Crab is nutrient-dense but low in calories. Crabs, like all other shellfish, are high in chromium, which helps insulin digest sugar and, as a result, reduces blood glucose levels. All shellfish, including crab, contain high levels of selenium. Selenium is an antioxidant that counteracts the carcinogenic effects of cadmium, mercury, and arsenic, which can cause tumors in humans. Higher levels of selenium in the blood have been shown to reduce the risk of cancer. In reality, lysate produced from the horseshoe crab's blue blood is used to identify spinal meningitis and treat cancer. Learn more about is crab good for diabetics by reading further.