Introduction to Fat-Soluble Vitamins

Fat-soluble vitamins fit into the micronutrient category, specifically under the vitamin category.
Micronutrients (including vitamins) do not contain kilocalories (energy), unlike macronutrients.

Water-soluble vitamins: There are nine such vitamins, and most exist in various chemical forms.
Fat-soluble vitamins: There are four fat-soluble vitamins, namely Vitamins A, D, E, and K.
- While this list appears simple, each name is an umbrella term for several different compounds.

Water-soluble vitamins: Presence in the body is more transient.
- Consumption is required every 1 to 2 days due to their rapid excretion (primarily in urine).
Fat-soluble vitamins: Presence in the body is less transient.
- They are stored in the adipose (fat) tissue.
- Can be consumed less frequently without immediately causing deficiency.

Both water-soluble and fat-soluble vitamin absorption occurs in the small intestine.
Water-soluble vitamins: The absorptive process is relatively simple, and they travel easily in the circulatory system.
Fat-soluble vitamins: Absorption and transport process are similar to that of fat.
- Challenge: Fats and water do not mix easily, preventing fat-soluble vitamins from traveling well in the watery circulatory system immediately after absorption.
- They first travel via the lymphatic system, where fats can more easily associate with the fluid.
- Dietary fat facilitates absorption: The absorption efficacy of fat-soluble vitamins drastically increases in the presence of dietary fat.
  - Summary: If fat is consumed simultaneously with fat-soluble vitamins, a greater amount of the vitamins will be absorbed.
  - Application Challenge: How might the absorption of fat-soluble vitamins differ if consumed with a meal versus a nutritional supplement pill (which typically lacks fat)? (Answer: Absorption is likely higher with a meal containing fat).

Any condition that decreases fat absorption significantly increases the risk of fat-soluble vitamin deficiency, as their absorption is heavily dependent on fat.

Inflamed bowel: Conditions like celiac disease, inflammatory bowel disease (IBD), or irritable bowel syndrome (IBS) impair fat absorption, thus impairing fat-soluble vitamin absorption.
Surgical removal of part of the gastrointestinal (GI) tract: This increases the risk of fat malabsorption.
- Examples: Gastric bypass surgery, removal of a diseased part of the bowel or intestine.
Diminished pancreatic function: Fat digestion is a complex process requiring emulsifiers and digestive enzymes.
- The pancreas releases pancreatic lipase into the small intestine for fat digestion.
- Conditions like chronic pancreatitis (inflammation of the pancreas) or surgical removal of part of the pancreas (due to cancer) compromise the production of pancreatic lipase.
Diminished gallbladder function: Bile chemically emulsifies fat to expose it to digestive enzymes.
- Conditions like cholecystitis (inflammation of the gallbladder) or surgical removal of the gallbladder (due to gallstones or chronic cholecystitis) result in diminished emulsification.

Fat malabsorption: In any of the above scenarios, there is a high risk for fat to move through the GI tract undigested.
Steatorrhea: The presence of a large amount of undigested fat in the stool.
- Indicator: Steatorrhea increases the likelihood that a fat-soluble vitamin deficiency exists or will soon develop.

Fat-soluble vitamins reach toxic levels more readily than water-soluble vitamins.
- This is primarily because they are more readily stored in adipose (fat) tissue, while water-soluble vitamins are more readily excreted in the urine.
Supplement use: Consumption of fat-soluble vitamin-containing supplements increases the risk of toxicity.
Vulnerable populations: Certain patient populations face especial danger from fat-soluble vitamin toxicity.
- Pregnant individuals.
- Individuals requiring blood-thinning medications (e.g., Vitamin K can interact).
- Those with various cardiovascular and metabolic conditions.
This concept will be reviewed in more depth when discussing each fat-soluble vitamin individually.