Transcript Lecture notes on macronutrients

Macro-nutrients in normal human
growth and development
MACRONUTRIENTS
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Proteins
Fats
Carbohydrates -sugars and starches
Alcohol
Introduction to Proteins
• Protein is the most abundant nitrogencontaining compound in the body.
• It generally comprises about 10% of dietary
energy but its significance is more than
simply as an energy source
• It is the major functional and structural
component of all cells of the body
• All enzymes, membrane carriers, blood
transport molecules, intracellularmatrix,
hair, finger nails, etc are proteins.
• In addition many hormones and a large
proportion of membranes are proteins
• Furthermore their constituent amino acids
act as precursors of many coenzymes,
hormones, nucleic acids essential for life
• The characteristic element in protein is
nitrogen which constitutes about 16% of its
weight.
• Carbon, oxygen and hydrogen are also
abundant in protein
• Sulphur and phosphorus make up small
proportions as well
Functions of Proteins
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Table 1.
Structural
body
skeleton and supporting tissues, and skin
tissue
connective tissue
cells
cellular activity
Protective
barrier acting as non-specific defences such as keratin in skin
inflammation
acute phase response
immunity
cellular immunity
Transport/Communication
plasma proteins albumin, transferrin,
hormones
insulin, glucagon, growth hormones
Enzymic
Exracellular
digestive, clotting, haemoglobin, O2 and CO2 transport
Metabolic
glycolysis, protein synthesis, citric acid cycle, urea cycle
Protein Structure
• Proteins are macromolecules consisting of
long chains of amino acid sub-units.
• The structure of the amino acid is as shown
on page 30 Geissler (2011) Human
Nutrition (12th edition).:
• The amino acids are joined together by
peptide bonds. Two amino acid units joined
together are referred to as di-peptides.
However chains can consist of thousands of
amino acids linked together referred to as
polypeptide chains.
• Polypeptide chains do not exist as straight
long chains but instead fold into definite
three-dimensional structures, the exact
shape depending on their particular function
and interaction with other molecules.
Insulin Hormone
• The insulin hormone is a small polypeptide
chain.
• Many hormones can have as few as ten
amino acids
Essential Amino Acids
• “essential” amino acids are so called because they cannot
be synthesised by humans and therefore must be ingested
in the diet, these include
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leucine,
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isoleucine,
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lysine,
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methionine,
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phenylalanine,
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tryptophan,
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threonine a
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valine.
Non-Essential Amino Acids
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The others - alanine, glutamic acid & aspartic acid are “non-essential”
as they can be synthesised by the body. A further 9 –
tyrosine,
glycine,
serine,
cysteine,
arginine,
glutamine,
asparagine,
proline and
histidine
are termed conditionally essential as they are derived from the
metabolism of other amino acids or complex nitrogenous metabolites.
If these are absent then they cannot be synthesised.
Digestion & Absorption
• After ingestion, proteins are denatured by the acid
of the stomach and then pass to the small intestine.
• There they are hydrolysed by a variety of
proteolytic enzymes.
• These enzymes originate in the pancreas and
include trypsin, elastin and carboxypeptidase.
• The resultant mixture of free amino acids and
small peptides are transported to the gut by a
series of carrier systems.
• After hydrolysis of the peptides the free
amino acids are then secreted into the
bloodstream or further metabolised in the
gut itself.
• Protein absorption may be incomplete for
various reasons, including their physical or
chemical structure may be resistant to
proteolytic attack.
• In addition if gut function is impaired such
as intestinal inflammation 100% absorption
may not occur.
• Absorbed amino acids pass into the portal
vein and then on to the liver, where a
proportion are taken up by the liver.
• The remainder pass through to the systemic
circulation and are transported into the
tissue’s cells.
Protein Synthesis and
Degradation
• Protein synthesis is a continuous process that takes place in
almost all cells of the body.
• In normal adults about 4g protein/kg body wt are
synthesised daily: hence about 300g/day in men and
250g/day in women.
• In newborn infants the rate is approximately 12g/kg body
weight falling to 6g/kg at 1 year reflecting the huge
demands for protein for growth an development in newborn infants.
