Vitamins are classified as being either water-soluble or fat-soluble. There are four fat-soluble vitamins -- A, D, E and K. Excess fat-soluble vitamins accumulate in your body, while the water-soluble vitamins are excreted. Fat-soluble vitamins are stored in your fat tissues and your liver.
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Micelles Make Things Happen
You need to take in enough fats to assist in the absorption of fat-soluble vitamins. Inadequate intake of healthy fats or the fat-soluble vitamins, or fat-malabsorption issues can result in deficiencies, while taking megadoses of these vitamins can be toxic. Fat-soluble vitamins are carried in your digestive system in a droplet called a micelle, which allows the vitamins to be absorbed by your body's intestinal absorption cells, called enterocytes. The micelle allows fat-soluble vitamins to disperse into your enterocytes, much like soap disperses in water.
The Bile Salt Bubble
The micelle is round like a bubble, with the outside made up of bile salts. Bile salts have one hydrophilic or water-liking end and one hydrophobic or water-fearing end. They form a round droplet with their hydrophilic ends facing outward and the hydrophobic ends facing inward. The center of the micelle contains the fat-soluble vitamins. The micelle protects the fat-soluble vitamins from water as they cross into the enterocytes.
Micelles constantly break down and then reform to transport fat-soluble substances into your enterocytes. Once micelles are in an enterocyte, they break down and chylomicrons are formed. Chylomicrons are lipoproteins, which are particles designed to transport lipids or fats in your blood. The outside of a chylomicron is composed of phospholipids that, like bile salts, are also present in bile and have one hydrophilic and one hydrophobic end.
Lymphatic System -- Drop-Off Location
Once the vitamins have entered the chylomicron, the chylomicron then transports them into your lymphatic system, a subsystem of the circulatory system and a vital part of your immune system that helps maintain bodily fluids. Once in the lymphatic system, the fat-soluble vitamins are transported in the blood by different mechanisms to carry out their functions in your body or they are transported back to your liver or fat tissue for storage.
REFERENCES & RESOURCES
- Harvard School of Public Health: Vitamins
- Colorado State University Extension: Fat-Soluble Vitamins, A, D, E, and K
- Department of Health and Human Services: Fat-Soluble Vitamin
- Encyclopedia Britannica: The Human Digestive System: Fats
- Colorado State University: Vitamin E
- Encyclopedia Britannica: Lymphatic System
- University of Washington: Digestion and Absorption of Fats
- Encyclopedia Britannica: Micelle (Chemistry)
- MedlinePlus: Bile
- Food and Agriculture Organization of the United Nations: Vitamin K
- Office of Dietary Supplements: Vitamin A -- Fact Sheet for Health Professionals
- Lincoln Public Schools: Vitamin Structure and Function