NEW YORK » Dr. Mehmet Oz says last week’s attack by 10 doctors who accused him of promoting "quack treatments" on his TV show was spurred by his vocal support for labeling genetically modified foods — a stance he says some if not all of those accusers oppose.

Oz devoted the first half of his syndicated show on Thursday to his response to what he called "a brazen letter from 10 mysterious doctors" sent to Columbia University, where Oz serves as vice chairman of the surgery department and performs heart surgery at Columbia’s affiliated hospital. The letter accused him of an "egregious lack of integrity" and urged the university to remove him from its faculty.

A Columbia spokesman defended Oz by saying the school is "committed to the principle of academic freedom."

But the letter set off a new round of criticism of Oz, who in the past has been slammed for promoting questionable cure-alls and last June appeared before the Senate’s consumer protection panel, where he was scolded for claims he had made on his show about weight-loss aids — claims he says he has since stopped making.

New Yorker staff writer Michael Specter, who profiled Oz for that magazine two years ago, said on NPR this week that, whereas every doctor’s first obligation is to do no harm, Oz "does harm every time he goes on the air by recommending things for which there is no evidence, and things often that he knows not to be true."

Meanwhile, New York Times columnist Frank Bruni described Oz as "a carnival barker" and "a one-man morality play about the temptations of mammon and the seduction of applause…."

Launched in 2009 by Oprah Winfrey, "The Dr. Oz Show" has repeatedly been criticized for mixing rigorous and dubious science. But Oz, who sees his show as a forum for discussing a range of health practices to better inform his audience, wondered aloud what accounted for the timing of last week’s letter.

His conclusion: His own continuing opposition to any legislation that would strip government agencies of their ability to label genetically modified foods in the marketplace.

Oz reiterated his conviction that consumers have the right to know whether food at their stores originates from genetically modified organisms. Those on the other side of the issue include food producers and agricultural companies who say GMOs are safe and that labeling them would increase prices while unnecessarily frightening consumers.

He noted that the letter’s lead author was Dr. Henry Miller, a senior research fellow at the Hoover Institution think tank at Stanford University who specializes in biotech and has campaigned against such labeling.

Oz characterized the letter as a smear intended to silence him and vowed, "We will not be silenced."

"My interest was and is solely to protect the academic respectability of a prominent medical institution," Miller said in a statement in response to Thursday’s program.

Dubbed "America’s doctor" by Winfrey, Oz has built a brand empire that also includes a magazine and best-selling books. But, whatever the reason, viewership for his TV show has eroded in the past few years. For this season through March, the audience has averaged 1.97 million viewers — roughly half the 3.80 million viewers who were tuning in during the 2011-12 season.

Even so, "he still has a pretty significant following," said Brad Adgate, a ratings analyst for Horizon Media, adding that, even if Oz’s current show were to end, he has other potential outlets, including the cable network of his benefactor, Winfrey.

"His current numbers would match those of a top-tier cable network in prime time," Adgate said.

EDITOR’S NOTE — Frazier Moore is a national television columnist for The Associated Press. He can be reached at fmoore@ap.org and at http://www.twitter.com/tvfrazier. Past stories are available at http://bigstory.ap.org/content/frazier-moore.