Manganese is absorbed with about 4% efficiency from the intestines and is carried into, and in the blood by the protein transmanganin. Regulation of body manganese levels is controlled primarily through variable excretion, rather than absorption. Manganese is essential for the enzymatic incorporation of xylose and galactose in glycoproteins. These glycoproteins are the cells’ natural barrier to invading viruses and coat every cell of the human body. Glycoproteins also make up the great majority of all secretory proteins. Manganese and magnesium are used interchangeably in many enzymes as cofactors. Aminopeptidases, which break down protein structure, are secreted by the intestinal mucosal cells for digestive purposes and utilize either magnesium or manganese as the cofactor element of their structures. Manganese is an important constituent of activated arginase, an enzyme found in the liver, and mediates the conversion of arginine to urea. Manganese is a constituent of the enzyme, pyruvate carboxylase, which plays an important role in gluconeogenesis: the conversion of various non-carbohydrate substances into glucose for subsequent use. Manganese is also involved in the transfer of high energy phosphate groups by phosphotransferase enzymes. These enzymes are found in various metabolic pathways, including several steps of glycolysis. Manganese tends to activate the lipid metabolizing enzyme lipoprotein lipase, otherwise called the "clearing factor." Manganese seems to be important for the enzymatic activity of glycosyltransferases, which play a vital role in mucopolysaccharide synthesis. Impaired activity of these enzymes, due to manganese deficiencies, can result in abnormal cartilage formation. High calcium and phosphorus intake decreases manganese absorption. Excretion of manganese is through the intestine via bile.