Adding shellfish to your diet increases your intake of lean protein, and shellfish fit into a healthy diet. While they also have a few health disadvantages -- for example, low-quality shellfish can contain toxins that affect your nervous system -- shellfish purchased from reputable retailers make smart additions to most diets. They also provide beneficial micronutrients and increase your consumption of vitamins and minerals.
Calories and Protein
Shellfish contains a moderate amount of calories -- a 3-ounce portion of cooked lobster contains 151 calories, while equivalent servings of cooked crab and oysters contain 141 and 173 calories, respectively. Some of these calories come from shellfish's protein content, and your body uses this protein to make hormones, antibodies and enzymes as well as proteins needed for new cell growth. You need several grams of protein daily -- 56 grams for men and 46 grams per women, according to the U.S. Department of Agriculture. A serving of cooked lobster contributes 32 grams toward this goal, while cooked crab and oysters offer 30 and 19 grams of protein per serving.
Incorporating shellfish into your diet helps you consume more vitamin B-12, also called cobalamin. Your cells need cobalamin to control gene activity, a physiological process that helps ensure healthy cell behavior and fights cancer growth. It also supports nerve cell communication and makes heme, the chemical essential for red blood cell function. Shellfish provide generous amounts of vitamin B-12, which makes it easy to consume the 2.4 micrograms you need daily. Cooked oysters provide 30 micrograms of vitamin B-12, while crab and lobster contain 5.7 and 2.4 micrograms, respectively.
Zinc and Copper
Shellfish also come loaded with minerals, particularly zinc and copper. Every cell in your body contains zinc, and you need it for proper wound healing, immune function and energy production. Like zinc, copper helps you make energy and nourishes your immune system, and it also helps your nerves function. You need only small amounts of copper and zinc daily -- 8 milligrams of zinc for women, 11 milligrams for men and 0.9 milligram of copper daily for both men and women. A single serving of cooked oysters provides your entire daily requirement for both vitamins. Cooked crab and lobster also provide all of the copper you need in a day, and they contain 6.5 and 6.9 milligrams of zinc per serving, respectively.
Risks from Sodium
Consume shellfish in moderation to avoid consuming too much sodium -- some types of shellfish, especially lobster and crab, are high in salt. A 6-ounce serving of cooked lobster contains 826 milligrams of sodium -- 40 percent of your 2,300 milligram daily limit -- while an equivalent portion of crab contains 672 milligrams, or 29 percent of your limit. Oysters provide a lower-sodium option, at 282 milligrams of sodium -- 12 percent of the daily limit -- per serving. Keeping your sodium intake low promotes lifelong health, while a high-sodium intake means you'll face a higher risk of gastric cancer, kidney stones and heart disease.