Practically zilch. It's one of the healthiest snacks around. There will be some fat if you go for salmon/tuna/egg toppings as these are oily fish but they are good for you oils and you are only eating small quantities.
Tuna or Maguro sushi with fat marbled thru it is just so expensive...but most japanese only eat sushi on special occasions and seldom more than once a week...if u have a bad stomach and go to the doctor guess what his first Q is ....have u eaten sushi lately?...it can be that bad sometimes especially the cheap stuff...