Just as my subject line states

How can I get my rats to have shiny coats?
My ratties look a lil "dull" lately... They eat lots of veggies and fruits along with lab blocks so they get a good diet...
Is there something I can put in their food to make their coats better or is there a better shampoo to use?
Or may it just be the winter time "blahs"?
Any tips appreciated!!