• Protein turnover like BMR is related to the size, shape and
body composition of the individual.
• Under normal conditions protein degradation occurs at the
same pace (except in infants).
• This continuous breaking down and resynthesis of proteins
is referred to as “PROTEIN TURNOVER”
• Normally there is a balance between protein synthesis and
breakdown but in times of pathological conditions such as
starvation, injury, infection, cancer, diabetes etc,
substantial rates of protein loss can occur.
• This can eventually become life-threatening.
Nitrogen Balance
• About 10-15g of nitrogen are excreted in
the urine of a healthy adult daily, mostly in
the form if urea and smaller amount of
ammonia (arises from oxidation of amino
acids).
• In addition uric acid, creatinine and some
free amino acids are also excreted
• On average about 16% of protein is
nitrogen.
• Therefore by measuring the urinary
excretion of nitrogen and multiplying by
6.25 the approximate protein content of
food can be obtained.
• However account must also be taken of losses
from stools (about 5-10%)
• In pathological conditions such as, haemorrhage,
burns and fistulas large losses can occur resulting
in negative nitrogen balance
• The body can achieve nitrogen balance
again and responds by reducing the amount
of urea excreted.
• The equilibrium can be restored within 5
days.
• Maintaining nitrogen balance is imperative
for maintaining tissue and tissue protein
integrity
Body Protein Stores
• The body of a 70kg man contains about 11kg of protein
and the approximate distribution is as follows:
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Tissue
Muscle
skin
blood
liver
brain
kidney
% total body protein
43%
15%
16%
1.8%
1.5%
0.3%
• Despite the wide variety of different
enzymes and proteins within an organism,
almost 50% of total protein content is in 4
proteins:
MYOSIN
ACTIN
COLLAGEN
HAEMOGLOBIN
Dietary Sources
• 2 classifications
• Low biological value (LBV) proteins are
from plant sources - pulses (legumes) such
as beans, peas, lentils. Also nuts are a rich
source
• High biological value (HBV) are proteins
from animal sources, includes eggs, milk,
meat, cheese.
• Broadly speaking animal sources of protein
account for 60-70% of protein intakes in
western countries and in developing
countries plant proteins normally account
for 70-80% of total protein intakes.
Dietary Reference Values
• See table labelled
• Table 7.1. Dietary Reference Values for
Protein.
Introduction to Carbohydrates
• Green plants synthesize carbohydrates from carbon dioxide
and water under the influence of sunlight (photosynthesis).
• The primary products are sugars which are soluble in water
and hence are easily transported in tissue fluid of plants
and animals to provide fuel to cells.
• Sugars are polymerized to polysaccharides which can
easily be stored or incorporated into cell-wall structures.
Structure
• Monomers and polymers of carbohydrates.
• Carbohydrates contain a carbon backbone, and
many polar hydroxyl (-OH) groups and are
therefore very soluble in water.
• Large molecules called polysaccharides consist of
many small, ring-like sugar molecules.
• The sugar molecules attached are together by
lycosidic bonds in a linear or branched array to
form sugar polymers (see page 26 Gissler )
Two main categories
• Plant foods contain two chemically distinct types of
polysaccharide
• 1) The storage polysaccharide starch (e.g. root vegetables
such as potatoes): a polymer of glucose linked by alphaglucosidic linkages
• 2) Non-starch polysaccharide (NSP) (dietary fibre): does
not contain the alpha-glucosidic linkages
• Animal tissue also contain carbohydrate polymers
in the form of glycogen which is found in many
tissues e.g. glycoproteins in the gut which provide
a protective lining for epithelial tissues.
• The amount present in animal tissues is small and
is not important in dietary energy terms
Functions of carbohydrates
• Provide large sources of metabolic fuel and energy stores
• form structural components of cell wall in plants and the
exoskeleton of arthropods
• form parts of RNA and DNA
• form integral features of many proteins and lipids
(glycoproteins & glycolipds), especially in cell membranes
where they are essential for cell-cell recognition and
molecular targeting.
