Krill are shrimp-like crustaceans that are approximately 1 to 6 centimeters long. They live is the ocean, where they feed mainly on phytoplankton. They're near the bottom of the food chain and are eaten by whales, seals, penguins, squid and fish. Krill oil, the oil that's found naturally in krill, is extracted and sold as a nutritional supplement.

Krill oil contains omega-3 fatty acids, which is the main reason it's becoming popular as a nutritional supplement. It also contains an antioxidant called astaxanthin. The algae that krill eat produces the bright red pigment astaxanthin that gives krill and other crustaceans such as lobster and shrimp their reddish-pink color.

Antioxidants protect our body cells from damage from free radicals, unstable substances that are thought to contribute to certain chronic diseases. Unlike many other antioxidants, astaxanthin crosses the blood-brain barrier, where it could theoretically protect the eye, brain and central nervous system from free radical damage.

People use krill oil for the same reasons they use fish oil, flax oil or other omega-3 fatty acids. Unlike fish oil, krill oil doesn't cause fishy burps or an aftertaste, a common side effect of fish oil. Also, krill oil contains higher amounts of astaxanthin than fish oil. Here are some specific conditions for which it's used.