Fat Soluble Vitamins are hydrophobic (fat loving) nutrients that are necessary in small amounts to sustain a healthy life. Excess fat soluble vitamins can be stored in the liver and fat (adipose tissue). Therefore, these vitamins do not need to be eaten every single day as stores of the vitamins can sustain a person for some time. This is why a deficiency may take months to develop as the stores of the vitamin can take a long time to be depleted. This ability to store vitamins in tissue can also lead to vitamin toxicity. This toxicity is a vitamin overdose caused by taking a vitamin in excess. Since fat soluble vitamins can be stored in the body they can accumulate to the point of causing illness.
Fat soluble vitamins are usually absorbed as part of chylomicrons (fat lobules) in the small intestine after being digested by pancreatic enzymes. Therefore, if fat absorption is inhibited the digestion of fat soluble vitamins will also be limited. This malabsorption can result in a deficiency of fat soluble vitamins as they are being excreted in the feces rather than being absorbed. This condition of excess fat excretion in feces is called Steatorrhea. It results in bulky, foul-smelling bowel movements with an oily appearance that float.
Common causes of Steatorrhea on the USMLE Step 1:
- Cystic Fibrosis (Thick mucus –> decreased ability to secrete pancreatic enzymes)
- Pancreatitis or Pancreatic Cancer (–> decreased ability to secrete pancreatic enzymes)
- Celiac Disease & Crohn’s Disease (damages portions of small intestine responsible for fat absorption)
- Liver disease, biliary cirrhosis and sometimes gallstones (–> less bile for fat absorption)
Water Soluble Vitamins are hydrophilic (water loving) nutrients that are necessary in small amounts to sustain a healthy life. Unlike fat soluble vitamins they are generally not stored in body tissues. If you take in excess water soluble vitamins, your kidneys excretes most of the excess in urine. This is why some people say the US has the most expensive urine in the world. We spend so much on expensive vitamin supplements and much of it just ends up in the toilet. B12 and Folate are exceptions as they are water soluble vitamins that are stored in the liver. Excreting excess in the urine and not storing much in the body means that water soluble vitamins need to be consumed daily and that deficiencies can become apparent quickly. Also toxicity of water soluble vitamins is much less common than toxicity for fat soluble vitamins, because excess water soluble vitamins don’t easily build up in the body. Patients with renal damage may reach toxicity of water soluble vitamins at much lower doses as their excretion of excess water soluble vitamins in impaired.
* Usually on Step 1 they will give you the vitamin letter/number and the name in parenthesis. They wouldn’t usually give you one without the other, so perfectly memorizing both is not necessary. I would just learn the letter/number as that is easier.
Here is a mnemonic to help remember which vitamins are fat soluble & which are water soluble.
Picture Used: Derivative of “At the beach – male abdominal obesity” by TiborVégh available at http://commons.wikimedia.org/wiki/File:At_the_beach_-_male_abdominal_obesity.JPG by Creative Commons 3.0 Attribution