Energy Balance & Metabolism

Energy Balance &

MetabolismBy the end of this unit you should be able to:-•Define energy balance•Describe regulation of food intake•Identify metabolic rate and factors affecting it•Describe thermoregulation.•List disorders of body temperature•List disorders of energy balance

MetabolismThe tem metabolism means change. It refers to

all chemical and energy transformations that occur in the body. Energy intake and output are balanced under steady state conditions.

Oxidation of food stuff in the body is complex , slow and stepwise. Oxidative reactions are termed catabolism.

Energy released from food oxidation appears as heat

All chemical reactions that involve take up of energy are termed anabolism.

Under normal conditions 75-80% 0f the chemical energy released from food oxidation appears as heat

The remaining 20-25% of the released chemical energy is stored in the form of ATP.

ATP is called the energy currency of the cell because the transfer of energy from food stuff to functional systems of the cell can be done only through ATP.

P-creatine can not transfer energy between food and functional cellular elements, but it can transfer energy interchangeably with ATP

Overall energy utilization by cells

GlucoseGlycogen

Lactic acid[ Pyruvic acid

Acetyl CoA

Deaminated amino acids

Other substrates

CO2+ H2O

ATP

AMP

Phosphocreatine

Creatine + Po4

Energy for :-1. Synthesis & growth2. Muscular contraction3. Glandular secretion4. Nerve conduction5. Active absorption….etc.

Energy balanceIt is the balance between energy input (.i.e. intake)

and energy output (.i.e. expenditure).In healthy adults , energy intake equals energy

output , accordingly body weight remains constant.Variations in energy balance over long periods

causes:-◦ Positive energy balance (Energy intake > energy output) .e.g. During child growth , Overweight & obesity

◦ Negative energy balance (Energy intake > energy output) .e.g. inanition , anorexia & cachexia

Malnutrition is a disease characterized by deficiency of one or more of the essential constituents of food.

Estimation of energy intake

Energy intake = The ingested amount of CHO, fat and protein in gm x its physiological heat value.

Physiological heat value is the amount of heat liberated from oxidation of food stuff inside the body.

Physical heat is the amount of heat liberated from oxidation of food stuff outside the body.

Bomb Calorimeter

Physical heat

Physiological heat

CHO 4.1 C/gm 4 C /gm

Protein 5.3 C/gm 4 C /gm

fat 9.3 C/gm 9C/gm

calorie; is the unit used to express energy amount. It is the amount of heat required to raise the temperature of one gm of water from 15 -16 °CCalorie or Kilocalorie =1000 calorie

Respiratory quotientIt is the ratio of CO₂ production to O ₂

utilization in the same time. (RQ=Volume of CO₂ produced/ Volume of O ₂ consumed) .

Importance of respiratory quotient1. Indicates the type of food stuff oxidized in

the time of estimation.o RQ 1 →CHOo RQ 0.7 → fat oxidationo RQ 0.8 → protein oxidation

Immediately after meals ,RQ is 1 After 8-10 hours, RQ is 0.7 In untreated diabetes mellitus, RQ is 0.7

RQ between 0.7- 1 represent the approximate ratio of CHO to fat metabolism.

2. Indicates the transformation of one food substance into another .e.g. when CHO is transformed to fat , RQ increase above 1. Why?

3. Indicates the main food stuff consumed by different organs .e.g. 0.97-0.99 .

o RQ for brain =1o RQ for skeletal muscle =0.85o RQ for the heart =0.7

4. Used in determination of energy output ( metabolic rate by the indirect calorimetric method).

5. Excess RQ = CO₂ produced during exercise & recovery- CO₂ produced during equal period of rest / O ₂ consumed during exercise &recovery - O ₂ consumed during equal period of rest.

