Denim is gaining on yoga pants, and athleisure needs to 'watch out'

Americans are falling in love with denim again — and it's coming at the expense of yoga pants.

Amid debate over whether the "athleisure" trend is starting to fizzle, search data from Google Trends shows that interest in denim is picking up. Meanwhile, fewer shoppers are on the hunt for yoga pants.

Jefferies analyst Randal Konik published the data in a note to investors Friday, saying brands like Lululemon should "watch out."

Source: Jefferies Oct. 7, 2016 research report by Randal J. Konik, citing Google Trends data

"We believe that the athleisure trend is plateauing, and we expect more conservative growth going forward," he said.

Data from Euromonitor "corroborates this thesis," Konik said. It shows that growth in the sports apparel market has recently slowed to 5 percent, from 7 percent in prior years. The category's rate of growth is expected to remain in the 4 percent to 5 percent range over the next several years, Konik noted.

After several years of decline, the fashion cycle appears to slowly be shifting back toward denim. Interest in the once ubiquitous item has been building thanks to new styles, which include rips and wider bottoms.

Retailers from Macy's to American Eagle have recently called out strength in their denim assortments — just as Lululemon reported a traffic decline in the quarter ended July 31.

Earlier this year, Lululemon CEO Laurent Potdevin told CNBC that he, like many of the brand's critics, thinks the athleisure bubble will pop. However, he argued the burst will hurt those companies that hopped on the trend, and don't have the technical know-how to deliver as high-quality of merchandise.

"The people that are not in it for the right reasons will go away," Potdevin said.