During a turn, will slip necessarily lead to altitude lost?
Think of a slip or skid that the aircraft is flying sideways (referred to as uncoordinated).  This does increase drag, and therefore it requires either more power, or given sufficient speed, increased pitch to maintain altitude compared to flying straight through the air in coordinated flight.

A "skid" is when the aircraft nose is pointed inside a turn where the pilot feels centrifugal force outside the turn, like in a skidding car.  In a slip, the nose is pointed outside the turn whereby the pilot feels a force of falling down into the turn.  In straight flight, whether the nose is pointed left or right, uncoordinated flight is called a slip either way: a forward slip for losing altitude, and a side slip for landing.