Compared to the other groups of nutrients, vitamins are required in the smallest amounts. And unlike minerals, vitamins are complex substances. Vitamins are classified as either fat-soluble (vitamins A, D, E, K) or water-soluble (B-vitamins and vitamin C). Fat-soluble vitamins depend on the presence of dietary fat and normal fat absorption for their uptake and utilization in the body. Water-soluble vitamins simply depend upon the presence of water for absorption.

It is important to provide balanced amounts of vitamins and other nutrients in complete diets. Mineral or vitamin supplements, or supplements of any kind, are not necessary for healthy animals consuming a complete and balanced pet food. In fact, adding supplements to diets that are already complete and balanced may create imbalances with detrimental effects.

• Vitamin A
  Vitamin A has a number of functions necessary for the health and well-being of animals including a role in normal vision, growth, immune system function and reproduction. The source of vitamin A is beta-carotene, which animals must convert to the actual vitamin for it to become active and function as vitamin A. Too much can cause dangerous conditions in animals, and result in deformed bones, weight loss and more.
• Vitamin D
  Although vitamin D (cholecalciferol) is considered a vitamin, it is also considered a hormone and is one of three major hormones involved in the regulation of calcium in the body. Its primary functions are to help in the mineralization of bone and to increase the absorption of calcium and phosphorus from the intestine. Vitamin D can be acquired in the diet, or it can be converted in the skin following exposure to ultraviolet radiation from sunlight.
• Vitamin E
  Vitamin E is used to describe a family of chemical compounds called tocopherols. It is also known for its action as a biological antioxidant, meaning a lack of vitamin E in the diet could result in damage to the wall or membrane of cells throughout the body. Vitamin E works with other nutrients (selenium and cysteine) as an antioxidant to minimize damage to cells from oxidation.
• Vitamin K
  The most common forms of vitamin K in the diet are called menadione and phylloquinone, which come from green, leafy plants and vegetables. The major function of this vitamin is as a clotting agent within the blood.
• B-Complex Vitamins
  B-complex vitamins are those vitamins originally identified as B1, B2, B6, B12 and others. Unlike the fat-soluble vitamins, these vitamins are not stored to any extent in the body and are required in small amounts in the daily diet. Although these nutrients don't provide energy in and of themselves, they are critical in the metabolism of protein, carbohydrates and fat, which results in energy for body processes.
• Vitamin C (Ascorbic Acid)
  This vitamin is also a water-soluble vitamin and has a primary metabolic role in the body of all mammals involving the synthesis or production of collagen. While ascorbic acid is essential in the diet of humans, other primates and guinea pigs, dogs have no dietary requirement for this vitamin.