RQ above 1 RQ below 1

1. Transformation of CHO into fat

2. Hyperventilation a.Muscular exercise b.Fever c.Metabolic acidosis

1. During recovery from muscular exercise due to increased O2 consumption to oxidize Lactic acid.

2. Metabolic alkalosis.3. Diabetes mellitus

Factors affecting RQ

Regulation of food intake & Energy Storage

Stability of total body mass and composition requires that energy intake match energy expenditure.

Increased energy intake is stored as fatDecreased energy intake causes loss of

total body fatThere are control systems that regulate

food intake, energy expenditure and energy stores.

Nervous regulation of food intake

A. Hypothalamic centers (appestat); The hypothalamus contains:-

1. Feeding center (lateral hypothalamic nuclei).2. Satiety center (ventromedial hypothalamic nuclei).3. Other nuclei of hypothalamus play a major role

in control of food intake .e.g. Destruction of paraventricular nucleus → excessive

eating. Destruction of dorsomedial nucleus →decreased

eating. Arcuate nucleus is the site of convergence of multiple

hormones from GIT & adipose tissue

The hypothalamic nuclei also influence the secretion of several hormones that are important in regulation of energy balance and control like thyroid hormone & glucocorticoid.

The hypothalamus receives input from:-1. Neural signals from the GIT.2. Chemical signals from gut hormones.3. Chemical signals from the blood.4. Cerebral cortex .e.g. sight , smell & taste

B. Other areas1. Amygdala & prefrontal cortex.

◦ They are parts of the limbic system◦ Lesion in these areas produces omniphagia.

2. Brain stem centers which control mechanical aspects of feeding like salivation, swallowing, chewing.

HYPOTHALAMUS &

ENERGY BALANCEFine tuning of energy input &output is the

function of hypothalamus

Regulation of food intakeA. Short term regulation( regulate

quantity of food intake):-1. Filling of GIT inhibit feeding(stretch

inhibitory signals are transmitted via the vagi to suppress feeding).

2. GIT hormones : CCK → inhibit feeding center directly. GLP & insulin → inhibit feeding. Ghrelin →increase feeding. (It is released from

stomach & reaches peak level before time of meals)

3) Oral factors as chewing, salivation, swallowing and tasting meter the food as it passes through the mouth and after a certain amount of food has passed , it inhibit the feeding center. The inhibition caused by this metering system is short( 20- 40 min)

4) Psychological factors ◦ cultural & environmental◦ Past experience to smell & taste of

food.

B. Long term regulation of food intake (maintain constant nutrient stores):-

1. Blood concentration of glucose, fat & amino acids (↓level of these substrates stimulate feeding).

2. Body temperature (Interaction of thermo-regulatory center & food regulating center).

3. Feedback signals from adipose tissue. leptin hormone is released from adipose

tissue & act on leptin receptors in the hypothalamus to induce:-

1. ↓ production of appetite stimulators like NPY.2. ↑ production of appetite depressors like CRH.3. ↑ sympathetic activity.4. ↓ insulin release from b cells of pancreas

Metabolic rate In a healthy adult person energy intake

is equal to energy output ( expenditure).Not all energy in food is transferred to

ATP, instead a large portion of this energy becomes heat. Then still more energy becomes heat as it is transferred from ATP to the functional systems of the cell.

Metabolic rate; is the rate of heat liberation by the body.

Overall energy utilization by cells

GlucoseGlycogen

Lactic acid[ Pyruvic acid

Acetyl CoA

Delaminated amino acids

Other substrates

CO2+ H2O

ATP

AMP

Phosphocreatine

Creatine + Po4

1. Synthesis & growth2. Muscular contraction3. Nerve conduction4. Active transport.

Measurement of metabolic rate of the body

1. Direct calorimetry; It determines the total quantity of heat

liberated from the body in a large specially constructed calorimeter (respiration calorimeter) in a given time.

Direct calorimetry is expensive and difficult to perform and is used for research purpose.

