Macronutrients

Question 1

What are the macronutrients?

Answer 1

Fat
Protein
Carbohydrates

Question 2

What is the term for dietary fat?

Answer 2

Triacylglycerols

Question 3

What are triacylglycerols used for?

Answer 3

TAG is the primary energy substrate stored in adipose tissues to sustain animals during fasting. Adipose tissue releases fatty acids during fasting

Question 4

What controls the influx of triacylglycerols into adipose tissue?

Answer 4

Adipose tissue lipoprotein lipase

Question 5

When is adipose tissue lipoprotein lipase upregulated?

Answer 5

In the presence of insulin

Question 6

How is dietary fat broken down in the mouth?

Answer 6

Lingual lipase secreted by serous glands

Question 7

How is dietary fat broken down in the stomach?

Answer 7

Gastric lipase secreted by gastric cells in the fundic mucosa

Question 8

In whom are the enzymes of fat digestion important?

Answer 8

Neonates more than adults

Question 9

Describe how cholic and chenodeoxycholic acid are important in the small intestine

Answer 9

Bile acids from liver – cholic and chenodeoxycholic acid. Molecules are amphipathic - act as a detergent has both hydrophobic and hydrophilic properties - to solubilize lipids to form mixed micelles

Question 10

Describe the enzymes acting in the small intestine to digest fat

Answer 10

Small intestine – pancreatic lipase secreted from the pancreas alongside the cofactor pro-colipase
Pro colipase is activated in the intestinal lumen by trypsin to form colipase
Colipase stabilises pancreatic lipase increasing its efficiency
Pancreatic Lipase hydrolyses TG to form monocyglycerol, fatty acids and glycerol

Question 11

Describe how fats are absorbed in the small intestine

Answer 11

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

Question 12

How is excess carbohydrate stored?

Answer 12

Dietary CHO excess stored as glycogen or fat in the presence of hormone Insulin

Question 13

What is dietary carbohydrate?

Answer 13

Many CHO in diet including polysaccharide (starch and non starch), disaccharide ( sucrose and lactose) and monosaccharide (glucose and fructose)

Question 14

Describe the digestion of dietary carbohydrate

Answer 14

Starches are digested by α- amylases in saliva and in pancreatic secretions to form maltose, maltotriose and α–dextrins
Further digestion then occurs by brush border enzymes – gluco-amylase, α-dextrinase, sucrase, and maltase to form monosaccharides – glucose and fructose

Question 15

Describe the absorption of carbohydrate

Answer 15

Absorption of CHO is limited to the monosaccharides - glucose, galactose and fructose.
It occurs in the small intestine
Capacity to absorb fructose is limited

Question 16

Describe NSP resistant starch, soluble fibres and insoluble fibres

Answer 16

NSP (Fibre) resistant starch and most oligosaccharides are not digested and absorbed in the small intestine
Soluble fibres (pectin/gum) are fermented by bacteria in the colon
Insoluble fibres make up the cellulose ‘roughage’ in the diet

Question 17

Describe what happens to non starch polysaccharides in terms of microbial fermentation

Answer 17

Microbial fermentation leads to the production of:
Gas – CO2-H2-CH4
Short chain fatty acids - acetate (enters peripheral circulation ), propionate (taken up by the liver) and butyrate (used by colonic cells)
This also enhances microbial growth

Question 18

Is there a store for protein?

Answer 18

No- amino acids are always being turned over

Question 19

What is the daily requirement for protein in an average adult?

Answer 19

0.8g/kg/day

However the amount varies depending on who you are and what you do

Question 20

How do you express a protein requirement?

Answer 20

Protein percent of the diet 
Amino acid percent of the diet
Amino acid percent of total protein
Digestible protein percent of the diet
Ideal Protein ratios (relationships among amino acids) 20 amino acids of which 9 must be consumed in the diet

Question 21

Why is protein required?

Answer 21

Maintenance = nutritional requirements to stay alive (does not require positive BW gain)

Growth = positive tissue accretion

Reproduction = tissue specific growth related to reproduction, reproductive function (milk, eggs, reproductive tissue)

Question 22

Describe the digestion of proteins

Answer 22

Pepsinogen released by chief cells in stomach
HCL released by parietal cells HCL & pepsinogen released in response to gastrin and vagus nerve
Forms Pepsin Active @ pH 2-3, inactive pH>5
Only protease which can break down collagen.
Action terminated by neutralisation by bicarbonate in duodenum.

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

Question 23

Describe the absorption of amino acids

Answer 23

Absorption of amino acids is dependent on the electrochemical gradient of Na+ across the epithelium.
The transporters bind amino acids only after binding sodium.
The fully loaded transporter undergoes a conformational change that dumps Na+ and the amino acid into the cytoplasm. The transporter then re-orients back to its original form.
The baso lateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into the blood (not dependent on sodium gradients).

Question 24

Describe the absorption of peptides

Answer 24

There is 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
The vast bulk of di- and tri-peptides are digested into amino acids by cytoplasmic peptidases.

Question 25

Describe the absorption of in-tact proteins

Answer 25

Absorption of intact proteins occurs rarely.
Very few proteins can get through the gauntlet of soluble (lumen) 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 26

When can in-tact protein be absorbed?

Answer 26

Shortly after birth, neonates can absorb intact proteins in the small intestine.
Most of these intact proteins are immunoglobulins which can be absorbed from the very first milk (colostrum) and this imparts early neonatal passive immunity.
“Closure” is when the small intestine loses the capacity to absorb intact proteins.