Fitbit cut its first quarter outlook and said it expects to invest in its corporate wellness business, new products and migrate to SAP in 2016.

The company's fourth quarter was well ahead of expectations, but the first quarter outlook was light.

For the fourth quarter, Fitbit reported earnings of $64.2 billion, or 26 cents a share, on revenue of $711.6 million, up from $370 million for the same quarter a year ago. Non-GAAP earnings were 35 cents a share, a dime better than estimates.

But the first quarter outlook fell short of expectations even though Fitbit's 2016 guidance was in line. Fitbit said first quarter earnings would be between break even and 2 cents a share on revenue of $420 million to $440 million. Wall Street was looking for earnings of 23 cents a share on revenue of $484 million.

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For 2016, Fitbit sees revenue for $2.4 billion to $2.5 billion and non-GAAP earnings of $1.08 to $1.20 a share.

So what's going on for Fitbit, which moved 21.4 million devices in 2015? Here's a look at the moving parts:

For the first quarter, Fitbit is launching two products globally with the Blaze and Alta. Marketing campaign expenses will cut into earnings.

Fitbit also said it will have additional manufacturing costs for those devices in the first quarter. Fitbit added that its sales channel shipments may be lumpy.

For 2016, Fitbit expects to smooth out its results due to new products and geographic expansion.

But the year will have investments too. "Operating margins reflect strategic investments expected in 2016 to further develop Fitbit's Digital Health strategy, expand corporate wellness offerings, support 2017 and 2018 product roadmaps and further build out back office infrastructure to support increasing scale and global breadth, including migrating business systems to SAP," said the company in a statement.

Add it up and Fitbit has some uncertainty ahead. For starters, the company is ramping manufacturing and migrating back-end systems to SAP. Those items alone would pose challenges. New products with uncertain demand over the long haul will be other significant wild cards.