Is it healthy to keep staff's lean all year round?

Due to her fussy eating, my staff is very lean. Even for a sbt bitch she is heavily muscled but holds hardly any body fat. I was wondering if it’s healthier for her to hold more fat for at least a few months of the year? The reason is that her body will have to work harder in the colder months, thus using more energy and fat except the fat isn’t there which would surely lead to muscular hypertrophy?