she can eat normal 食 and drink blood... while bella was pregnant she drank human blood to satisfy nessie's thirst and after she was born she began to drink human blood but after she grew up a little she was able to go hunting so bella would make her compete with jacob who would catch a bigger animal so that nessie wouldn't get used to human blood. normal human 食 is not interesting for nessie and although she can eat it she doesn't find it tasty..
When Nahuel was questioned about his diet, he 発言しました that he lived off a mixture of human blood and normal food, and that he could live off either at need (he didn't know yet that drinking animal blood was a possibility). I'd guess that Nessie would be the same.
