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Third stage
Medicine
Lec-1
د
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وائل
1/1/2014
NUTRITIONAL FACTORS IN DISEASE
NUTRITIONAL FACTORS IN DISEASE:
Obtaining adequate nutrition is a fundamental requirement for survival of every individual
and species. The politics of food provision for humans are complex, and constitute a
prominent factor in wars, natural disasters and the global economy. In recent decades,
economic success has been rewarded by plentiful nutrition unknown to previous
generations, which has led to a pandemic of obesity and its serious consequences for
health. Yet in many parts of the world, famine and under-nutrition still represent a huge
burden. Quality, as well as quantity, of food influences health. Inappropriate diets have
been linked with diseases such as coronary heart disease and cancer. Deficiencies of simple
vitamins or minerals lead to avoidable conditions such as anaemia due to iron deficiency or
blindness due to severe vitamin A deficiency
Physiology of nutrition:
Nutrients in the diet can be classified into:
1)macronutrients: which are eaten in relatively large amounts to provide fuel for energy,
and
2) micronutrients: e.g. vitamins and minerals, which do not contribute to energy balance
but are required in small amounts because they are not synthesized> in the body.
Energy balance:
The laws of thermodynamics dictate that energy balance is achieved when energy intake =
energy expenditure.
Energy intake is determined by the 'macronutrient' content of food. Carbohydrates, fat and
protein provide fuel for oxidation in the mitochondria to generate energy (as adenosine
triphosphate (ATP). The energy provided by each differs:
carbohydrates (4 kcal/g)
fat (9 kcal/g)
protein (4 kcal/g)
Energy expenditure has several components:
1)The basal metabolic rate (BMR) describes the obligatory energy expenditure required to
maintain metabolic functions in tissues and hence sustain life. It is most closely predicted
by fat-free mass (i.e. total body mass minus fat mass), which is lower in females and older
people Extra metabolic energy is consumed during growth, pregnancy and lactation, and
when febrile. Metabolic energy is also required for thermal regulation, and expenditure is
higher in cold or hot environments.
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2) digestion of food accounts for approximately 10% of total energy expenditure, with
protein requiring more energy than other macronutrients
3)muscular activity: which can vary considerably with occupation and lifestyle .Physical
activity levels are usually defined as multiples of BMR.
Regulation of energy balance is coordinated in the hypothalamus, which receives afferent
signals that indicate nutritional status in the short term (e.g. the stomach hormone ghrelin,
which falls immediately after eating and rises gradually thereafter, to suppress satiety and
signal that it is time for the next meal) and the long term (e.g. the adipose hormone leptin,
which increases with growing fat mass and stimulates satiety). The hypothalamus responds
with changes in many local neurotransmitters that alter activity in a number of pathways
which influence energy balance ,including hormones acting on the pituitary gland ,and
neural control circuits which connect with the cerebral cortex and autonomic nervous
system.
Responses to undernutrition:
In response to starvation, reproductive function is suppressed, BMR is reduced, and there
are profound psychological effects, including energy conservation through lethargy. These
adjustments can 'defend' body weight within certain limits. However, in the low-insulin
state of starvation , fuels are liberated from stores initially in glycogen (in liver and muscle),
then in triglyceride (lipolysis in adipose tissue, with excess free fatty acid supply to the liver
leading to ketosis) and finally in protein (proteolysis in muscle).
Responses to overnutrition:
In response to over-nutrition, BMR is increased, and extra energy is consumed in the work
of carrying increased fat stores, so that body weight is again 'defended' within certain
limits.
In the high-insulin state of over-nutrition, excess energy is invested in fatty acids and stored
as triglycerides; these are deposited principally in adipose tissue but they may also
accumulate in liver (non-alcoholic fatty liver disease) and skeletal muscle. In the absence of
hypothalamic function (e.g. in those with craniopharyngioma;) or in rare patients with
mutations in relevant genes (e.g. in leptin or melanocortin-4 receptors), loss of satiety
signals, together with loss of adaptive changes in energy expenditure, result in relentless
weight gain.