Respiration Calorimeter

2. Indirect calorimetry; In this method , heat liberated by the subject

is calculated by:-1. Measurement of oxygen consumption in

liters in a period of time using :-◦ Modified Benedict apparatus (closed

circuit method).◦ Douglas bag

(open circuit method).2. Multiplying the volume of the utilized

oxygen by the energy equivalent of oxygen. Energy equivalent of O₂ is the amount of heat

liberated by consumption of 1 L O₂ . Metabolic rate (Kcal /unit time)=

O₂ consumption L/unit time x 4.82 Kcal

Open circuitDouglas bag

Closed circuitModified Benedict

apparatus

Factors affecting metabolic rate

1. Muscular exercise ( Maximal labor increase metabolic rate 3.5 times that of inactivity).

2. Food ingestion (specific dynamic action of food).

It is the obligatory increase in energy output during assimilation of food(Digestion , absorption & processing of food), thus reducing the amount of calories obtained from food. It equals 30% for protein, 6 % for CHO & 4% for fat and 10% for mixed diet.

It start 1 h after eating, reach a peak after 4-5 h &last for 12 h after eating . Its cause is uncertain but could be due to:-1. Increased sympathetic activity after food ingestion.2. SDA of proteins is due to their

deamination by the liver and their stimulatory effect on cellular metabolism.

3. SDA of carbohydrate is due to glycogenesis by the liver

4. SDA of fat is due to their stimulatory effect of on cellular metabolism.

3. Environmental temperature ( high or low environmental temperature increase metabolic rate).

4. Body weight , height , surface area. Large animals have higher metabolic rate than

small animals but when calculated /Kg or m² surface area, small animals have higher MR

5. Sex , age, growth, pregnancy ,lactation 6. Emotional excitement (due to ↑ catecholamines)7. Hormones (thyroid hormone, testosterone , growth

hormone and catecholamines).8. Fevers ↑body temperature 1°C→↑ metabolic rate 10-

14%9. Sleep (metabolic rate becomes minimal)10. Starvation ( ↓ metabolic rate up to 40%)

Energy output can be partitioned into separate measurable components which include:-1. Energy used for performing

essential metabolic functions of the body ( basal metabolic rate, 50-70% of energy output).

2. Energy used for performing various physical activities (25 %).

3. Digestion, absorption and processing of food (8% ).

4. Maintenance of body temperature.

Energy used for doing basal

metabolic processes in cells.

•Basal Metabolic rate (75%)

Energy used for muscle contraction

during physical activity

• Heat of physical activity (25%)

Energy used for digesting ,

absorbing and processing food

•SDA of food (8%)

Energy used for maintaining body

temperature•Heat for maintenance of body

temperature

Energy output

BMRPhysical activityfood intakeBody temp.

Basal metabolic rate is defined as;- The minimum amount of energy required

to exist . It accounts for 50-70% of daily energy expenditure in most sedentary individuals.

BMR is measured under the following conditions;

1. 12 hours after eating2. After a night restful sleep.3. No psychic or physical stress4. Air temperature 20-25°C.BMR is expressed in terms of calories/ hIn a 70 Kg man , it is estimated to be 65-70C/h

Skeletal muscles even under basal conditions account for 20-30 % of BMR. This explains ↓ BMR in:-1. Old age2. females

BMR is usually corrected for body surface area in m² obtained from body weight & height.

Significance of BMR; BMR investigates the metabolic rate in absence

of the major factors affecting it, so it is valuable in:-1. Judging the metabolic rate of an individual.2. Comparison of the metabolic rate of different

individuals BMR is expressed as % ↑ or↓ from expected normal

value.

Factors affecting BMRA-physiological factors

1. Age ; BMR shows decline with age 1C/10 y between age 20-60 y. Why?

◦ In newborn BMR is 1.5-2 times> BMR of adults. ◦ At age 20 y it reaches the adult value (40 C /m²/ h).

2. Sex; BMR in males is 7 % higher than females .Why?

3. Race ;BMR is lower in Africans & Asian people than European.

4. Physical habits ; BMR is higher in athletes than sedentary individuals.

2. Dietetic habits prolonged ingestion of protein increase

BMR about 10% more than BMR with mixed diet.

