CARBOHYDRATES AND PROTEINS Flashcards
What is the cheapest source of calories providing 50% or more of calories in most diets?
Carbohydrates
What is the general elemental composition of carbohydrates?
Cx(H2O)y
What is the recommended daily carbohydrate intake for males and females?
Males: 220-330 g/day, Females: 180-230 g/day
What are the principal dietary carbohydrates?
Polysaccharides, disaccharides, and monosaccharides
Which polysaccharides are primarily digested in the gastrointestinal tract?
Starches (glucose polymers) and their derivatives
What is the structure of amylopectin?
A branched molecule
What is the structure of amylose?
A straight chain with only α1,4 linkages
What are the common dietary disaccharides?
Lactose (milk sugar) and sucrose (table sugar)
What are the common dietary monosaccharides?
Fructose and glucose
What is sorbitol, and how does it affect blood sugar?
Sorbitol is formed when the aldehyde group of glucose is hydrogenated to an alcohol group, slowing its absorption and diminishing its effect on blood sugar concentration.
What is the major storage form of polysaccharides in animals?
Glycogen
What is the structural similarity between glycogen and amylose?
Both are straight-chain polymers.
What are examples of non-starch polysaccharides?
Cellulose, hemicellulose, pectins, gums, alginates
What makes cellulose indigestible by humans?
It is composed of beta-1,4-linked glucose molecules in straight chains.
What is the primary enzyme responsible for carbohydrate digestion in the mouth?
Salivary alpha-amylase
At what pH is salivary alpha-amylase deactivated?
pH of 4 or lower
Which enzyme is more efficient at digesting carbohydrates in the small intestine?
Pancreatic alpha-amylase
What are the end products of alpha-amylase digestion?
Maltose, maltotriose, and α-limit dextrins
What enzymes hydrolyze disaccharides and small glucose polymers into monosaccharides?
Lactase, sucrase, maltase, and α-dextrinase
What are the monosaccharide products of lactose digestion?
Glucose and galactose
What are the monosaccharide products of sucrose digestion?
Glucose and fructose
What is the enzyme responsible for breaking down maltose?
Maltase (glucoamylase)
What is the substrate and product of trehalase?
Substrate: Trehalose, Product: Glucose
What is the glycemic index range for ‘good’ carbohydrates?
≤ 55
What glycemic index value is considered ‘bad’?
> 70
What is the clinical importance of butyrate produced by dietary fiber fermentation?
Butyrate serves as a fuel for colonocytes and supports their development.
What receptors are involved in the transport of short-chain fatty acids in the colon?
GPR43, SLC5A8, SLC16A1, and GPR109A
What is the significance of receptors SLC5A8 and GPR109A?
They are critical in the development of colonocytes.
What is the preferred energy substrate of colonocytes?
Butyrate
What is the role of butyrate in gene transcription?
Inhibits histone deacetylases, modulating the epigenetic profile and transcription of selective genes in the colon
What is the carbon source for ketone body generation in colonocytes?
Butyrate
What is the function of dietary fiber in fecal bulk?
Increases mass of fecal flora, leading to increased fecal bulk
How does fiber affect absorption of sugars and fats?
Delays absorption of sugars and fats
What component of fiber lowers serum cholesterol by binding bile salts?
Lignin
What type of carbohydrate transport depends on sodium concentration?
Glucose and galactose transport via SGLT1
What transporter is responsible for glucose and galactose uptake from the gut?
SGLT1 (sodium-glucose cotransporter-1)
What transporter facilitates fructose entry into enterocytes?
GLUT5
How is glucose transported out of enterocytes into the interstitium?
Via GLUT2
What is the most common congenital disaccharidase deficiency?
Congenital Sucrase-Isomaltase Deficiency
What are common symptoms of lactose intolerance?
Diarrhea, bloating, and flatulence after ingestion of sugar
What test is used to diagnose lactose intolerance?
Breath test detecting excessive gas
What causes secondary lactase deficiency?
Mucosal injury from infectious gastroenteritis, parasitic infections, celiac disease, or Crohn’s disease
What genetic defect causes glucose-galactose malabsorption?
Autosomal recessive defect in SGLT1 on chromosome 22q13
What is the treatment for glucose-galactose malabsorption?
Fructose, as it is not affected by the defect
What syndrome results from GLUT2 mutation?
Fanconi-Bickel syndrome
What are complications of excessive fructose consumption?
Hypertriglyceridemia,
hyperuricemia, and
gout
What is the clinical importance of butyrate in colon health?
It serves as a fuel for colonocytes and supports the gut-associated immune system
What are the transporters involved in short-chain fatty acid transport into the epithelium?
GPR43, SLC5A8, SLC16A1, and GPR109A
What transporter is specific to butyrate uptake?
GPR109A
What is the treatment for congenital sucrase-isomaltase deficiency?
Sacrosidase
What enzymes are inhibited by excessive gastric acid in ZES?
