a sudden the incidence of fights, disorder, of attacks against our staff started to drop tremendously. The word got around — we knew it would. And we'll often hear from inmates, 'Please, please, I won't do that anymore. Don't put me in the disciplinary pod. I don't want to eat nutraloaf.' "

Scientists say it's the monotony of eating the loaf that's the real punishment. Marcia Pelchat, a physiological psychologist at the Monell Chemical Senses Center in Philadelphia, says humans have evolved to crave a variety of food.

"Having to eat the loaf over and over again probably makes people miserable. They might be a little nauseated by it, they're craving other foods," says Pelchat.

And it can sometimes stop prisoners from eating altogether. "It's very difficult to consume enough calories to keep your weight up if you're on a boring diet," says Pelchat.

That's why human rights advocates say it's unethical to use food as punishment in this way.

"Given that food is clearly recognized as a basic human need to which prisoners are constitutionally entitled, restrictions on food, taking away food has always been sort of legally right on the line," says David Fathi, director of the National Prison Project for the American Civil Liberties Union.

There's no guidance from the government on using the loaf, but the American Correctional Association, which accredits prisons and sets best practices for the industry, discourages using food as a disciplinary measure.

The Federal Bureau of Prisons says it has never used the loaf in its facilities. Still, the loaf persists in other parts of the corrections system, and no agencies or organizations are keeping track of where and how often it's used.

So Benson Li, the former president of the Association of Correctional Food Service Affiliates and the food service director at the Los Angeles County Jail, offered to help us find that out.

At a recent meeting of the association, Li conducted an informal survey at the request of NPR. About 40 percent of the prisons and jails that responded said their use of the loaf is diminishing, 30 percent said they do not use nutraloaf, and about 20 percent said their use was about the same or slightly growing.

Li says that, overall, the results suggest that the loaf is gradually being phased out.

"[Prisons and jails] are using less or some of them are using sparingly — maybe just two to three times in the last year," he says.

Li says he thinks one of the reasons for this is that prisoners have been challenging the loaf in the courts.

"You have seen a lot of different inmate claims and lawsuits against the Eighth Amendment in different states," he says.

One of the provisions of the Eighth Amendment is that "cruel and unusual punishment" not be inflicted on prisoners. So the prisoners who are filing these suits are hoping the courts will rule that chewing on loaf day after day is unconstitutional. And, believe it or not, there is precedent: In the 1970s, the Supreme Court ruled that a potato-y prison paste called grue should be outlawed under the Eighth Amendment.

The loaf has held up better than grue. Of the 22 cases brought since the beginning of 2012 alone, none have succeeded. But Li's informal survey suggests that the court cases are making the corrections industry increasingly squeamish about serving it.

And Fathi of the ACLU says this is part of a bigger transformation happening in the industry.

"The fading of the use of nutraloaf is part of a larger long-term trend toward professionalization and, in most respects, more humane conditions of confinement," he says.

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Food As Punishment: Giving U.S. Inmates 'The Loaf' Persists