The question has been asked too many times not to deserve an answer.
The bottom line is that sushi can be quite healthy or unhealthy depending on what ingredients are used, how it is prepared and how much you eat.
Fresh fish, particularly salmon, trout and tuna, can have plenty of protein and omega-3 fatty acids. Depending on the type of fish, fish can also have varying amounts of Vitamin A and D, calcium, and magnesium.
Fresh vegetables frequently appearing in sushi can have important vitamins and minerals. Asparagus can provide vitamins A, B2, B6, C, E and K, fiber, numerous minerals such as copper, phosphorus, potassium, zinc and iron.
Cucumbers offer a host of nutrients such as pantothenic acid, potassium, manganese, vitamin C, phosphorus, magnesium and vitamin B1. Carrots have beta-carotene, fiber, vitamin K, potassium and antioxidants. Shiitake mushrooms are good sources of selenium, iron, dietary fiber, protein and vitamin C and have anti-oxidants.
However, calling something sushi does not mean that it is healthy.
To make sushi more healthy, make sure you do the following:
Minimise the use of sauces
Sauce is like makeup, hiding the true appearance and taste of the fish, vegetables and rice, especially when these are not very fresh or high quality. Be suspicious when you see a sushi menu short on sushi without the sauce. Many sauces are heavy on salt, sugar, fat or calories.
Be careful about any additional adjectives attached to the name of the sushi. The “spicy” versions of sushi frequently have sauces that are primarily mayonnaise. In some restaurants, “crazy” and “wild” can also mean “spicy.” “Creamy” sushi rolls can have added butter or other types of fat. “Crunchy” rolls often have a fatty batter. In general, I don’t eat anything named “angry,” “vengeful,” “spiteful” or “jealous.”
Remember deep fried sushi is deep fried
An increasingly common type of sushi is deep-fried sushi. Yes, deep frying can seem synonymous with “make tastier.” But it also can add calories, salt and fat.
Check the ingredients
Calling something sushi does not change its ingredients. Bacon in sushi is still bacon. Salt on sushi is still salt. Ask the restaurant or chef specifically what is going into a sushi dish. Is the dish just fish and rice and vinegar? What else is added? How much salt? How much sugar?
Look for lower rice ratios
Sushi rice tastes different because of the added vinegar, salt and sugar. A piece of sushi can range from being largely fish or vegetables to just a tiny piece of fish sitting on a mound of rice. The fish is the expensive part of sushi, so some restaurants will try to stuff you with rice instead.
Choose the right fish
Fish may have mercury, polychlorinated biphenyls (PCBs), prolybrominated diphenyl ethers (PBDEs) and other harmful chemicals. The trouble is many of the pollutants that humans dump into the oceans and other bodies of water end up in fish.
As Consumer Reports indicates, fish that tend to have the lowest amounts of mercury are the five S’s (salmon, shrimp, scallops, squid and sardines), oysters and tilapia. Fish that tend to be high in mercury include king mackerel, Chilean sea bass, bluefish, halibut, Spanish mackerel and tuna.
Also, fish farmers frequently add chemicals to make the fish larger and more attractive, so you may want to inquire whether the fish is farm-raised or wild.