• NSP: Important in maintaining normal gut function and
prevention of constipation
Digestion & Absorption
• To be absorbed in the gut, carbohydrates must be broken
down into their constituent monosaccharide units.
• A variety of hydrolytic enzymes capable of splitting the the
bonds between the sugar residues are secreted by the
mouth, pancreas and enterocytes (liver cells).
• These enzymes ensure that 95% of carbohydrates are
digested and absorbed within the small intestine.
• Non-starch polysaccharides (fibre) are the skeletal remains
of plant cells that are resistant to the digestive enzymes in
humans.
• Instead they undergo fermentation in the colon by the
action of micro-organisms.
• Fermentation is the process whereby micro-organisms
breakdown monosaccharides and amino acids to derive
energy for their own metabolism.
• A number of end-products result including the gasses
carbon dioxide, hydogen, and methane. In addition shortchain fatty acids (SCFA) are produced.
• Theses SCFA are undergoing intense research at present
because of their potential health benefits: includes,
reduction of colorectal cancer risk, reduced risk of
cardiovascular disease through cholesterol lowering.
Classification
• Can be classified according to the degree of
polymerization (DP):
• Monosaccharides (DP 1)
• Disaccharides (DP 2)
• Oligosaccharides (DP 3-9)
• Polysaccharides (DP > 9)
• Non-starch polysaccharides (DP > 9)
• Sugar alcohols
Nutrient Example
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Monosaccharides - glucose, fructose
Disaccharides - sucrose, lactose
Oligosaccharides - raffinose, inulin
Polysaccharides - starches
Non-starch polysaccharides - fibre
Sugar alcohols - sorbitol (naturally occuring
or comercially synthesised)
Food Sources
glucose - fruit & vegetables
fructose - honey
sucrose -Sugar beet or cane
lactose - milk
raffinose, inulin - legumes
starches - cereal grains, potatoes,
fibre - wheat bran e.g. weetabix, all bran, porridge
Sugar alcohols - cherries or commercially prepared (substitute
for sucrose for diabetic diet as it is absorbed more slowly)
Recommended Dietary Intake
• From birth carbohydrates provide a large part of
the energy in human diets, with approximately
40% of the energy in mature breast milk being
supplied as lactose.
• After weaning, carbohydrates are the largest
source of energy (40-80%) in human diets, with
most being derived from plant material.
• Sugars and Starches
• Non-milk extrinsic sugars 10% of total dietary energy
• Intrinsic and milk sugars and starch 37% of total dietary
energy
• NSP
• 18g/day from a variety of foods whose constituents
contain it as a naturally integrated component
Clinical Conditions Involving
Carbohydrate Metabolism
• DIABETES is characterised by a failure to maintain the
concentration of blood glucose within the normal range.
• 2 types present:
• Type 1 diabetes - insulin dependent - presents with acute
symptoms induced by high blood glucose.
• Classic presentation includes thirst, polyuria and weight
loss.
• Usually arises early in life.
• Treatment is by insulin injection
• Type 2 diabetes - non-insulin dependent -usually
presents in middle-aged obese subjects.
• Symptoms improved by diet, weight loss and
exercise.
(see page 425 Diabetes Mellitus Gissler and Powers)
Introduction to Fats
• The average daily intake of fat in a Western diet ranges
from 50 to 100g and provides 35-40% of total energy.
• It consists mainly of triglycerides, also known as
triacylglycerols, which form the principal component of
visible oils and fats, and minor quantities of phospholipids
and cholesterol esters.
• The physical properties of dietary fat, such as their
hardness at room temperature (melting point) and
subsequent metabolic properties once in the body are
determined by the number of double bonds in their
constituent fatty acids, (degree of saturation or
unsaturation) and length of fatty acid carbon chain.
FUNCTIONS OF FAT
• Source of dietary energy, both short-term and as long-term reserves in
adipose tissue in the form of triacylglycerols and in breast milk also
composed mainly of triacylglycerols
• Provides essential fatty acids
• Acts as an insulator in the body maintaining body heat
• Structural role contributing to the architecture of cells : in animals
phosphoglycerides are the major lipids, in plants glycosylglycerides
predominate.
