Sushi is made with a short-grain, starchy rice. That's what helps it stick together in a pretty little roll. Technically, sushi refers to a rice dish that rolls sticky rice, flavored with rice-vinager and a bit of sugar, around some type of filling. It can be rolled with seweed, or not, according to your preference.
White rice, especially the short-grained variety, is not the most healthy food. It rapidly sends sugar into the blood, forcing the body to increase blood insulin. This in turn has the effect in healthy people of removing too much sugar from the blood, both through metabolization, and conversion of blood sugar into fat which is stored for times of starvation. It stresses the pancreas and, if done on a regular basis (think of eating lots of potatoes with every meal), can be a factor that leads to type-2 diabetes.
Eating sushi doesn't have to be unhealthy. But as with all foods, it has to be eaten sparingly. All foods have both healthy and unhealthy effects on the body. The absolutely best form of dieting is to teach yourself about good nutrition, and eat healthy, forever. It requires a life-style change. You've heard it all before, whole grains, including such things as flax seed, oatmeal, whole wheat, etc., deep-green veggies, leafy veggies, deep orange veggies (again in moderation) brightly colored foods such as peppers, berries, lugumes (the more color, the higher the nutritional content, say kidney beans versus great white, though neither are nutritional slouches), dairy, meats (lean and grilled, or broiled are the healthiest ways to eat meat, and red meat can also contribute to adult onset diabetes).
In short, eat a small anount of a wide variety of veggies, fruits, meats (inclucing fish), and whole grains. Eat reasonable portions. Train yourself to eat when you're hungry, and eat slowly. Take the time to savor each mouthfull. Then, increase your caloric output through exercise, or activity. Then, you won't have to diet. And you will be able to do so much more with your life, and enjoy it.
Seeeeeya; Goodweed of the North