they already hit it regarding the calories...and also, even if you don't eat the oil with the fish, it's a resource. Might not be pleasant, but any food oil can be used for some other cooking aps. the fish water is worthless.
NEVER drain your tuna?!!?!!? you need to EAT that oil!
here's why:
The oil mixes with some of the tuna's natural fat (Omega-3s), so when you drain oil-packed tuna, some of its Omega-3 fatty acids also go down the drain. Since oil and water don't mix, water-packed tuna won't leach any of its precious Omega-3s, unfortunately this also has the effect of lowering the calorie-to-weight ratio so important in a survival situation.
Every cell in our body is surrounded by a cell membrane composed mainly of fatty acids. This membrane checks that the proper amounts of necessary nutrients enter the cells and ensures efficient excretion of harmful toxins and byproducts of metabolism. Made up of fat, cell membranes depend on the type of fat we eat. To maintain integrity and fluidity, cells need a healthy membrane. This ensures good communication with other cells and efficient ability to hold water and vital nutrients. All the fats you eat get incorporated into cell membranes, and the type of fatty acids dictates how your cells respond and grow. Saturated fats are solid at room temperature, while unsaturated fats stay liquid even in the refrigerator. Researchers believe that diets containing large amounts of saturated or hydrogenated fats produce cell membranes that are hard and lack fluidity, while diets rich in Omega-3 fats promote membrane fluidity.
Omega-3 fats are also famous for their ability to improve blood flow, prevent cardiovascular disease, reduce the risk of arrhythmia, lower the amount of lipids in the blood, and prevent extreme blood clotting. Researchers found that Omega-3 fatty acids affect cell growth by activating an enzyme called sphingomyelinase, which ultimately causes cancer cell death. Additionally, fish is full of other anti-inflammatory compounds known as resolvins which, unlike drugs, such as aspirin, ibuprofen and the COX-2 inhibitors, do not have negative side effects on our gastrointestinal or cardiovascular systems.
To stay healthy and safe from premature aging or onset of chronic disease, eating tuna (or other cold water fish) twice a week is a great option. This truly nutrient-dense food gives you high quality protein, no carbohydrates, minerals selenium, magnesium, and potassium; the B vitamins niacin, B1 and B6; and perhaps most important, the beneficial Omega-3 essential fatty acids. Besides the healthy fats, fish has more easily digestible complete protein than most foods, high amounts of vitamins A, B, D, and K, as well as calcium, iodine, magnesium, phosphorus, potassium, and iron. Tuna is also a very good source of Vitamin B6 which, along with Folic Acid lowers levels of artery-damaging homocysteine and prevents atherosclerosis.
With all the fabulous benefits tuna has to offer, moderation is still the key to success. Consider that overdoses of nourishing fish nutrients, just like underdoses, may also impair immune function, elevate LDL cholesterol, alter blood lipids and blood clotting, and worsen Type II diabetes. Moreover, fish oil naturally contains high levels of the two potentially toxic vitamins, A and D.
P.S.
If you are think canned foods are less nutritious than fresh options, you are not far from truth. But canning may not be as bad as you think. Canning foods is a very effective method of preservation. The process protects against the growth of micro-organisms and kills harmful bacteria, but the high heats used for canning rob foods of vast amounts of nutrients, especially vitamin C, Thiamin and Riboflavin which are sensitive to light. You still benefit from the mineral supply of the canned foods, and the macro-nutrient nourishment may even be better than what you find in fresh products.
So... canned food is not always worse than fresh or frozen. If the food's overall quality is high, it is best in all forms - canned, frozen, or fresh.