Michaels recently caused a stir after telling a women's health magazine that she doesn't understand people's fascination with the trendy diet, which promotes high-fat and low-carb meals, encouraging its followers to load up on foods like bacon, avocados, eggs and cheese, but avoid bread, bananas and beans.
"Why would anybody think this is a good idea?" she asked.
Roker, who's struggled with his weight all his life and follows the keto diet himself, didn't take kindly to Michaels' comment.
"This from a woman who promoted on camera bullying, deprivation, manipulation and more weekly in the name of weight loss," Roker tweeted, referring to Michaels' time on "The Biggest Loser."
He later added on the "Today" show, "My point is — what works for you works for you, and there's science on both sides that say it's not a great idea and science that says it is a good idea."
But Roker's harsh tweet shocked Michaels.
"I was pretty upset," she told Inside Edition. ".... This was a guy I did segments with on the 'Today' show for over a decade."
Michaels, who outlines her own diet and exercise program in her book "The 6 Keys," added that she'd like an apology from Roker.
"I don't think there's any scenario where a debate about nutrition should become personal, name-calling and mudslinging," she said.