Digestion, absorption and fate of macronutrients Flashcards
What is soluble fibre?
dissolves in water to form a gel-like material (helps lower cholesterol and blood glucose). Undergoes fermentation in colon to produce short chain fatty acids
What is insoluble fibre?
promotes movement of material through digestive system
What process are major macronutrients digested by?
hydrolysis (requires enzymes)
What do disaccharides require for digestion?
brush border hydrolases
Where are mono/di/polysaccharides digested?
in small intestine as monosaccharides
Where are oligosaccharides digested?
large intestine, absorbed as SCFA
Cause of primary lactose intolerance
loss of lactase at weaning (lactose not hydrolysed, metabolised in bowel into SCFA and gases
How is lactose intolerance diagnosed?
H2 in breath (lactose malabsorption)
Cause of secondary lactose intolerance
infection/illness causes reduced lactase expression
Name sugar transporters
GLUT family - passive, SGLT family (sodium glucose transport) - active
Describe the fate of dietary carbohydrate
glucose uptake by liver, muscle and other tissues, glycogen synthesis, oxidative disposal
How are trans fats produced?
hydrogenation of unsaturated fatty acid
How are triacylglycerols digested?
TAG emulsified by bile acids to form small micelles. Hydrolysed by pancreatic lipase which liberates fatty acids and forms 2-monoglycerol
How is dietary fat absorbed in the small intestine?
fatty acids and monoglycerols enter enterocytes where they are re-esterified to form new triacylglycerols
What happens to consumed cholesterol?
half enters absorptive cells where some is esterified into cholesteryl esters. Remainder lost in faeces
How is dietary fat transported?
in the form of chylomicrons
What are chylomicrons?
large lipoproteins produced in enterocytes used to transport dietary fats
Composition of chylomicrons
mainly triacylglycerols and cholesterol esters in core. Surface composed of unesterified cholesterol, phospholipids and apoproteins
Describe how energy from fatty acids is converted into ATP energy
oxidative disposal of dietary fatty acids by forming acyl coenzyme A
How do dietary fatty acids accumulate in postprandial state?
in the form of triacylglycerol in adipocytes
Functions of dietary fat
energy provision (oxidative disposal), energy storage (adipose and muscle triacylglycerol), structural role in cell membranes (phospholipids, cholesterol), provides EFAs, carry fat-soluble vitamins
How are proteins digested in the stomach?
acid denatures proteins and unfolds them, zymogen (pepsinogen) activated to pepsin which hydrolyses 20% of proteins (including collagen) into di and tripeptides or amino acids
How are proteins digested in small intestine?
by pancreatic proteases
Examples of pancreatic proteases
trypsin, chymotrypsin, carboxypeptidase, elastase
How are amino acids absorbed into the bloodstream from small intestine?
amino acids enter enterocytes via Na+ cotransporters and are then transported into the bloodstream by facilitated diffusion
How are di- and tripeptides absorbed into the bloodstream from the small intestine?
di and tripeptides enter enterocytes by cotransport with H+ via PepT1. Digested by cytoplasmic peptidases into amino acids (or some remain intact)
When does intact protein absorption occur?
in newborns for a few days to acquire passive immunity by absorption of immunoglobulins from colostral milk
Role of insulin in the fate of dietary amino acids
insulin increases amino acid uptake in tissues, increases protein synthesis enzyme activity, reduces protein catabolic enzyme activity
Which factors act to increase protein synthesis?
insulin, IGF (GH), testosterone, adrenergic influences, exercise
Which hormones increase protein catabolism?
thyroid hormones, cortisol, (absence of insulin)
Function of macronutrients
provide energy, building blocks, essential nutrients (cannot be made in body)
Order the macronutrients from most to least energy dense
fats (9kcal/g), alcohol (7kcal/g) protein (4kcal/g), carbohydrates (4kcal/g)
What percentage of the energy requirement does the British Nutrition Foundation recommend be obtained from carbohydrate, fat and protein?
48% from carbohydrate, 36% from fat, 16% from protein
How does the CVD risk and mortality of obese unfit individuals compare to obese fit individuals (fat but fit)?
obese fit individuals have a lower risk of CVD and CVD mortality
Causes of nutrient imbalance
inadequate intake, malabsorption, medical conditions, social factors
Examples of medical conditions that can lead to nutrient imbalance / deficiencies
Crohn’s disease, cystic fibrosis, bariatric surgery, parenteral or tube feeding, allergies, HIV infection, renal or hepatic disease (alter protein storage), psychiatric illnesses (e.g. EDs)
What are essential nutrients?
nutrients needed in the body that cannot be synthesised in the body
Examples of essential amino acids (EAAs)
isoleucine, phenylalanine, leucine, lysine, valine, methionine, threonine, tryptophan, histidine
Examples of non-essential amino acids
alanine, asparagine, glutamine, serine, arginine, cysteine, glycine, aspartic acid, glutamic acid, proline, tyrosine
Examples of essential fatty acids
n-6 PUFA: linoleic acid (LA), LC PUFA
n-3 PUFA: alpha-linoleic acid (ALA), AA, EPA
Function of essential fatty acids
cell membranes, biologically active compounds (e.g. eicosanoids), growth and development
Where can omega 3 fatty acids be obtained from?
oily fish
What is the recommended intake of carbohydrates for a non-physical active adult?
2.6-3.8 g/kg/BW
What is the recommended intake of fat for a non-physical active adult?
0.8-1.2g/kg/BW
What is the recommended intake of protein for a non-physical active adult?
0.8g/kg/BW
