Carbohydrates are neutral compounds composed of carbon, hydrogen, and oxygen that serve as the primary sources of energy in the human body. They can be divided into simple carbohydrates, which include monosaccharides (e.g., glucose, fructose, galactose) and disaccharides (e.g., sucrose, lactose), and complex carbohydrates, which include starch polysaccharides (e.g., starch, glycogen) and nonstarch polysaccharides (e.g., glucan, cellulose). Monosaccharides are directly absorbed by enterocytes. Disaccharides and polysaccharides require degradation into monosaccharides to be absorbed by hydrolytic enzymes, which are secreted by the salivary glands in the mouth (salivary amylase), intestinal villi (maltase, lactase), and pancreas (pancreatic amylase). Conditions that decrease the secretion of these enzymes result in malabsorption and maldigestion. Glucose and galactose are absorbed via sodium-glucose linked transporters (SGLTs) in the intestinal epithelial cells and translocated into the circulation via glucose transporter 2 (GLUT2). Fructose is absorbed into enterocytes via glucose transporter 5 (GLUT5) through facilitated diffusion. The uptake of glucose into other cells is mediated by glucose transporters 1 to 5. Intracellularly, monosaccharides are further metabolized by a series of enzymatic reactions that release ATP.
For details on the carbohydrate glucose, see “” and “ .”
- Compounds consisting of carbon, oxygen, and hydrogen
- Classified as simple (e.g., glucose, fructose, sucrose) or complex sugars (e.g., starch)
- Monosaccharide: a simple carbohydrate that cannot be further broken down by simple hydrolysis (e.g., glucose, fructose, galactose)
- Disaccharide: two monosaccharides linked by a glycosidic bond (e.g., sucrose, maltose, or lactose)
- Polysaccharide: multiple monosaccharides bound by glycosidic bonds (e.g., glycogen, cellulose, starch, , )
Glycosidic bond: linkage between a carbohydrate and another molecule (e.g., carbohydrate and alcohol)
- Two forms:
- 1,4-α-glycosidic bond (OH group below the plane of the ring), e.g., maltose
- 1,4-β-glycosidic bonds (OH above the plane of the ring), e.g., lactose, cellulose
- Two forms:
Lactase is the only enzyme in the human body that can cleave β-glucosidic bonds, but it only cleaves those of the disaccharide lactose. There are no enzymes in the digestive tract that can cleave the β-glycosidic bonds of polysaccharides. Cellulose (fiber) therefore remains undigested in the intestine.
Carbohydrates in food
- Sources: table sugar, cereals, fruits, and vegetables
- Monosaccharides: absorbed directly by enterocytes
- Polysaccharides: broken down by enzymes into monosaccharides via hydrolytic cleavage of α-
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Absorption of glucose
Glucose enters intestinal epithelial cells and proximal renal tubular cells via SGLT. In all other cells of the body, glucose uptake occurs via specific membranous glucose transporters (e.g., GLUT2, GLUT5).
- Sodium-dependent glucose cotransporter 1 (SGLT1): a specific transporter, located on the luminal side of mucosa cells and the proximal straight tubule in the kidney
- Glucose transporters (GLUTs): a group of specific glucose transporters that are present in the plasma membranes of almost all cells of the body
Intestinal glucose absorption: via SGLT1
- Secondary active transport: The driving force of absorption is a sodium concentration gradient maintained by basal Na+/K+ ATPase, which transports sodium out of the cell.
- Sodium moves down its concentration gradient into the cell, taking a glucose molecule with it each time.
- Transport into the blood: via GLUT2 (circulates unbound in the blood)
- Glucose uptake into cells: passive transport via facilitated diffusion
Renal glucose reabsorption
- Free filtration of glucose by the kidneys
Complete reabsorption in the proximal tubules via two types of SGLT (urine normally is glucose-free)
- Reabsorbs the remaining glucose (∼ 2%) as well as galactose in the PCT
- One molecule of glucose is absorbed together with two molecules of sodium
- Reabsorbs ∼ 98% of urinary glucose in the proximal convoluted tubule (PCT)
- One molecule of glucose is absorbed together with one molecule of sodium.
- Reabsorption also relies on a sodium concentration gradient via Na+/K+ ATPase.
|Overview of the most important glucose transporters|
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BRICK LIPS: Brain, RBCs, Intestine, Cornea, Kidney, Liver, Islet cells, Placenta, Spermatocytes (insulin-independent glucose uptake)
Metabolism of glucose
See “ “ for:
- Glucose degradation
- Glucose synthesis: See “.”
- See ““ for glucose storage.
- See “ATP synthesis pathways and caloric values.” for
Absorption of galactose
Galactose is part of lactose (found in milk products).
- Lactose is cleaved in the small intestine by lactase.
- Free galactose is absorbed by enterocytes via SGLT1.
- Free galactose is transported into blood via GLUT2.
- Galactose circulates to the liver for further metabolism.
Breakdown of galactose
- Galactokinase activates galactose: galactose + ATP → galactose-1-P + ADP
- Galactose-1-phosphate uridyltransferase: galactose-1-P + UDP-glucose → UDP-galactose + glucose-1-P (can be fed into )
- UDP-galactose 4-epimerase: UDP-galactose → UDP-glucose
High blood levels of galactose also result in conversion to the osmotically active galactitol via aldose reductase. In individuals with galactokinase deficiency, excess galactitol forms in the lens of the eye and leads to early-onset cataracts.
Absorption of fructose
- Sucrose is cleaved in the small intestine by sucrase-isomaltase.
- Freed fructose is absorbed into enterocytes via facilitated diffusion by GLUT5.
- Freed fructose is transported into the blood via GLUT2.
- Fructose circulates to the liver for further metabolism.
Breakdown of fructose (fructolysis)
- Fructokinase activates fructose: fructose + ATP → fructose-1-P + ADP
- Aldolase B splits hexose into two trioses: fructose-1-P → dihydroxyacetone-P + glyceraldehyde
- Trioses are converted to glyceraldehyde-3-P:
- Glyceraldehyde-3-P is fed into .
- Fructose can be produced from glucose via sorbitol (osmotically active sugar alcohol) without using ATP.
- In the body, fructose is the primary source of energy for spermatozoa.
- Tissues that have both aldose reductase and sorbitol dehydrogenase (liver, ovaries, seminal vesicles) will not accumulate sorbitol.
- Tissues/cells that do not have sorbitol dehydrogenase activity (e.g., lens, retina, kidneys, Schwann cells) accumulate sorbitol.
- Excess sorbitol causes osmotic damage and explains changes seen in hyperglycemic diabetic patients such as diabetic cataracts, diabetic retinopathy, diabetic nephropathy, and diabetic neuropathy.
- Maldigestion and malabsorption
- Disorders of glucose metabolism
- Disorders of galactose metabolism:
- Disorders of fructose metabolism