I don't know what marmite is lol, Tuna is ok as a treat once in a while. cats are obligate carnivores and meat is what is good for them. What isn't is dry food. Please check the links at the bottom of the post
Nutrition since there are so many bad things out there is very important to your cat’s health
Contrary to what you may have heard, dry foods are not a great thing to feed a cat.
Dry foods are the number 1 cause of diabetes in cats as well as being a huge contributing factor to kidney disease, obesity, crystals, u.t.i’s and a host of other problems. Food allergies are very common when feeding dry foods.
The problems with it are that they are loaded with carbohydrates which many cats (carnivores) cannot process. Most of the moisture a cat needs is gotten
out of the food and 95% of it is zapped out of dry foods in the processing. Also, most use horrible ingredients and don't use a muscle meat as the primary ingredient and use vegetable based protein versus animal. Not good for an animal that has to eat meat to survive.
You want to pick a canned food w/o gravy (gravy=carbs) that uses a muscle meat as the first ingredient and doesn't have corn at least in the first 3 ingredients if at all. Fancy feast is a middle grade food with 9lives, friskies whiskas lower grade canned and wellness and merrick upper grade human quality foods.Also, dry food is not proven to be better for teeth. Please read about cat nutrition.
http://www.newdestiny.us/nutritionbasics.html
http://www.catinfo.org/feline_obesity.htm
http://maxshouse.com/feline_nutrition.htm#Dry_Food_vs_Canned_Food.__Which_is_reall