• Performs metabolic role: cholesterol, a component of dietary fat, is
metabolised into a group of hormones, the steroid hormones. These
include testosterone, progesterone and cortisol.
• Fat-soluble vitamins are stored in fat in the liver or in adipose tissue
• Make foods more palatable
Structure of fats
• All lipids are composed of a carbon skeleton with
hydrogen and oxygen substitutions.
• Nitrogen, sulphur and phosphorus are also present in some
lipids.
• The main dietary component of fats are fatty acids varying
in length from one to greater than 30 carbons.
• Short-chain fatty acids are those with < 8 carbons.
• Medium-chain fatty acids are those with 8-14 carbons.
• Long-chain fatty acids are those with > 14 carbons
Nomenclature of fatty acids
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About 21 different fatty acids are found in the diet in appreciable amounts, the
most prevelant being palmitic, stearic, oleic, linoleic and arachidonic acids.
• Increasingly fatty acids are described by a notational system which
summarizes their structure. In this scheme, the total number of carbon atoms in
the fatty acid is expressed as C12, C16 etc.
• The total number of double bonds is then shown after a colon, for example
palmatic acid is C16:0 and linoleic acid is C18:2
• The position of the first double bond is defined in relation to the methyl end of
the carbon chain, with the carbon atom at the methyl end termed the omega (or
n) carbon.
e.g. linoleic acid has its first double bond between the sixth and seventh carbons
from the methyl end, so its full notational name is C18:2 omega-6, or C18:2 n6
Classification
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fats can be classified into 3 groups: see figure 5
Saturated- have no double bond, contain carbon atoms linked only by single
bonds. They are principally derived from storage fats of animals or products
derived from them, milk,,butter, cream or cheese. e.g. palmitate. SFAs tend to
elevate LDL cholesterol and hence total serum cholesterol
Monounsaturated- have 1 double bond. Concentrated sources are olive oil and
rapeseed oil. MUFAs are regarded as beneficial s they do not have a
hypercholosterolaemic effect, and when substituted for SFA , lower the LDLcholesterol. e.g. oleic acid
Polyunsaturated- have more than 1 double bond. They are divided into 2 types n6
(omega-6) & n3(omega-3) and they have distinctively different features. The
parent fatty acids in each of these groups,. linoleic acid(n6) and alpha-linoleic
acid (n3), are termed essential fatty acids because humans lack the enzyme to
synthesize them and therefore require a dietary source, principally from plant
foods such as oils, nuts, and seeds.
• Long-chain fatty acids derived from those
essential fatty acids are precursors of many
metabolic intermediaries such as
prostaglandins, leukotrienes, and
thromboxanes and hence have variable
influences on inflammatory processes,
immune response and blood clotting.
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Most dietary polyunsaturated fat is in the form of n6 fatty acids, principally
linoleic acid derived from vegetable olis, such as sunflower, corn etc.
n6 have a hypocholesterolaemic effect and are encouraged as a substitute for
saturated fatty acids.
n3 polyunsaturated fatty acids have minimun effect of blood cholesterol , the
main interest in n3 fatty acids lies in their ability to influence thrombotic and
inflammatory function. Are principally found in fish oils, and hence fish oils
are recommended as a protective mesure against cardiovascular disease.
2 long-chain n3 derivatives EPA (ecosapentaneoic acid) and
DHA(doscahexaenoic acid) have generated a lot of interest recently because of
their important physiological effects, they may be important in inflammatory
disorders such as rheumatoid arthritis and crohn’s disease
Dietery Sources
• Saturated fats: animal sources such as butter,
cheese, meat
• monounsaturated fats: olive oil
• polyunsatured fats: sun flower oils and spreads
Digestion and Absorption
• Fat digestion begins in the stomach, where a churning action helps
form a coarse fat emulsion. This mixture enters the small intestine and
is modified by mixing with bile and pancreatic juice.
• Triacylglycerols, which form the bulk of fats in the diet, must be
broken down into partial glycerides and fatty acids by a pancreatic
lipase enzyme in the small intestine before they can be absorbed.
