Macronutrients Flashcards
Function of Carbohydrates
Energy substrate Builds macromolecule Glycogenesis Spares protein and fat NSP - substrates for fermentation in colon, fibre for faecal bulk, excretion dead cells, increase transit time
Dietary fat
95% of dietary fat intake
Triacylglycerol (TAG)
Single double bond - monounsaturated; multiple - polysaccharide
Functions: TAG is the primary energy substrate stored in adipose tissue to sustain animals during fasting; influx of TAG into adipose tissue largely mediated by the action of adipose tissue lipoproteins lipase (LPL); this is unregulated in the presence of insulin, fatty acids, during fasting
Fats carry essential fat soluble vitamins A, D, E, K
Serves as a component of cell membranes and precursor of steroid hormones and vit D
Fat broken down by lingual lipase secreted by serous glands
Protein
Dynamic nature of protein
In health requirements 0.8g/kg/day
Older adults 1.2g/kg/day
Acute illness demand can exceed this - negative nitrogen balance due to inflammation induced catabolism
Overview of digestion and absorption
Adaptations to GI tract facilitate different functions
Stomach - acidic environment - mucous and bicarbonate ions
Small intestine - peristalsis becomes segmented to squeeze food against intestinal wall and maximise digestion and absorption
Sequence of sphincters - cardiac, pyloric, ileocaecal valve, internal and external sphincters
Parasympathetic NS important for GI function
Brain and Buccal
Mastication
Swallowing
Taste - sweet, sour, salty, bitter and umami
Fat digestion in stomach
Stomach –gastric lipase secreted by gastric cells in the fundic mucosa. An acid enzyme that does not require bile acid (from liver) or colipase (from pancreas) for optimal functioning
These enzymes are important
o in neonates contributing to up to 50% of lipid hydrolysis
o in adults up to 30% of lipid hydrolysis
Protein digestion in stomach
Chief cells produce pepsinogen converted to pepsin in presence of HCl - proteins -> peptides
Acid environment - denatures protein
Reduces bacterial load
Both vasovagal reflex and endocrine release of gastrin stimulate pepsinogen secretion during the gastric phase
Fat digestion
Bile acids from liver - chalice and chenodeoxycholic acid (molecules are amphipathic - acts as detergent - solubizes lipids to form mixed micelles)
Pancreatic lipase secreted from the pancreas alongside the cofactor pro-colipase
Pro collapse is activated in the intestinal lumen by trypsin to form collapse (which stabilised pancreatic lipase increasing its efficiency)
Pancreatic lipase hydrolyses to TG to form monoclycerol, fatty acids and glycerol
Pancreatic proteases and protein digestion
The two primary pancreatic proteases are trypsin and chymotrypsin. They are synthesized and packaged within secretory vesicles as inactive pro enzymes: trypsinogen chymotrypsinogen
The pro enzymes are activated by an enteropeptidase - enterokinase secreted by the mucosal membrane of duodenum
The secretory vesicles also contain a trypsin inhibitor to serve as a safeguard against trypsinogen converting to trypsin within the pancreas or pancreatic ducts
Pancreas peptidases and protein digestion
Endopeptidases and exopeptidase
Endopeptidases are in stomach and exocrine pancreas secretion
Exocrine - secretion by brush border enzymes
Pancreas and pancreatic alpha amylase carbohydrate digestion
Starch made up of 30% amylose and 70% amylopectin
Amylases begin breakdown of starch and glycogen into maltose and glucose
Absorption of nutrients in SI
Blood supply - 9 litres of fluid enter GI tract with 8 litres absorbed through small intestine
Large surface area to maximise secretion and absorption
Disaccharide, glucose, galactose and fructose digestion in SI
Disaccharides further digestion occurs by actin of brush border enzymes – gluco-amylase, α-dextrinase, sucrase, and maltase to form monosaccharides – glucose, fructose and galactose
Absorption of CHO is limited to the monosaccharides - glucose, galactose and fructose.
It occurs in the small intestine
Capacity to absorb fructose is limited
Fat absorption (fatty acids) in SI
Water soluble mixed micelles pass through the ‘unstirred water’ overlying the microvillus
Monocylglycerol and fatty acids cross apical membrane of microvilllus through passive diffusion and lipid protein transporter mechanisms
Protein absorption (amino acid) in SI
Pepsin – stomach Trypsin – pancreatic proteases Peptidases – brush border enzymes PEPT1: human peptide transporter 1 NHE3 – sodium-hydrogen exchanger 3
