No, you probably need Photoshop, Photoshop Elements or a good diet.
lol abt the diet part. its for people that ill be taking pics of.
Actually, yes - sortof: a new feature of Lightroom 5 supports "correcting" aspect ratio (see manual lens corrections in develop module) which can make people look skinnier all over - i.e. it's not "selective" (you can't target body regions, or individual persons) like the advanced tools in Photoshop (e.g. liquify), but if it's all you need..