Occurs mainly in the duodenum.
• Absorption occurs mainly in the jejunum.
• Under conditions of nutritional excess, fatty acids are absorbed by
adipose tissue where they are converted to storage lipids in the form of
triacylglycerols. The triacylglycerols can be mobilised at a later time,
when the carbohydrate energy reserves are low. Mobilisation involves
the conversion of triacylglycerols back to fatty acids.
Dietary Cholesterol
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Cholesterol is a wax-like substance belonging to the steroid family.
It is essential to life; it is a primary component of cell membranes and a
substrate for the synthesis of bile acids, steroid hormones and vitamin D.
Cholesterol is insoluble in water and is therefore bound to proteins in blood for
transport purposes, these proteins are called apoproteins or apolipoproteins.
These are water soluble proteins collectively called lipopoteins and can be
classified according to their density.
Chylomicrons- are the form in which lipids consumed in the diet are absorbed from the
gastrointestinal tract. Triglycerides are gradually removed by muscle and adipose tissue
under the influence of lipoprotein lipase. The chylomicron remnants are taken up by the
liver and reassembled into new lipoproteins.
Very low-density lipoproteins (VLDL) are constructed by the liver from chylomicron
structures and are composed mainly of triglycerides. VLDLs maintain a supply of
triglyceride to body tissues in a fasting state.
Low-density lipoproteins (LDL) are derived from VLDLs. As the triglyceride is removed by
body cells, the remaining cholesterol is concentrated within LDL for transfer to the
peripheral tissue. Sixty percent of total circulating cholesterol is contained in LDLs,
hence high levels of LDL are a risk for CHD
High-density lipoproteins (HDL) – are small dense particles derived from chylomicron
hydrolysis, and comprised of protein, cholesterol and phospholipid. They have an
important role transporting cholesterol back to the liver. Because they are associated
with cholesterol removal, high concentrations are beneficial in CHD
Normal reference ranges
• Total cholesterol < 5.0mmol/L
• LDL Cholesterol < 4.0mmol/L
• HDL Cholesterol > 1.15 mmol/L
Triglycerides
< 2.3mmol/L
Cholesterol lowering
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The influence of dietary cholesterol on blood cholesterol is relatively small
because most circulating cholesterol is of endogenous origin. Significant
effects of dietary cholesterol on cholesterol lowering are only achieved after
reduction of extreme levels of intake of cholesterol rich foods e.g. eggs, butter,
cheese, liver, offal, pate,
Both the total amounts of lipids in the blood and the amounts associated with
the various lipoprotein fractions, have prognostic significance.
Raised blood cholesterol is, along with smoking and hypertension, one of the
three principal risk factors for cardiovascular disease. Reducing cholesterol
levels reduces cardiovascular risk.
Guidelines for reducing
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Raised total and LDL levelreduce saturated fat intake
Substitute with mononsaturated fats
Increase fibre intake, fruits, vegs, oate and pulses
reduce intake of dietary cholesterol
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Raised triglyceridesIf obese, loose wt
Reduce saturated fat and total fat
consume more complex carbs, and less refined carbs
reduce alcohol intake
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Raised LDL and triglyceridesIf obese, loose weight
reduce saturated fat and total fat
substitute with monounsaturates appropriate to maintian body
weight
consume complex carbs, reduce refined carbs
Reduce dietary cholesterol
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low HDL levels
Encourage regular aerobic cholesterol
Moderate intake can be encouraged (1-2 units per day)
Ensure total fats not too low
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Drug treatment
• Lipid lowering drugs are being increasingly used in the treatment of
hyperlipideamia particularly in patients with established CHD or those
at risk of multiple risk factors. They are used to treat severe
hyperlipidaemia in adequately controlled by dietary measures
• Commonly used are statins lipostat/lipitor
References
• 1)Department of Health (1991). Dietary Reference Values for
Food Energy and Nutrients for the United Kingdom. HMSO,
London.
2) Chapter 2. Gissler and Power
• 3)MJ Gibney, HH Voster, KJ Kok (2002). Introduction to
Human Nutriton. Blackwell Publishing.