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Macronutrients Metabolism (((Menu labelling: Will it help in reducing…
Macronutrients Metabolism
Fat
Major sources: oils, nuts, animal fats (i.e. butter, fish oil) Fats are the biggest source of energy for the body Comprised of fatty acids
Types of fats
Simple lipids (i.e. triglycerides)
Derived lipids (i.e. cholesterol)
Compound lipids (i.e. phospholipids)
Classification of fatty acids
By length of carbon chain
Short chain (SCFA), < 8 carbons,
e.g. acetic, propionic, butyric acids, generally water soluble, generated by colonic bacteria
Medium chain (MCFA), 8
15 carbons, e.g. caprylic, capric , lauric, myristic acids; often intermediates in synthesis of long chain fatty acids Long chain (LCFA), >=16 carbons, of most interest in human nutrition
By number and position of double bonds
Monounsaturated = MUFA
Polyunsaturated = PUFA
Important examples in human nutrition: linoleic, and linoleic, linolenic, eicosapentanoic and docosahexanoic acids
By orientation of the double bonds
Most naturally occurring FAs have cis double bonds
Trans bonds can occur after bacterial fermentation, e.g. in cow’s rumen, or in industrial processing, e.g. to make solid margarine; trans FAs act more like saturated FAs chemically
Fed state
TGs taken up by tissues through lipoprotein lipase
Acetyl - CoA, product of b-oxidation of fat, is not needed for current energy requirements, therefore used for fat synthesis
Fasted state:
TGs broken down into
a) Fatty acids → acetyl CoA (beta oxidation) → citric acid cycle
b)Glycerol side chain-liver to make into glucose
Most tissues have limited capacity for β- oxidation so liver is major site
Can’t transport acetyl-CoA in blood so need to convert to ketone bodies: acetoacetate (volatile)
used by other tissues (brain, red blood cells) as fuels
when metabolising large amounts of fat (low carbohydrate diet, starvation) → exhaled acetone
Hormones and
metabolism
Fed State
increased
glucose and aa’s in portal blood →
increase
synthesis of glycogen from glucose in muscle and liver -
decreased
↓ gluconeogenesis
increase
insulin (from pancreas b cells causes: uptake of glucose and aa's into muscle and adipose tissue
Increased
uptake of triglycerides from blood lipoproteins into adipose, fatty acid, triglyceride and cholesterol synthsesis
increased muscle protein synthesis
Fasted State
↓ glucose and aa's in the portal blood leading to
decrease
in insulin and an increase in glucagon
increase
in glucagon (from pancreas islet a cells and a
decrease
in insulin cause: breakdown of glycogen in the liver for the release of free glucose into the bloodstream
increase
gluconeogenesis in the liver and kidney
↓ protein synthesis in muscle
Factors affecting nutrient absorption
Interactions between diet components
Fibre reduces absorption of many minerals
Calcium reduces iron, zinc and fatty acid absorption
Iron, zinc and copper compete for absorption
2. Physiological need
Iron absorption increases during pregnancy
Vitamin D - calcium- parathyroid hormone axis which enhances calcium and phosphorus absorption as needed
3. Diseases of the gut affecting absorption
Gastritis inflammation of gastric mucosa caused by infection, bile reflux, NSAIDs, smoking, excessive alcohol consumption
glandular atrophy: ↓ secretions ↓ intrinsic factor may cause ↓vitamin B12 absorption and lead to megaloblastic anaemia
Diarrhoea - irritation of intestinal mucosa
decreased absorption of many nutrients lack of water resorption can lead to dehydration
Inflammatory bowel disease Crohn’s disease and ulcerative colitis
Pancreatic insufficiency leads to malabsorption
Environmental enteropathy villous atrophy leads to malabsorption
Very common in low income countries
Formally called tropical enteropathy but wealthy tropical countries, e.g. Singapore, don’t have the problem Due to repeated infections and inadequate repair, coupled with deficient diet Lack of absorptive surface can lead to poor nutrient absorption and malnutrition
Metabolism
Catabolic reactions: large molecules are broken down into smaller simpler molecules usually accompanied by the release of energy using ‘ATP currency’ eg. glycogenolysis, glycolysis
Anabolic reactions: large molecules are synthesised from smaller molecules usually require the input of energy using ‘ATP currency’ eg glycogenesis, gluconeogenesis
Catabolism + Anabolism = Metabolism
Regulation of metabolism
Constant energy supply going from Fed → fasted → starved
Energy storage
Major store is fat
Some glycogen in liver and muscle
No storage form of protein so catabolize muscle if need amino acids
Protein Fed State (catabolism)
Broken down to amino acids
In infection, the liver and immune system are major drivers of protein synthesis and breakdown
Carbon skeletons of glucogenic amino acids used to synthesize glucose for brain and red blood cells
Carbon skeletons of ketogenic amino acids used to synthesize ketone bodies for use by other tissues
Excess nitrogen excreted as urea
Fed State (anabolism)
Proteins turn over regularly and need to replace sloughed mucosal cells,
Can only make a protein if all its amino acids are available
Insulin stimulates protein synthesis in fed state
Carbohydrates
Need to maintain constant blood glucose main fuel for brain since triglycerides can’t cross blood - brain barrier
Fed state
From gut most glucose goes through portal vein to hepatocytes where it is phosphorylated and moved on into catabolic and anabolic reactions
Some glucose enters systemic circulation and insulin secretion is stimulated (pancreas beta cells)
Fasted state
Glycogen stores in liver and muscle are limited so within hours of fasting need to move to catabolising fat and protein. Glucose formed from pyruvate
Low intensity can use mainly fat but rapid intensive exercise uses muscle glycogen
broken to monosaccharides
glycogenesis
Brain relies on glucose to function >60% of glucose, MUscles cannot produce glucose, RBCs cannot produce glucose b'se they lack mitochondria
Fibre
Edible plant or animal material not hydrolysed by the endogenous enzymes of the human digestive tract
Effects of fibre
Slows gastric emptying and reduces mixing of gastrointestinal contents with digestive enzymes
Reduced enzyme function and nutrient diffusion rate
Laxative effect
Promote short chain fatty acid production by colonic bacteria can be absorbed from colon
Promote bifidobacteria growth (good bacteria)
ATP is never stored: it is constantly cycled between ATP and ADP
A calorie is a calorie
Calorie
is defined as energy required to raise 1kg of water from 15 degrees to 16
Thermodynamically: A calorie is a calorie according to the 1st law of thermodynamics that energy can never be created or destroyed but only transformed
differences in chemical structure
there is a difference in metabolizable energy from different food products
Metabolizable energy: Energy available for use after losses in urine, faeces and sweat
Atwater factor:
eg. fiber decreases transit time
bulky hence absorption is not effective
intestinal mucosa erosion
energy expenditure
greater weight loss with decrease in CHO consumption
high protein consumption associated with high RMR due to high thermic effect from protein
Menu labelling: Will it help in reducing consumption and reduce calorie consumption 1. Impulsive eaters, impulsive decision makers 2. Limited understanding of the meaning of calories