is known to function in gene expression as a co-substrate, and have unique biosynthetic pathways in different organisms. Unlike humans, felines are able to synthesize Vitamin C in the liver, via the glucuronate pathway. The simple sugars glucose and galactose are used as input. This pathway naturally maintains ascorbic acid (active Vitamin C) at an adequate level; therefore it does not need to be separately included in their diet. Vitamin D Vitamin D is a dietary requirement for cats as they lack the ability to efficiently synthesize vitamin D3 using sunlight. This fat-soluble vitamin is required in cats for bone formation through the promotion of calcium retention, along with nerve and muscle control through absorption of calcium and phosphorus. Wild cats probably gather vitamin D from prey liver. Unlike dogs and humans, cats metabolize vitamin D2 and vitamin D3 slightly differently. A dose of Vitamin D2 is approximately 70% as effective as the same amount of vitamin D3. Vitamin E Vitamin E as an antioxidant in gastrointestinal health cat food diets can have a positive effect of improving the animal's immune function and prevent against infections. Vitamin E is a free radical scavenger that functions as a chain-breaking antioxidant to prevent free radical damage of cell membranes. Vitamin E aids in protecting cells from highly reactive oxygen species within the lungs, muscles, skin, brain, tissues and red blood cells. Supplementation of vitamin E in the diet benefits the immune system and improves resistance to infections and diseases. The National Research Council (NRC) suggested a ratio of vitamin E to polyunsaturated fatty acids be 0.6:1 to ensure enough vitamin E to combat any free radicals. Polyunsaturated fatty acids (PUFA) are prone to oxidative destruction in cellular membranes and increase the requirement for antioxidants. Vitamin E is an essential nutrient which needs to be included in the feline diet in order to protect the lipid components within cellular membranes of various tissues in the body. Vitamin E is able to accomplish this through working with selenium and acting as an antioxidant to prevent free radicals from interacting with these fatty acid membrane components, resulting in reduced levels of oxidative stress. This is particularly important in the skin as various oxidative environmental components can have very damaging effects if not protected by vitamin E. The nutrient profiles developed by AAFCO currently advise a minimum dry matter inclusion of 40 IU/kg of vitamin E