*2.4. Sterols*

Krill oil also contains an important fraction of sterols that range between 2.3–3.9% of total lipids [42]. Desmosterol and cholesterol represent the most abundant sterols of the total sterols [30]. Desmosterol is the precursor of cholesterol and represents 1.70–18.63% of total sterols [17]. The component variation is due to the different food intake of krill since crustaceans are not able to endogenously synthetize sterols but are supplied by diet and phytosterol dealkylation. Cholesterol has been reported as 81.33–82.34% of total sterols with a concentration of 18.95 to 31.96 mg/g of oil [43]. Compared to other fish oil, cholesterol amount in krill oil is even higher than hoki oil, which contains 5.15 mg/g of oil, 2.04 mg/g of tuna oil and 11.81 mg/g of egg yolk oil [17]. Considering the potential nutraceutical application of krill oil, a higher content of cholesterol may represent a matter of concern due to the onset of cardiovascular diseases, for example atherosclerosis, in a cholesterol rich diet. However, extraction methods could limit the content of total cholesterol in krill oil as reported by Bruheim and Cameron who demonstrated that using pure ethanol instead of ethanol mixed with water significantly reduced the cholesterol concentration. Minor percentages are represented by other sterols found in krill samples in 1977, such as

24-nordehydrocholesterol (0.1–1.7%), trans-dehydrocholesterol (1.1–1.5%), brassicasterol (0.5–1.7%), 24-methylenecholesterol (0.1–0.4%), and two stanols (0.1–0.2%) [44].

#### *2.5. Vitamins*

Krill oil contains a high amount of α-tocopherol (Vitamin E) which is characterized by a potent antioxidant effect. The concentration of this isoform ranges between 14.74 to 63.0 mg/100 g of oil and represents 90% of total tocopherols, while the other homologues of tocopherols (β-, γ-, δ-) are present only in traces (γ (0.25 to 3.67 mg/100 g of oil) and δ-tocopherol (0 to 0.65 mg/100 g of oil)) [45]. Besides vitamin E, vitamin A has also been found in frozen krill (about 0.11 mg/100 g of wet weight) [21]. The presence of vitamin A in krill samples represents an important feature contributing to making krill oil a very promising food supplement for the regulation of human immune function and for counteracting some infectious diseases. Depending on extraction methods, the vitamin A content of krill oil ranges between 16.4 to 28.5 mg per 100 g of krill oil, a concentration which is significantly higher than tuna oil (11.1 mg/100 g of oil) [45].
