Macronutrients: Carbs, proteins and fats [Theme 1] Flashcards
Dietary fat
- Hormone regulation
- Function
Triacyglycerol- primary energy storage in adipose tissue.
- Releases fatty acids during fasting
Influx into adipose tissue mediated lipoprotein lipase
- Regulated by insulin
Fat digestion in the mouth
- Gland
Lingual lipase
- Secreted by serous glands
Fat digestion in the stomach
- Cell secreted from
Gastric lipase
- Secreted from gastric chief cells
- In fundic mucosa
Optimum pH of 3-6
- No need for bile acid or colipase
Importance of fat digestive enzymes
Lingual lipase and gastric lipase accounts for:
- 50% of lipid hydrolysis in neonates
- 30% of lipolysis in adults.
Bile acids in fat digestion
These acids/ salts are released from the liver.
Amphipathic molecules: cholic and chenodeoxycholic acid.
They dissolve lipids to from mixed micelles.
Fat digestion in the small intestines
Pancreatic lipase is secreted with pro-colipase.
Pancreas and activated colipase hydrolyse TG.
Products of hydrolysis passively diffuse across the apical membrane of the microvilli.
- Also through lipid protein transporter mechanisms.
Pancreatic lipase
Digestive enzyme in pancreatic juice into the duodenum.
Released with pro-colipase which stabilises it when activated by trypsin into colipase.
Hydrolyses TG to—->
- MG
- Fatty acids
- Glycerols
Dietary carbohydrates.
- Polysaccharides
- Disaccharides
- Monosaccharides
Polysaccharides:
- Starch
- non starch: cellulose, chitins, glucans
Disaccharides:
- Lactose
- Sucrose
Monosaccharides:
- Glucose
- Fructose
Initial digestion of starch
In the salvia and pancreas by alpha-amylase.
- Forms maltose, maltotriose and alpha-dextrins.
Further digestion by brush border enzymes:
- Gluco-amylase
- Alpha-dextrinase
- Sucrase
- Maltase
- ——> releases glucose and fructose.
Sucrase
Brush border digestive enzyme that breaks down sucrose—–> glucose and fructose.
Maltase
Brush border digestive enzyme that breaks down maltose—–> 2 alpha glucose molecules
Carbohydrate absorption
Only monosaccharidies can be absorbed in the small intestines.
Glucose and galactose absorbed apically through Na+ co-transport.
- Fructose via GLUT-5
All monosaccharides absorbed basolaterally via GLUT-2.
Fructose absorption
Fructose enters the small intestinal membrane at the apical side via GLUT-5.
This capacity is limited so excess fructose passes to the colon and can cause osmotic diarrhea.
Carbohydrates no absorbed in small intestines
Non-starch polysaccharides- like fibre.
- Soluble fibre like pectin and gum are fermented by bacteria in the colon.
Most oligosaccharides.
Soluble fibre like pectin and gum are fermented by bacteria in the colon.
Fibre
A non-starch polysaccharide.
Both soluble and insoluble starch both enhance peristalsis and gastric motility.
- Reduces cholesterol absorption
- Balances population of commensal bacteria
Examples:
Pectin, Gum (soluble)
Non-starch polysaccharides
Dietary fibre
Responsible for maintaining weight and colon health.
They can be fermented microbialy to produce gas
Short chain fatty acids
These enhance microbial growth in the gut that is ferments NSP.
Acetate- enters peripheral circulation.
Propionate—-> enters liver
butyrate—-> used by colonic cells
Daily requirement for protein in diet
0.8g of protein per kg of weight a day.
Uses of protein in the body
Maintenance:
to keep us alive
Growth:
Positive tissue accretion (muscles, connective tissue, etc.)
Reproduction:
Growth in reproductive tissue (milk, eggs etc)
Protein value in diet
The more similar the source of protein is to the body proteins, the more likely it is to contain all the amino acids required.
Varying the range of protein source is very important for people who have diets missing food groups.
Protein digestion in the stomach
HCl and Pepsinogen released in response to gastrin and vagal stimulation.
Protein digested by pepsin.
- Pepsinogen is the inactive form released from chief cells in the stomach.
- Activated as pepsin in pH 2-3
HCl secreted by gastric parietal cells.
Digestion terminated by bicarbonate neutralisation.
Pepsin
Gastric protease released by gastric chief cells in its inactive form: pepsinogen.
- Triggered by gastrin and vagal stimulation.
It is activated in the presence of HCl where pH 2-3.
Pepsin is the protease that can break down collagen.
Protein digestion by the pancreas
Pancreas releases pro-enzymes:
- Trypsinogen
- Chymotrypsinogen
Pro-enzymes are activated by enterokinase secreted by Brunner glands in the duodenum:
- Trypsin
- Chymotrypsin
Trypsin inhibitor in the secretory vesicles prevent inappropriate activation of trypsin in the pancreas/ pancreatic duct.
Absorption of amino acids
Amino acids are enter the cell via cotransport with Na+, on the apical membrane.
The basolateral membrane contains transporters no dependent on Na+.
Absorption of Di/tripeptides
They enter the enterocyte in the small intestines apically, co transported with H+.
In the cell, they are cleaved by cytoplasmic peptidase into amino acids.
Amino acids leave basolaterally via transporters not dependent on H+.
Absorption of intact proteins
Only very few proteins can go through the membrane bound proteases intact.
- Normal enterocytes do not have the transporters to facilitate the transport.
Neonates can absorb intact proteins in the SI
- immunoglobulins from colostrum
Closure of small intestines
Occurs when the small intestine loses its capacity to absorb intact proteins.
Seen after babies stop being able to absorb immunoglobulins.
