Gwen Stefani sleevless shirt black pants kingston

Someone won't have to worry about gaining any weight over the holidays!

A very thin Gwen Stefani was snapped with sons Kingston and Zuma in Los Angeles on Tuesday, and the singer/fashionista rocked -- gasp! -- a muscle tee, baggy pants and a sweatshirt.

While the outfit may be casual, Gwen did wear a full face of makeup, but it looks like she forgot her sunglasses! She has to know that squinting can cause wrinkles ... even at 43 years old, she's not immune to aging!