Near the end of Kirk's career I guess obesity was at least an option, given the state of Scotty. ...Haha it looks like Stack Exchange has tackled this issue:  https://scifi.stackexchange.com/questions/18896/is-obesity-ever-addressed-in-star-trek The last answer shows that in TNG at least the replicator apparently actually does manage nutrition, at least to the extent of preferring to provide non-fattening desserts (presumably with artificial sweeteners in them).