Definition
This is a way of sorting the vitamins your body needs into a group that dissolves in oil and a group that dissolves in water. Vitamins A, D, E, and K are fat-soluble, while the B vitamins and vitamin C are water-soluble. This difference in solubility is the key factor that decides how they are absorbed, stored, and removed from the body.
Vitamins that like oil, vitamins that like water
Once vitamins enter the digestive tract, the route they take to be absorbed depends on which kind of liquid their molecules are attracted to. Vitamins A, D, E, and K are fat-soluble vitamins with a nonpolar character, so they don't mix with water molecules. They have to mix with fats and bile acids in the digestive juices to form tiny micelles (small clusters of fat droplets) before they can pass through the cells lining the small intestine.
Vitamin C and the B vitamins are the opposite. They are polar, so they mix well with water. They dissolve in the water in the digestive tract without any help from fat, and mostly enter the capillaries through carrier proteins in the intestinal lining.
So this difference in how they dissolve completely separates the doorway through which a nutrient enters the body. Fat-soluble vitamins travel through the lymphatic vessels and join the bloodstream gradually. Water-soluble vitamins ride the blood through the portal vein and enter the whole-body circulation quickly.
How they move and are kept in the body
Once inside the body, the way they travel and stay also splits sharply. Water-soluble vitamins ride the water-rich blood to cells all over the body and get used there. The B vitamins mostly become coenzymes, which help enzymes do their work. Any surplus left after use is filtered by the kidneys' glomeruli and tends to leave the body mostly in urine.
Fat-soluble vitamins, on the other hand, can't dissolve in the water in blood, so they travel attached to lipoprotein carriers. What is left over after use doesn't easily leave in urine. It mostly stays bound in the fat droplets of the liver and the fat tissue under the skin. Vitamin K is an exception, though. The body keeps only a small amount, and it is used up fairly quickly or removed through bile.
Fat tissue works as a kind of storehouse, so body levels can stay steady for a long time. But if the amount coming in keeps exceeding what the body can metabolize, too much builds up and toxicity can appear. Vitamin A can damage the liver, and vitamin D can push the calcium level in the blood too high.
To be more precise: an exception among water-soluble vitamins
It is easy to oversimplify and think that water-soluble vitamins never stay in the body and are flushed out as fast as you take them in. But just because something dissolves in water chemically doesn't mean every molecule disappears from the body within a day.
To be more precise, among the water-soluble vitamins, vitamin B12 can be stored in liver tissue in amounts that last for years. It binds to special carrier proteins and is kept stably in liver cells, so even if you stop taking it for a while, a deficiency doesn't appear right away.
Also, even though the route out through urine is open, a burden can build up if a level above the filtering limit stays in the body. For example, if the level of vitamin B6 gets too high, it can irritate peripheral nerve tissue. Each vitamin has a limit to what the body can metabolize, set by its own chemical traits.
Common misconceptions
Water-soluble vitamins never stay in the body and are all flushed out, so it doesn't matter how high the level in the body gets.
They are mostly removed in urine, but there are exceptions such as vitamin B12, which is stored in the liver for a long time. And if too high a level stays in the body, as with vitamin B6, it can affect nerve tissue.
Fat-soluble vitamins also get filtered out quickly in urine through the kidneys if you drink a lot of water.
Fat-soluble vitamins don't mix with water and bind to fat tissue and the liver, so they are hardly removed at all through the kidneys' water-filtering route.
Where you meet it
This is the process in which vitamin A forms a complex with bile acids and fats in the digestive tract and is absorbed into the lymphatic vessels.
This is what happens when vitamin C, a water-soluble vitamin, dissolves in the water in blood and is carried to cells, and the surplus is then removed through the kidneys.
This is the mechanism in which vitamin D, a fat-soluble vitamin, stays in the liver and fat tissue and is used over a long time to keep calcium in the body in balance.
In one sentence
Fat-soluble vitamins mix with fats, are absorbed through the lymphatic vessels, and stay in the body a long time. Water-soluble vitamins dissolve in water, are absorbed through the capillaries, and are mostly removed quickly, but both kinds have limits to what the body can metabolize.