Energy-yielding nutrients (macronutrients):
Carbohydrates:
The 'available' carbohydrates (starches and sugars) are broken down to monosaccharides
before absorption from the gut and supply over half the energy in a normal, well-balanced
diet. No individual carbohydrate is an essential nutrient, as carbohydrates can be
synthesised de novo from glycerol or protein. However, if the available carbohydrate intake
is less than 100 g per day, increased lipolysis leads to ketosis.
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Dietary guidelines do not restrict the intake of intrinsic sugars in fruit and vegetables or the
sugars in milk. However, intake of non-milk extrinsic sugars (sucrose, maltose, fructose),
which increase the risk of dental caries and diabetes mellitus, should be limited.
Some starches are digested promptly by salivary and then pancreatic amylase, producing
rapid delivery of glucose to the blood. Other starches are digested more slowly, either
because they are protected in the structure of the food, because of their crystal structure,
or because the molecule is unbranched (amylose). These differences are the basis for the
'glycaemic index' of foods. This is the area under the curve of the rise in blood glucose
concentration in the 2 hours following ingestion of 50 g carbohydrate, expressed as a
percentage of the response to 50 g anhydrous glucose. There is emerging evidence linking
high glycaemic index foods with obesity and type 2 diabetes .
Dietary fibre:
is plant food that is not digested by human enzymes in the gastrointestinal tract. Most
dietary fibre is known as the 'non-starch polysaccharides' (NSP) ,A small percentage of
'resistant' dietary starch may also pass unchanged into the large intestine. Dietary fibre can
be broken down by the resident bacteria in the colon to produce short-chain fatty acids.
1. This is essential fuel for the enterocytes and contributes to bowel health. The extent
of flatus formed is dependent on the food source.
2. They increase water holding capacity of colonic content and bulk of feaces thus
preventing constipation
3. Reduces risk of colonic cancer
4. Prevent diverticulosis
5. Slow gastric emptying and induce satiety
6. Reduce bile acid absorption ,hence cholesterol level
Fat:
Fat has the highest energy density of the macronutrients (9 kcal/g) and excessive
consumption may be an insidious cause of obesity . fatty acids(FA)are either saturated or
unsaturated .
Unsaturated FA are either polyunsaturated or monounsaturated FA.
Monounsaturated FA may take cis or trans configuration,oleic acid is an example of cis-
monounsaturated FA and elaidic acid is an example of trans- monounsaturated FA.
there are three 'essential' polyunsaturated fatty acids are linoleic acid alpha-linolenic acid
and arachidonic acid which humans cannot synthesise de novo.
Fish oils are rich in ω3 PUFA (e.g. eicosapentaenoic and docosahexaenoic acid)which
promote the anti-inflammatory cascade of prostaglandin production and occur in the lipids
of the human brain and retina. They inhibit thrombosis by competitively antagonising
thromboxane A2 formation.
Substituting saturated fat (i.e. from animal sources: butter, ghee or lard) with PUFA in the
diet can lower the concentration of circulating low-density lipoprotein (LDL) cholesterol and
may help prevent coronary heart disease.
Intake of trans fatty acids (TFA) (isomers of the natural cis fatty acids) has increased in
world diet, primarily due to use of oils that have been partially hydrogenated in the food
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industry. It is recommended that TFAs are limited to < 2% of the dietary fat intake, as they
are associated with cardiovascular disease.
Cholesterol is also absorbed directly from food in chylomicrons and is an important
substrate for steroid and sterol synthesis, but not an important source of energy.
Proteins:
Protiens should constitute 17% of the total energy.
Proteins are made up of some 20 different amino acids, of which nine are 'essential' i.e.
they cannot be synthesised in humans but are required for synthesis of important proteins.
The remaining amino acids can be synthesised in the body by transamination, provided
there is a sufficient supply of amino groups.
Protiens derived from animal source are rich in essential amino acids,and of higher
biological value than vegetable protiens.
Micronutrients:
These are vital components of diet which does not contribute to energy balance,but are
required in small amounts because they are not synthesized in the body. They include
vitamins,inorganic metals and ions.