Image source: Fitbit.

What: Shares of fitness wearables company Fitbit (NYSE:FIT) tumbled 22.3% in May, according to data provided by S&P Global Market Intelligence. The company's first-quarter results came in above analyst expectations, but earnings guidance fell well short of estimates.

So what: Fitbit reported first-quarter revenue of $505.4 million, up 50% year over year and about $62 million higher than analysts were expecting. Much of this growth was driven by new products: The Blaze and the Alta accounted for nearly half of Fitbit's total revenue. In total, Fitbit sold 4.8 million devices during the quarter, up from 3.9 million during the same period last year.

Non-GAAP EPS came in at $0.10, down from $0.27 during the prior-year period but $0.07 higher than the average analyst estimate. Major increases in spending drove down Fitbit's bottom line, with total operating expenses rising by 170% year over year. R&D spending more than tripled, and sales and marketing spending more than doubled.

This heavy spending seems to be the main cause of the poor performance of the stock in May. Fitbit guided for second-quarter non-GAAP EPS of $0.08 to $0.12, well below analyst expectations of $0.26.

Now what: The pessimism surrounding Fitbit stock is understandable. The company's fitness devices are at risk of eventually becoming irrelevant due to more powerful wearable devices, and the company's extremely heavy spending could signal that its impressive profitability in the past may be difficult to maintain in the future.

Despite Fitbit's solid first-quarter results relative to analyst expectations, investors weren't thrilled with the company's guidance. Going forward, Fitbit will need to translate these additional expenses into continued revenue growth. Otherwise, investors will probably continue to flee.

Timothy Green has no position in any stocks mentioned. The Motley Fool recommends Fitbit. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.