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Key S&P sector may be signaling a second-half growth slowdown, strategist warns

Materials stocks may be flashing a warning signal.

A recent reversal in the S&P 500 basic materials index could mean growth is about to hit a wall, Miller Tabak's chief market strategist, Matt Maley, told CNBC's "Trading Nation" on Tuesday.

"From March 2020 until May of this year, the materials stocks outperformed in a major way," up around 130% versus a 90% gain for the broader S&P 500, Maley said.

But materials stocks have stalled since their May 17 peak, falling 5% while the S&P has climbed 5.5%.

A reversal like that from such an economically sensitive sector combined with falling interest rates and oil prices could spell trouble for growth for the rest of 2021, Maley warned.

"We're having several things outside of the stock market, but also within the stock market, that are telling us that we may have to put on the brakes a little bit about our enthusiasm about growth in the second half," he said.

Add new mask mandates and slower-than-expected manufacturing growth to the mix, and investors should be thinking about protecting their portfolios, New Street Advisors Group founder and CEO Delano Saporu said in the same "Trading Nation" interview.

"Investors could start looking at the diversification in their portfolio, start looking at asset allocation, those other areas to put money in if we see some sort of slowdown in equities because we know equities have run up a lot since ... the trough in March of 2020," Saporu said.

"From my standpoint, I'm definitely looking at that for clients and for myself."

In an email to CNBC, Saporu said investors "need to have enough exposure to defensive/alternative assets," highlighting cyclical groups such as the health-care sector and cryptocurrencies such as bitcoin.