How are water-soluble vitamins different from fat-soluble vitamins?

Water-soluble and fat-soluble vitamins are distinguished primarily by their solubility, which dictates how the body absorbs, transports, stores, and excretes them.1

Absorption and Transport

• Fat-soluble vitamins (A, D, E, and K): These require bile salts for efficient intestinal absorption. They are primarily transported from the intestines via the lymph before entering the bloodstream.2
• Water-soluble vitamins (C and B vitamins): These are absorbed in the intestine and pass directly into the bloodstream to be carried to the tissues where they are utilized.3

Storage and Excretion

• Storage: The body has a limited capacity to store water-soluble vitamins, whereas it can store larger quantities of fat-soluble vitamins, primarily in the liver and fatty tissues.234
• Excretion: Water-soluble vitamins are generally excreted in the urine when intake exceeds requirements. In contrast, fat-soluble vitamins are not readily excreted in urine; they are broken down and excreted primarily through bile and feces.342

Toxicity

Because fat-soluble vitamins are stored in the body, they have a higher potential for toxicity when consumed in excess. Water-soluble vitamins are generally considered non-toxic, though exceptions exist, such as high doses of niacin or vitamin B6.43