A. Cassandra Becker, a dietetic intern at Tufts Frances Stern Nutrition Center, answers: Fish provides high-quality protein, as well as healthful omega-3 fats, essential amino acids, nourishing trace elements and vitamins, while being low in cholesterol and saturated fat. However, there are many risks associated with consuming raw fish, such as sushi, ceviche, sashimi and other dishes. Raw fish can increase the risk of foodborne illness caused by bacteria and parasites, a growing concern with globalization of the food supply. Cooking fish to a temperature between 129-133 degrees Fahrenheit for five minutes is the most effective method for killing parasites, although flash-freezing the fish at a temperature of -31 degrees Fahrenheit for 15 hours, as is often the case with sushi-grade fish, or by regular freezing at -4 degrees Fahrenheit for seven days before consumption, is also effective.
Although cooking does not remove mercury (or other contaminants such as polychlorinated biphenyls and dioxins), some varieties used in sushi, including shark and swordfish, are higher in mercury. Thus, sushi consumption has been associated with higher blood mercury levels.