Tuna fish is economical and packaged in cans, so it can be a convenient source of protein if you're on a budget or need a meal that travels well and doesn't require refrigeration. Tuna is relatively low in calories and fat and provides several other nutrients that make it unlikely to encourage body fat gain. However, you'll gain fat no matter what you eat if you consume too many calories, so consider tuna's role in your overall diet.
Tuna is relatively low in calories, which makes it unlikely to be fattening. A 3-oz. serving of tuna contains just 99 calories, which comprises less than 5 percent of the daily suggested intake of 2,000. Your overall calorie intake dictates whether or not you gain fat; a surplus of 3,500 calories causes you to gain approximately one pound. Tuna is unlikely to promote weight gain because it's low enough in calories to fit into your diet.
Tuna is rich in protein, as each 3-oz. serving of the fish contains 22 g of this nutrient. Protein is known for its muscle-building effects, but it can also aid in weight management. High-protein foods tend to be less fattening than others, as research from the May 2008 edition of "The American Journal of Clinical Nutrition" explains that protein is more filling than other nutrients and can help you burn calories more quickly.
Tuna lacks carbohydrates, which may make it a less fattening food than higher carbohydrate foods. While carbohydrate-rich foods can't make you fat if you don't consume a calorie surplus, research from the March 2010 edition of "Nutrition & Metabolism" indicates that lower carbohydrate diets are more effective for limiting body fat.
Tuna is very low in fat, as a 3-oz. serving contains less than 1 g. While dietary fat intake does not directly produce fat gain, dietary fat is the most calorie-dense nutrient, so fatty foods tend to be high in calories. Additionally, research published in the May 2001 edition of "International Journal of Obesity and Related Metabolic Disorders" suggests that fat is less filling than other nutrients and promotes a lower rate of calorie burning, so fatty foods may encourage fat gain. Tuna contains omega-3 fats, healthy fats that may reduce your risk of heart disease.
Tuna is a rich source of vitamin D, with more than 25 percent of the daily suggested intake in each 3-oz. serving. Vitamin D is important because it is required for growth and repair of bones. In addition, the March 2011 edition of "Hormone and Metabolic Research" indicates that increased vitamin D intake can promote higher levels of testosterone, a hormone that can increase muscle and reduce body fat.