Water-Soluble vs. Fat-Soluble Vitamins

Everyone agrees that vitamins are good for you, but when you hear that a food or a nutrient is rich in vitamins or minerals, what exactly does that mean? How does your body use the vitamins it acquires through the diet? How do vitamins make you healthy? The answers to such questions depend mainly on a single quality: whether or not the vitamin can dissolve in water. 


Let’s start with the basics: a vitamin is a chemical compound, one of several types of nutrients that we need to ensure proper metabolism. Some of these vitamins, our bodies can make; others, on the other hand, need to be obtained from the foods we eat (they are referred to as essential nutrients). 


Humans have 13 essential vitamins: Vitamins A, C, D, E, K, and the eight vitamins of the B complex.



Now, what does it mean to be “water-soluble?”

If a vitamin can be dissolved in water, it is often called a water-soluble vitamin. The B vitamins and vitamin C dissolve in water.


Being water-soluble has both major advantages and disadvantages.  


These vitamins are easily accessed by the body, readily absorbed from the digestive tract into the bloodstream. They are easily added to foods during processing, and thus many foods are fortified with B vitamins and vitamin C. 


It’s difficult to overdo it on the water-soluble vitamins because they’re eliminated in our urine and are therefore not stored in the body for long. They don’t accumulate enough to become toxic. 


Some of the same properties that make water-soluble vitamins easy to access also make them more susceptible to consumption and more easily destroyed by food storage and preparation processes like boiling. 


Heat and light can chemically alter these vitamins and render them useless! Produce that is rich in B or C vitamins must be kept refrigerated, and milk and grains must be kept away from brightly lit areas. They also get eliminated quickly by natural body filters like the kidney, and thus they must be replaced every day. 


The nine water-soluble vitamins have unique and critical functions


  • Vitamin B1, known as thiamine, helps enzymes break down sugar and amino acids for energy. 
  • Vitamin B2, or riboflavin, also helps with enzyme functions for energy breakdown. 
  • Vitamin B3 is also known as either niacin and nicotinic acid, depending on the form. It is responsible for assisting in DNA repair and in various tasks of metabolism.  
  • Vitamin B5, pantothenic acid, is necessary to make co-enzyme A, an extremely versatile molecule that helps with energy production and the synthesis of many molecules that help communicate messages throughout the body.  
  • Vitamin B6, also called pyridoxine, helps synthesize carbohydrates, amino acids, and lipids.
  • Vitamin B7, or biotin, is involved in carbon dioxide transport and is vital for hair and skin function. 
  • Vitamin B9 is folate or folic acid. It helps with DNA repair and cell reproduction. Folic acid is vital for the synthesis of several genetic elements: DNA, RNA, and amino acids. 
  • Vitamin B12, known as cobalamin, is involved in DNA synthesis for every cell in the human body. Two of its most important functions are the maturation of red blood cells and the synthesis of nerve cell insulation. 
  • Vitamin C, also known as ascorbic acid, is involved in tissue repair and immune function. 



What about fat-soluble vitamins?

If a vitamin cannot be dissolved in water, but it can be dissolved by fat, it is called an insoluble vitamin (using water as the measuring stick for solubility), or alternatively, a fat-soluble vitamin. Vitamins A, D, E and K are all fat-soluble. These vitamins have their own set of advantages and disadvantages. 


Fat-soluble vitamins are not usually eliminated or reduced by common food preparations. Since the body stores these vitamins in fat rather than eliminating them in the urine, small amounts of fat-soluble vitamins can exert significant effects. On the other hand, since these vitamins are stored in the body’s fat compartments, they present more risk for vitamin toxicity if taken in larger quantities.


The functions of fat-soluble vitamins are quite versatile

  • Vitamin A comes in the form of carotenoids like beta-carotene and in the form of retinol. It is crucial for night vision and color vision, as well as for cell repair.  
  • Vitamin D gets activated by sunlight and acts much like a hormone. It is the primary regulator for calcium and phosphate and has a central role in promoting bone health. 
  • Vitamin E is a group of eight compounds that act as antioxidants, which help protect cells against damage from reactive oxygen species.
  • Vitamin K helps build proteins responsible for blood coagulation and bone metabolism. 


All 13 of these vitamins are sold as supplements and have demonstrated clinical importance. If you’re looking to increase your vitamin intake, be sure to check in with your health provider about the best strategy for doing so.



Semba R. The discovery of the vitamins. International Journal for Vitamin and Nutritional Research. 2012;82(5):310-5.

Lykstad J, Sharma S. Biochemistry, Water Soluble Vitamins. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2019.

Institute of Medicine (US) Standing Committee on the Scientific Evaluation of Dietary Reference Intakes and its Panel on Folate, Other B Vitamins, and Choline. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington (DC): National Academies Press (US); 1998.

Johnson, E. J., & Mohn, E. S. (2015). Fat-soluble vitamins. World Review of Nutrition and Dietetics, 111, 38–44.


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