3. Climate BMR increase 10 % above normal in cold

countries. BMR decrease 10 % below normal in hot

countries. Why?4. Pregnancy , lactation ; Increase BMR

Why?5. Anxiety & tension ; Increase BMR. Why?

Factors affecting BMRB-Pathological factors

1. Disorders of thyroid gland (Hypothyr-oidism →↓BMR, hyperthyroidism →↑BMR).

2. Disorders of adrenal cortex (Cushing disease ↑BMR , Addison disease↓ BMR).

3. Hypoparathyroidism →↑BMR.4. Body temperature. (fever→↑BMR,

Hypothermia →↓BMR). 5. Prolonged starvation →↓BMR. Why?6. Diseases

◦ Diabetes insipidus →↑BMR.◦ Heart failure→ ↑BMR.◦ Leukemia→ ↑BMR◦ Shock & nephrotic syndrome→↓BMR.

Calculation of energy requirement

1. Determine the individual´s energy output and supply him with an equal amount of calories:-

A. basal energy requirements Male ~1600-1800 C/day Female ~ 1300-1500 C/day Average =1700 C/day

B. Energy required for work; obtained from tables ~ 1000 C/ day.

2. 10% of the calculated energy output is added to compensate for SDA of food.

Metabolism during muscular exercise

Muscle contraction requires much energy, the production of which needs sufficient O₂ supply & nutrients.

O₂ availability at the cellular level is the most important factor for an efficient muscular performance.

Sedentary individuals can increase their basal O₂ consumption 8-12 times while trained athletes can increase it 16-20 times.

Energy sources during exercise

1-phosphogen system1. The phosphogen system;

◦ It consists of the high energy phosphate compounds (ATP& P~creatine). Each high energy phosphate bond stores 7.3 C

ATP→ADP+ energy ADP →AMP+ energy ADP+P~creatine → ATP+creatine◦ It is the immediate source of energy for

skeletal muscle contraction .◦ Its amount is small. It supplies energy to

the muscle for few seconds.

Energy sources during exercise2-Glycogen lactic acid system

Most of the energy required for resynthesis of ATP & Creatine pohospahate is derived from glucose or glycogen.

Stored glycogen in the muscle is split into glucose which then undergoes anaerobic glycolysis to form pyruvate & ATP.

In the presence of O₂, pyruvate is completely oxidized in the mitochondria to Co₂ & H₂o.

In absence of O₂, pyruvate is converted into lactate ( which diffuses to the blood) & ATP.

Lactic acid accumulation cause acidosis & fatigue.

Energy sources during exercise

2-Aerobic system

It is slower than the other two systems.Fat & CHO are oxidized in the mitochondria to

release large amount of energy. It allows muscle to contract for an unlimited time. Second windOn starting prolonged exercise, athletes

experience an uncomfortable sense of dyspnea due to lactic acid accumulation. Why?

After a variable period, dyspnea disappear and exercise continues comfortably. Why?

This stage is reached rapidly by athletes.

Oxygen DebtDuring muscular exercise , the body tries to

supply sufficient O₂ to the exercising muscles.In most cases, particularly during severe

exercise, O₂ supply is not sufficient for aerobic resynthesis of the energy stores.

Accordingly anaerobic breakdown of glucose occurs with lactic acid accumulation in the muscle leading to fatigue.

The body is now in a state of O₂ debt and it has to supply the O₂ which it was unable to supply during the period of exercise. Such O₂ debt will be paid in the recovery period.

O₂ debt occurs during exercise and is paid during recovery. i.e. during recovery , O₂ consumption by the muscle remains increased until O₂ debt is paid back.

O₂ debt = O₂ consumption during recovery period - O₂ consumption during an equal period of rest.

O₂ consumption during recovery periods starts from the end of exercise till basal O₂ consumption is reached.

O₂ debt is used for:- Oxidation of lactic acid Replace O₂ derived from myoglobin.