Nutrition, Digestion, & Absorption

Harper's Biochemistry 32nd Edition; Special Topics A

Glycemic Index

The increase in blood glucose after a test dose of a carbohydrate compared with that after an equivalent amount of glucose is known as

1 or 100%

Glucose, galactose, lactose, maltose, isomaltose, and trehalose have an index of ?

Hydrolysis

The digestion of carbohydrates to oligo, mono, and disaccharides is through a process called

Fructose, sugar alcohols, sucrose

Sugars that are absorbed less rapidly and have lower glycemic index

between near 1 or 0

glycemic index of starch

insulin

Foods that have a low glycemic index are beneficial since they cause less fluctuation in _______ secretion

salivary and pancreatic amylases

The hydrolysis of starch is catalyzed by

dextrins

• (then a mixture of glucose, maltose, and maltotriose and small branched dextrins)

The hydrolysis of starch yields ?

α(1 → 4) glycoside bonds

salivary and pancreatic amylases catalyze random hydrolysis of ? *what bonds?

sucrase-isomaltase

a bifunctional enzyme catalyzing hydrolysis of sucrose and isomaltose

• disaccharidases

• maltase

• sucrase-isomaltase

• lactase

• trehalase

located on the brush border of the intestinal mucosal cells, where the resultant monosaccharides and those directly ingested are absorbed

TRUE

TRUE OR FALSE: Congenital deficiency of lactase occurs rarely in infants

Inuit

Congenital deficiency of sucrase-isomaltase occurs among the ______, leading to sucrose intolerance, with persistent diarrhea and failure to thrive when the diet contains sucrose

Lactose Intolerance

lactase activity begins to fall after weaning and is almost completely lost by late adolescence, leading to what condition?

SGLT 1

Glucose and galactose transport protein

sodium-dependent process

Glucose and galactose are absorbed through what process?

carrier-facilitated diffusion

Other monosaccharides are absorbed by what type of diffusion?

FALSE (down)

TRUE OR FALSE: Because they are not actively transported, fructose and sugar alcohols are only absorbed up their concentration gradient

osmotic diarrhea

Large intakes of fructose and sugar alcohols can lead to ?

triacylglycerols and phospholipids(less)

The major lipids in the diet are ?

micelles (4-6 nm)

These lipids are hydrophobic molecules and are hydrolyzed into small droplets called ______ before they can be absorbed

A, D, E, and K

fat-soluble vitamins

lingual and gastric lipases

Hydrolysis of triacylglycerols is initiated by?

sn-3 ester bond

lingual and gastric lipases attack what type of bond

1,2-diacylglycerols and free fatty acids

Hydrolysis of triacylglycerols forms?

emulsyfying agents

1,2-diacylglycerols and free fatty acids can act as?

Colipase

Pancreatic lipase requires this protein for activity

2-monoacylglycerols and free fatty acids

major end products of luminal triacylglycerol digestion

primary ester links

Pancreatic lipase is specific in targeting what kind of bond?

1 and 3

Pancreatic lipase targets what positions in triacylglycerols?

TRUE

TRUE OR FALSE: Inhibitors of pancreatic lipase are used to inhibit triacylglycerol hydrolysis in the treatment of severe obesity.

25%

Pancreatic esterase only allows approximately _____% of ingested triacylglycerol to be hydrolyzed to glycerol and fatty acids before absorption

Bile salts

What permits emulsification of the products of lipid digestion into micelles together with dietary phospholipids and cholesterol

enterohepatic circulation

bile salts remain in the intestinal lumen, where most are absorbed from the ileum into the ?

• Fatty acid and glycerol

• triacylglycerols

• Monoacylglycerol pathway

Within the intestinal epithelium:

• 1-monoacyglycerols are hydrolyzed to ?

• 2-monoacylglycerols are reacylated to ?

• VIA WHAT PATHWAY?

hepatic portal vein

Glycerol released in the intestinal lumen is absorbed into the ?

triacylglycerol

glycerol released within the epithelium is reutilized for the synthesis of?

Normal Phosphatidic acid pathway

glycerol is synthesized into triacylglycerol in the epithelium through what pathway?

Chylomicrons; lymphatics → bloodstream

Long-chain fatty acids are esterified to triacylglycerol in the mucosal cells and secreted as _______ into _________

hepatic portal vein; free fatty acids

Short and medium chain fatty acids are mainly absorbed into the ________ as _________?

Cholesterol

________ is absorbed dissolved in lipid micelles and is mainly esterified in the intestinal mucosa before being incorporated into chylomicrons

Plant sterols and stenols

What competes with cholesterol for esterification, but are poor substrates, resulting to an increased amount of unesterified cholesterol in the mucosal cells

Denaturation

Process of breaking down protein

TRUE

TRUE OR FALSE: Native proteins are resistant to digestion because few peptide bonds are accessible to the proteolytic enzymes without prior denaturation by heat in cooking and by the action of gastric acid.

• 72%

• 22%

• 6%

Percentage of each pathway:

• monoacylglycerol pathway

• phosphatidic acid pathway

• hepatic portal vein (free glycerol)

Endopeptidases and Exopeptidases

two main classes of proteolytic digestive enzymes

Endopeptidases

hydrolyze peptide bonds between specific amino acids through out the molecule

Endopeptidases

First enzymes to act to proteins, yielding a larger number of smaller fragments

Pepsin

what type of endopeptidase catalyzes hydrolysis of peptide bonds adjacent to amino acids with bulky side-chains

*it is present in the gastric juice

Trypsin

What type of endopeptidase catalyzes hydrolysis of lysine and arginine amides

*it is present in the pancreas

Chymotrypsin

What type of endopeptidase catalyzes hydrolysis of amides of aromatic amino acids (phenylalanine, tyrosine, and tryptophan)

*it is present in the pancreas

Elastase

What type of endopeptidase catalyzes hydrolysis of amides of small neutral aliphatic amino acids (ex. Glycine, Alanine, Valine)

*it is present in the pancreas

Exopeptidases

What catalyzes the hydrolysis of peptide bonds, one at a time, from the ends of peptides.

