It's not a sitcom, but Jillian on Good Day Live eats low carb. She mentions it quite a bit.

Also there was a story about Donatella Versace in Vogue last month where she said that she doesn't eat many carbs.

And as we speak there is a man on Regis and Kelly talking about cutting out the bad carbohydrates. I'm not really listening or watching but I did hear him say that much.

I think we'll be seeing more and more references.