Pancreatic alpha-amylase and other pancreatic enzymes
How does fructose malabsorption manifest in IBS patients?
Fructose is not absorbed efficiently, leading to gastrointestinal symptoms
What happens to glucose and galactose in congenital SGLT1 defect?
They are not absorbed, leading to severe diarrhea and dehydration
What is the consequence of GLUT2 mutation in carbohydrate metabolism?
Fasting hypoglycemia,
glycogen deposition, and
tubular nephropathy
Why is fructose a suitable carbohydrate for patients with glucose-galactose malabsorption?
Fructose is absorbed via GLUT5 and is independent of SGLT1
What short-chain fatty acid serves as a preferred energy substrate for colonocytes?
Butyrate
How does fiber affect satiety?
Satiety is achieved more rapidly
How is lactose intolerance treated?
Supplementation with lactase
What are the clinical manifestations of congenital lactase deficiency?
Severe watery diarrhea upon receiving lactose-containing formula
How does excessive gastric acid in ZES affect carbohydrate digestion?
Deactivates pancreatic enzymes, leading to malabsorption and diarrhea
What happens to fructose upon entry into enterocytes?
It is phosphorylated, converted to glucose, and transported as glucose
How does SGLT2 function in glucose transport?
It transports glucose out of the renal tubules
What are the clinical consequences of consuming >50g of fructose?
Increased triglycerides, hyperuricemia, and potential for gout
What transporter moves glucose and galactose into enterocytes?
SGLT1
What is the role of GLUT5 in carbohydrate metabolism?
Facilitates the transport of fructose into enterocytes
What happens to glucose in Fanconi-Bickel syndrome?
Glucose is stored excessively in the liver as glycogen, leading to hepatomegaly
What is the relationship between sodium levels and glucose transport?
Higher sodium levels drive more glucose into cells via SGLT1
Why does lactose intolerance cause bloating and flatulence?
Undigested lactose is fermented by bacteria in the colon, releasing gas
What is the primary source of essential and non-essential amino acids for cellular metabolism?
Proteins
What condition results from a deficiency of proteins due to non-availability of essential amino acids?
Negative nitrogen balance
What percentage of energy in an average Western diet comes from proteins?
10-15%
How much endogenous protein is secreted daily in the form of digestive secretions and desquamated cells?
30 g/day
What is the difference between plant-based and animal-based proteins in terms of biological value?
Animal-based proteins have high biological value; plant-based proteins have low biological value.
At what pH does pepsin optimally cleave peptide linkages in the stomach?
1.6 – 3.2
Which enzymes act as endopeptidases in the small intestine?
Trypsin, chymotrypsin, and elastase
What is the role of collagen in protein digestion?
It is a major constituent of connective tissue, and its fibers must be digested for meat protein digestion.
Which pancreatic enzyme cleaves aromatic amino acids from the carboxy terminal end of proteins?
Carboxypeptidase A
Which pancreatic enzyme cleaves arginine or lysine from the carboxy terminal end of proteins?
Carboxypeptidase B
What type of peptidase cleaves amino acids from the carboxy terminal of 3-8 amino acid peptides?
Amino-oligopeptidase
What transporter facilitates the absorption of dipeptides and tripeptides into intestinal cells?
PepT1 transporter
Which condition is associated with defective secretion of pancreatic enzymes, causing generalized malabsorption?
Cystic fibrosis
What is the genetic cause of Hartnup disease?
A congenital defect in the transport mechanism for neutral amino acids in the intestine and renal tubules.
What happens in lysinuric protein intolerance due to a defect in the basolateral membrane (BLM)?
Impaired exit of cationic amino acids leads to protein malnutrition, hyperammonemia, and multi-organ disease.
What is the treatment recommendation for celiac disease?
Gluten-free diet
What is the most common congenital defect in amino acid transport causing renal and intestinal symptoms?
Cystinuria
Which transporter system is used for sodium-independent transport of neutral amino acids?
System L
What happens when PepT1 expression is induced in colon cancer?
It facilitates the transport of bacterially derived peptides, promoting the inflammatory process.
How is dietary protein digested in the stomach?
Pepsin cleaves peptide linkages, hydrolyzing bonds between aromatic amino acids and a second amino acid.
Which brush-border enzyme cleaves proline-containing dipeptides with a free alpha amino group?
Dipeptidyl aminopeptidase IV
Why are enteral diets based solely on free amino acids hyperosmolar?
Free amino acids increase osmolality, unlike dipeptides and tripeptides, which improve solubility and stability.
Which amino acid transporter is sodium-coupled and primarily transports glutamine, asparagine, and histidine?
System N
Which genetic polymorphism reduces PepT1 substrate affinity?
Substitution of phenylalanine at position 28 with tyrosine
What is the role of gamma-glutamyl transpeptidase in protein digestion?
It cleaves gamma-glutamyl bonds and transfers glutamine to amino acid or peptide acceptors.