Carboxypeptidases

A type of Exopeptidase that is secreted in the pancreatic juice, release amino acids from the free carboxyl terminal

Aminopeptidases

A type of Exopeptidase that is secreted by the intestinal mucosal cells, release amino acids from the amino terminal

Dipeptidases & tripeptidases

type of Exopeptidases that are seen in the brush border of intestinal mucosal cells catalyze the hydrolysis of di- and tripeptides

zymogens

The proteases are secreted as inactive ________

gastric acid and activated pepsin

Pepsinogen is activated to pepsin by?

enteropeptidase

In the small intestine, trypsinogen, the precursor of trypsin, is activated by ___________, which is secreted by the duodenal epithelial cells

Trypsin

What can activate chymotrypsinogen to chymotrypsin, proelastase to elastase, procarboxypeptidase to carboxypeptidase, and proaminopeptidase to aminopeptidase

free amino acids, di- and tripeptides, and oligopeptide

The end product of the action of endopeptidases and exopeptidases is a mixture of ?

sodium-dependent active transport

Free amino acids are absorbed across the intestinal mucosa by what type of transport

FALSE

TRUE OR FALSE: Relatively large peptides cannot be absorbed intact

Calbindin

intracellular calcium-binding protein required for calcium absorption

Vitamin D

acts to recruit calcium transporters to the cell surface which increases calcium absorption rapidly

Phytic acid (inositol hexaphosphate) or phytate

plant compound seen in cereals; binds calcium in the intestinal lumen, preventing its absorption

• Phenylalanine

• Valine

• Threonine

• Tryptophan

• Isoleucine

• Methionine

• Histidine

• Leucine

• Lysine

9 ESSENTIAL AMINO ACIDS

calcium oxalate (insoluble)

a high intake of oxalate can sometimes cause calcium deficiency due to the formation of ________

phytase

yeast contains an enzyme, ______ , that dephosphorylates phytate, so rendering it inactive

ferritin

Inorganic iron is transported into the mucosal cell by a proton-linked divalent metal ion transporter, and accumulated intracellularly by binding to ________

Ferroportin

Iron leaves the mucosal cell via a transport protein called

FALSE; free transferrin

TRUE OR FALSE: Iron can only leave the mucosal cell if there is free transthyretin in the plasma to bind to.

Hepcidin

• a peptide secreted by the liver when body iron reserves are adequate

• it downregulates the expression of the ferroportin gene

FALSE; decreased

TRUE OR FALSE: In response to hypoxia, anemia, or hemorrhage, the synthesis of hepcidin is increased

approximately 10%

Because of the mucosal barrier, what is the percentage of the dietary iron that is actually absorbed in the body?

Fe2+ (reduced)

Inorganic iron is absorbed in the ____________ state

Vitamin C

most effective compound/reducing agent that enhances iron absorption

Calcium

the absorption of both inorganic and heme iron is impaired by ?

TRUE

TRUE OR FALSE: Alcohol and Fructose also enhance iron absorption

FALSE

TRUE OR FALSE: Heme iron from meat is absorbed separately and less available than inorganic iron

OBESITY

Food intake in excess of energy expenditure leads to

body mass index = weight (in kg)/height2

commonly used as a way of expressing relative obesity

20 to 25

desirable range of BMI

• heat output

• consumption of oxygen

Energy expenditure can be determined by two ways:

• directly by measuring ________

• indirectly by measuring ____________

~20 kJ/L

Value of energy expenditure per liter oxygen consumed, regardless of whether the fuel being metabolized is carbohydrate, fat, or protein

respiratory quotient [RQ]

• an indication of the mixture of metabolic fuels being oxidized

ratio of the volume of carbon dioxide produced: volume of oxygen consumed

²H₂ ¹⁸O

dual isotopically labeled water that permits estimation of total energy expenditure over a period of 1 to 3 weeks

Basal metabolic rate (BMR)

the energy expenditure by the body when at rest, but not asleep, under controlled conditions of thermal neutrality

12 hours

Basal metabolic rate (BMR) is measured about how many hours after the last meal?

weight, age, and gender

BMR depends on what 3 aspects

Total energy expenditure (TEE)

This depends on the BMR, the energy required for physical activity, and the energy cost of synthesizing reserves in the fed state.

TRUE

TRUE OR FALSE: It is possible to estimate an individual’s energy requirement from body weight, age, gender, and level of physical activity

Adipose Tissue

The decrease in BMR with increasing age, even when body weight remains constant, is the result of muscle tissue replacement by ?

FALSE

TRUE OR FALSE: women have a significantly higher BMR than men of the same body weight and age because women’s bodies contain proportionally less adipose tissue

Physical activity ratio (PAR) or metabolic equivalent of the task (MET)

The most useful way of expressing the energy cost of physical activities is as a multiple of BMR also known as ?

Physical activity level (PAL)

the sum of the PAR of different activities, multiplied by the time taken for that activity, divided by 24 hours

diet-induced thermogenesis

considerable increase in metabolic rate after a meal

synthesizing reserves of glycogen, triacylglycerol, and protein

the major part of diet-induced thermogenesis is the energy cost of ?

secreting digestive enzymes and of active transport of the products of digestion

the minor part of diet-induced thermogenesis is the energy cost of ?

both adults and children

Marasmus can occur in what population