Macronutrients

Question 1

Macro nutrients

Answer 1

Fats

Carbohydrates

Protein

Question 2

Micro nutrients

Answer 2

Vitamins

Minerals

Trace elements

Question 3

Dietary fat

Answer 3

TAG is primary energy substrate stored in adipose tissues to sustain animals during fasting

Influx of TAG into adipose tissue largely mediated by the action of adipose tissue lipoprotein lipase

Upregulated in presence of insulin

Adipose tissue releases fatty acids during fasting

Question 4

Fat digestion: mouth and stomach

Answer 4

Mouth- lingual lipase secreted by serous glands

Stomach- gastric lipase secreted by gastric cells in the fundic mucosa

Question 5

Fat digestion: small intestine

Answer 5

Bile acids from liver- cholic and chenodeoxycholic acid

Amphipathic- act as detergent has both hydrophobic and hydrophilic properties

Pancreatic lipase secreted from pancreas alongside cofactor pro-colipase

Pro-colipase activated in intestinal lumen by trypsin to form clipase

Colipase stabilises pancreatic lipase

Pancreatic lipase hydrolyses TG to form monocyglcerol, fatty acids and glycerol

Question 6

Carbohdyrate

Answer 6

Polysaccharide, disaccharide, monosaccharide

Stored as glycogen or fat in presence of insulin

Question 7

Carbohydrate digestion

Answer 7

Starches digested by a-amylase in saliva and in pancreatic secretions to form maltose, maltotriose and a-dextrins

Fruther digestion occurs by brush border enzymes- glucose amylase, a-dextrinase, sucrase and maltase to form monosaccharides glucose and fructose

Question 8

Carbohydrate absorption

Answer 8

Limited to the monosaccharides- glucose, galactose and fructose

Occurs in the small intestine

Capacity to absorb fructose is limited

Question 9

Non starch polysaccharides and other carbohydrates

Answer 9

NSP (fibre) resistant starch and most oligosaccharides are not digested and absorbed in the small intestine

Soluble fibres fermented by bacteria in colon

Insoluble fibres make up cellulose ‘roughage’ in the diet

Question 10

Non starch polysaccharides

Answer 10

Microbial fermentation leads to production of:

• gas (CO2-H2-CH4)
• short chain fatty acids (acetate, propionate, butyrate)
• also enhances microbial growth

Question 11

Protein

Answer 11

No store of protein

Adults excrete about 80g nitrogenous waste each day

Daily requirement 0.8/kg/day

Question 12

Protein requirements

Answer 12

Maintenance- nutritional requirements to stay alive

Growth- positive tissue accretion

Reproduction- tissue specific growth related to reproduction, reproductive function

Question 13

Protein digestion

Answer 13

Dietary protein , few exceptions, are not absorbed

Through gastric and pancreatic proteins, protein digested within lumen of intestine into medium and small peptides

Question 14

Protein digestion by pepsin in stomach

Answer 14

Pepsinogen released by chief cells in stomach

HCL released by parietal cells

HCL and pepsinogen released in response to gastrin and vagus nerve

Forms pepsin (active pH 2/3, inactive pH >5)

Question 15

Protein digestion by pancreatic proteases

Answer 15

Trypsin and chymotrypsin

Synthesised and packaged within secretory vesicles as inactive pro enzymes (tripsonogen and chymotrypsinogen)

Pro enzymes activated by enterokinase

Question 16

Enterokinase

Answer 16

Enteropeptidase

Secreted by mucosal membrane of duodenum

Question 17

Sequence of enzyme activation

Answer 17

1. Trypsinogen
2. Trypsin
3. Trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase
4. Trypsin, chymotrypsin, elastase, carboxypeptidase

Question 18

Absorption of amino acids

Answer 18

Dependent of electrochemical gradient of Na+ across epithelium

Transporters bind amino acids after binding sodium

Fully loaded transporter undergoes conformational change dumps Na+ and amino acid into cytoplasm

Transporter then re-orients back to its original form

Baso lateral membrane of enterocyte contains additional transporters which export amino acids from the cell into the blood

Question 19

Absorption of peptides

Answer 19

Virtually no absorption of peptides longer than three amino acids but there is abundant absorption of di and tri peptides, probably by a single transport molecule with hydrogen

Vast bulk of di and tri peptides are digested into amino acids by cytoplasmic peptidase

Question 20

Absorption of intact proteins

Answer 20

Occurs rarely

Very few can get through the gauntlet of soluble and membrane bound proteases intact

‘Normal’ enterocytes do not have the transporters needed to carry proteins across the plasma membrane and they can’t permeate tight junctions

Question 21

Absorption of intact proteins at birth

Answer 21

Neonates can absorb intact proteins in the small intestine

Most are immunoglobulins which can be absorbed from the very first milk and this imparts early neonatal passive immunity

Close is when the small intestine loses the capacity to absorb intact proteins