Nutrition and Health: Energy

  • Created by: kjaneway1
  • Created on: 04-04-18 09:28


·         the capacity of physical system to perform work

·         in the biological world energy exists in several forms

o   chemical

o   mechanical

o   light

o   Electrical

o   Heat

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Units of energy

·         calorie – 1kcal defined as the amount of heat required to rise in temperature of 1 kg of water from 14.5 to 15.5° at atmospheric pressure

·         food energy is measured in kilocalories

·         joule – energy used to move the mass of 1 kg through 1 m by the force of 1 N.

·         SI unit equals joule

·         in nutrition use kilojoules megajoules

·         1 Kcal = 4.18 kj

·         1 KJ = 0.24 Kcal

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sources of energy

·         lipids

·         carbohydrates- made via photosynthesis

·         proteins

·         alcohol

·         energy content of food products is measured using a bomb colorimeter

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Gross energy

·         the amount of energy in food products

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Digestable energy

·         the amount of energy use is available for digestion

·         DE equals GE – faeces energy

·         a bomb colorimeter is used to determine DE

·         GE food intake – GE of faeces output

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Metabolisable energy

·         total food energy available to the body

·         ME accounts for energy losses faeces and urine

·         ME equals GE – (E lost in faeces and urine) or ME equals DE – (E lost in urine)

·         measured by bomb colorimeter on food faeces and urine

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Atwater factors (1902)

·         determined ME separately for carbohydrate, fat, protein and alcohol.

·         Used to predict wholefood ME content as an alternative to burning food, stall in urine.


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atwater factors - inaccuracy

- the amount of energy absorbed by human body is not constant 

- foods with high percentage of water - dehydration can impact energy per weight unit 

- fibre content 

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atwatet factors - values

- the complete oxidation of fat in a bomb calorimeter released 9.4 kcal (39.5 kj) 

- complete oxidation of carbohydrate relases 4.1 kcal 

- complete oxidation of protein relesases 5.6 kcal 

- correction for digestive loss and for urea synthsis 

mean rounded values 

fat = 9kcal 

carbohydate = 4 kcal 

protein =4 kcal 

alcohol = 7 kcal 

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energy density

- is the amount of energy in the partical weight or volume of food and is generally expressed as the energy (kcal) per gram of food (kcal/g) or per ml (kcal/ml)

- so lower energy density lower energy content per g or ml of food

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energy -why do we need it?

- 80- 85% is required for maintenance of the body's functions (60- 75%) (heart beating; brain and nervous system; biochemical reactions; muscle contraction and function) and diet induced thermogenesis (10%)

- voluntary work about 15-35% of the average persons energy expenditure 

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transfering food energy to cellular energy

- chemical energy - energy in food (carbohydrates, fat, protein) 

-cellular energy - ATP - sourse of energy for cells 

- ATP is a form of energy cells can use; fundemental energy molecule used as a sourse of energy for verious biochemical processes 

- NADPH - nicotinamine adenine dinucleotide phosphate is used mainly in reactions involving biosynthis 

- neither ATP nor NADPH can be stored 

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The ADP - ATP cycle

-  when extracting energy from nutrients, the formation of ATP from ADP and Phosphate captures energy 

- Breaking a phosphate bond in ATP to ADP and Phosphate releases energy for biosynthis and work 

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Energy exteraction from food

stage 1 - digestion, absorption and transport 

stage 2 - breakdown of molecules 

stage 3- complete oxidation (ATP production) 

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catabolic reactions

glycogen - glucose - co2 + h20 produces energy on the breakdown of glucose to co2 and h2o 

triglyceride - glycerol and fatty acids - co2 and h2o energy is produced on the breakdown of glycerol and fatty acids

protein - amino acids - co2 and h2o amd urea energy is produced on the breakdown of amino acids  

catabolism - reactions that break doen compounds into small units 

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anabolic reactions

glucose + glucose need energy to form glycogen 

glycerol and fatty acids need energy to for a triglyceride 

amino acid + amino acid need energy to form protein 

anabolism - reactions that build complex molecues from smaller ones 

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how much energy do we need?

- The level of energy intake that will balance energy expenditure when the individual has a body size and composition and a level of physical activity consistent with long-term good health; and that will allow for the maintenance of economically necessary and socially desirable physical activity. In children and pregnant or lactating women, the energy requirement includes the energy needs associated with the deposition of tissues or secretion of milk at rates consistent with good health

- the estimated averagw requirement EAR values for energy is a standard deviation curve 

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components of energy expenditure

- physical activity - 20 - 35% 

- BMR ( basal metadolic rate) 60 - 75% 

- TEF ( thermic effect of food) 5 - 10% 

 these percentage changes due to how active a person is. 

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Basal metabolic rate (BMR)

- is the amount of energy used by the body at rest but not asleep under controlled condition of thernal neutrality in the post absortive state ( 12 hours after last meal) 

- the energy requirement for basic physiological function eg respiratipn, heartbeat, body temperature and muscle tone 

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basal energy expenditure

kidneys - 8% 

sceletal muscle 22%

adipose tissue - 4%

liver - 21% 

brain 20% 

heart 9% 

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BMR measurements conditions

- completely rested , lying down and fully awake 

- fasted for 10-12 hours before the measurements 

- free from stress 

- about 12 -18 hours after significant physical activity #

- the enviroment should be thermo-neutral 22-26c 

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factors influencing BMR

- body composition 

- body size 

- age 

- gender

- physiological factors 

- enviromental themperature 

- stress 

- physical activity 

- caffeine, smoking 

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Resting metabolic rate (RMR)

- the amount of energy needed to maintain basic physiological function 

- measured under less strictly controled conditions than BMR 

- measured in postprandial state 

- about 10% higher than BMR 

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thermoc effect of food (TEF)

- also known as diet induced thermogenesis 

- the increase in metabolic rate after a meal 

- the amount of energy which is needed to digest absorb and metabolise comsumed food 

- the major part is the energy cost of synthesizing reserves of glycogen and triglycerides and protein synthsis 

- for mixed diet 5 -10% 

- the highest for protein and the lowest for fat 

- the cosr of synthesizing glycogen from glucose is about 5% of the ingested energy 

- while the cost of synthesizinf triglycerides from glucose is about 20% of the ingested energy 

- fat 2-3%, carbohydrate 6-8% and protein 20-30%

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Energy expenditure of physical activity

- the amount of energy needed for physical movement and work 

- associated with all the activities that are part of daily living 

- includes exercise and lifestyle physical activity 

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