Absorption

What is absorption?

Absorption is the uptake of nutrients from the gastrointestinal tract into the blood. In order to be absorbed, certain nutrients need to be broken down (digested) into smaller nutrients, for example, proteins into amino acids, starch into glucose and fats into fatty acids.

Sites of Nutrient Absorption

Most nutrients are absorbed in the upper two parts of the small intestine: duodenum and jejunum.

Nutrients that can be absorbed in the stomach ^[1-p.51]:

• Water
• Minerals: copper, fluoride, iodide, molybdenum
• Alcohol

Nutrients that can be absorbed in the duodenum ^[1-p.51]:

• Monosaccharides (glucose, fructose, galactose) ^{[2-p.212; 3 ]} and to a lesser extent amino acids ^[3] and fatty acids ^[3]
• Minerals: calcium, copper, iron, magnesium, phosphorus, selenium
• Vitamins A, B1 (thiamin), B2 (riboflavin), B3 (niacin), B7 (biotin), B9 (folate), D, E and K

After surgical removal of the duodenum, iron and calcium malabsorption can develop ^[4-p.542].

Nutrients that can be absorbed in the jejunum ^[1-p.51]:

• Lipids (fats, cholesterol)
• Monosaccharides: fructose, glucose, galactose and probably tagatose
• Amino acids and small peptides
• Vitamins A, B1 (thiamin), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), D, E and K
• Minerals: calcium, chromium, iron, magnesium, manganese, molybdenum, phosphorus, potassium ^[8], zinc

About 90% nutrients is absorbed in the first 100-150 centimeters of the jejunum ^[5]. When the jejunum is affected by a severe disease or surgically removed, and ileum remains intact, no malabsorption should occur ^{[4-p.542; 5]}.

Nutrients that can be absorbed in the ileum ^[1-p.51]:

• Water (most of water is absorbed in the ileum)
• Vitamins B9, B12, C, D and K
• Minerals: magnesium, potassium ^[8]

When the ileum is affected by a disease or surgically removed, the malabsorption of proteins, fats, fat-soluble vitamins (A,D,E and K), vitamin B12, calcium, magnesium, zinc and water can develop ^[4-p.542].

Nutrients that can be absorbed in the colon ^[1-p.51]:

• Water
• Minerals: calcium, sodium, chloride ^[6,7], potassium ^[8]
• Short-chain fatty acids (SCFA) (acetate, propionate and butyrate), which are produced during fermentation of undigested or nondigestible carbohydrates and some amino acids by beneficial intestinal bacteria ^[1].
• Substances produced by large intestinal bacteria: vitamin B1 (thiamin), vitamin B2 (riboflavin), vitamin B7 (biotin), vitamin B9 (folate), vitamin K

Surgical removal of the colon can affect the absorption of water but not other nutrients ^[4-p.542].

Dietary carbohydrates (except dietary fiber), proteins, and fats are completely absorbed in the small intestine, so, in a healthy person, none of them should appear in the stool.

Soluble dietary fiber and polyols (sorbitol, xylitol, etc.), which are not absorbed in the small intestine are fermented by colonic bacteria and their breakdown products are absorbed in the colon.

Transport of Nutrients Across the Intestinal Wall

Transport of most nutrients across the intestinal wall (absorption) is not regulated, so it depends only on common transport principles, like osmosis and electro-chemical gradient, but not on nutrient blood levels.

• Active transport (against the osmotic gradient; energy required): glucose, galactose, amino acids, calcium, iron, vitamin C, vitamin B1, folic acid, bile acids, and, partially, sodium.
• Facilitated transport (with the help of transport substance, no energy required): fructose, vitamins B2 and B12.
• Passive transport (down the osmotic gradient, no energy required): most other nutrients

Water transport is enhanced by active transport of glucose and amino acids in the jejunum and sodium transport in the ileum ^[8].

About 10 liters of water enters the small intestine every day: ~2 liters from the diet, and the rest from saliva, bile, pancreatic and intestinal juices. From this, 9 liters are absorbed in the small intestine, so only about 1 liter enters the colon, where some of it is absorbed, and about 150 mL is excreted with the stool ^[8].

Glucose is absorbed in the jejunum with the help of sodium-dependent glucose transporter SGLT1 (S = sodium, GL = glucose, T = transport) only together with sodium (glucose-sodium co-transport). Galactose is absorbed by the same mechanism as glucose.

Fructose absorption depends on the amount of transport protein GLUT5 in the small intestinal wall. Healthy people can absorb up to 50 grams of fructose at one sitting, but those with low amount of GLUT-5 only 0 to 20 grams.

Amino acids are absorbed in the small intestine with the help of amino acid transporters and sodium, by the same mechanism as glucose ^[9].

Sodium is absorbed in the small intestine and colon by various mechanisms, such as a co-transport with glucose or amino acids. Chloride transport is mainly accompanied with sodium transport.

Potassium is absorbed down the electrochemical gradient. In the colon, potassium may be absorbed, but it is usually also secreted, so net absorption does not regularly occur ^[8].

Iron from animal foods (ferrous [Fe2+] or ‘heme iron’ from hemoglobin and myoglobin) is absorbed better than non-heme (ferric, Fe2+) iron from plant foods (22% vs. 2%) ^[8]. Iron absorption increases when body iron stores (ferritin) are low (like after bleeding or menstruation) and decreases when they are high ^[8].

Calcium absorption in the small intestine is dependent on vitamin D and stimulated by parathormone (PTH), which increases when the blood calcium level falls. Calcium absorption is also stimulated by pregnancy, a growth hormone and insulin, and inhibited by thyroxine and cortisol. In general, only about 30% of dietary calcium is absorbed ^[8].