Energy of human body



Energy is a prime requisite  for body function and growth. When a child's  intake  of  food  falls below a standard  reference,  growth slows, and of low levels  of intake persist, adult stature will be reduced. Similarly, if adults fail  to meet their food requirements  they lose  weight.  This may leaf to reduced  ability  to work,  to resist  infection, and to enjoy  the normal satisfaction  of life. This underlines  the need for an adequate  intake  of food which is the source of all energy.  

Measurement of energy  

The  energy  value  of   foods has long been expressed  in terms of the kilo_ calorie  (kcal). The " kilo_ calorie " is generally  expressed as " Calorie"  written with  capital  "C". This  has been replaced by the " joule" which has been  accepted  internationally . The use of " kilocalorie" to  measure  energy still continues. The conversion factors  

1 kcal = 4. 184 kJ ( kilo Joule)  1kJ= 0.239 kcal

1000 kcal = 4184 kJ                 1000 kJ= 239 kcal

1000kcal = 4.184 MJ              1MJ =  239 kcal 

(MJ means millions Joules) 

 The dietary  sources  of energy  are: proteins,  fat carbohydrates  and dietary  fibre. They supply energy at the following  rates :

Proteins             4 kcal/ g (or 17 kJ)

Fat                      9 kcal/ g (or 37 kJ)

Carbohydrates   4 kcal/ g (or 17 kJ) 

Dietary  fibre      2 kcal/ g (or 8 kJ) 

If  an adequate  energy  supply is not provided,  some protein  will be burnt to provide energy. This is considered wasteful, as they  are no longer available  for their essential  body building  function. 

Reference man and women 

 Energy intake recommendations are formulated  for a " reference  man" and a  " reference  woman" whose  profiles  are  described, and then  necessary   adjustments  are made for subjects who deviate  from the  standard reference. This  procedure  was first  devised by the FAO Committee on calorie  requirements in 1950 and has been  in use ever since. 

 An  Indian reference  man is between  18  and 29 years of age and weights 60 kg., with a height of 1.73 metre and a BMI of 20.3. He is free from  disease  and physically  fit  for active work. On each working  day he is employed  for 8 hours in occupation  that  usually  involves  moderate  activity. While not at work he spends 8 hours in bed, 4 to 6 hours sitting  and moving around and 2 hours  in walking  and in active recreation  or household  duties. 

 An Indian  reference  women is between  18 and 29 years  of age, healthy and  weights 55 kg,  with a height of 1.61m and a BMI of 21.2. She is free  from  disease  and  physically  fit for active  work. She may be engaged  for 8 hours in bed, she spends 4 to 6 hours sitting  or moving  around  only through  light  activity  and 2 hours in walking  or in active recreation  or in household duties. 

Energy requirements 

  The energy  requirement of an individual  might  be defined  as that  level  of  energy  intake  in relation  to  expenditure  which is least likely  to result in obesity  or heart disease or  which is most likely  to prolong  active life. The two  standard  deviation  value is not added to the average  requirement. This  is because  the energy  intake and expenditure  of an individual  are finely  balanced, and any surplus  energy  consumption  will be stored as  fat and a continuous  excess of intake will lead to obesity.  Adults  and even growing  children  are known to adapt either intake to suit their output, or output to suit intake over a very wide range. We do not have a proper  understanding   of the lower limit of adaptation.  

Broadly, the total energy  requirement  of an individual  is made  up of three components; 

(a) Energy  required  for basal  metabolism.  This is about 1 kcal/ hour for every kg  of  body weight  for an adult; 

(b) Energy  required  for daily  activities  such as  walking , sitting, standing , dressing  and undressing, climbing  stairs, etc; 

(c) Energy expenditure  for  occupational  work. This is further  classified  as light  work ( an office  clerk) moderate  work  and heavy work ( manual physical  labour). 

    The first  components is  nearly  the same  for all individuals . It is the latter  two components that vary  depending  upon  the type of activities   

  Procedures  for calculating  total energy  expenditure   are given in the WHI expert  Committee report on energy  and protein requirements.  

Factors  affecting energy requirement 

 Energy  requirements  vary from one  person to another  depending  upon  interrelated  variables  acting  in a complex  way, such as age, sex, working  condition, body composition,  physical  activity,  physiological  state etc. All these factors lead   to differences   in food intake.  

Energy  requirements  have  been  laid  down by various expert groups of FAO and WHO. It  has become customary  for countries to lay down  their own standards. Thus  there are  British  standards,  American  standards in Canadian  standards, etc. The  standards  in  India  are those  recommended  by the Indian Council  of Medical Research . These standards  are revised from time to time in the light of newer  knowledge.  

Vulnerable  groups 

(a) Pregnancy  and lactating   mothers: The energy  requirements   of women  are increased  by pregnancy  ( +350 kcal daily throughout  pregnancy) and  lactation   ( +600 kcal daily during the first  6 months, and + 520 kcals  daily during  the next  6 months) over  and  above their normal requirements.  This is to provide for their extra energy  needs associated  with the deposition  of tissues  or the secretion of milk  at rates consistent  with  good health.  

(b) Children: Because  of their  rapid growth rate, young  children  require  proportionately  more energy  for each kilogram  of body weight than  adults .

A problem  that arises is in recommending intakes in communities where a large number of children are underweight  because  of malnutrition. In order  to provide for " catch _ up growth " during  childhood,  intake should  be based on age rather than weight where practical. The ICMR standards  are based on age, and not on body  weight  ( except during  the first year of life). 

 Children  above  the  age of 13 years need as much energy as adults. This is  because  they show a good deal of physical  activity,  almost equal to hard work by adults. This is also the age when puberty  sets in and  there is a   spurt in growth and an increase  in  metabolic  rate.  This fact should be borne in  mind when planning  dietaries for children.  

(c) Adults: The energy requirements  decrease with age because  of a fall in BMR  and a decrease  in physical  activity  in most  persons. In general,  there is a 2 per cent  decline  of resting  metabolism  for  each  decade for adults. The FAO/ WHO Committee  suggested  that  after the age of 40 years, requirements  should be reduced  by 5 per cent  per each decade until  the age of 60, and by 10 per cent each decade thereafter.  

(a) Rounded  off to the nearest  10 kcal/ day. 

(b) GWG _ Gestational  weight gain. Energy  need in pregnancy  should  be adjusted for actual bodyweight,  observed  weight gain and activity  pattern  for the population. 

(c) WG_  Gestational  weight  gain  remaining  after delivery.  

Nutritional Individuality 

 In normal individuals  at all ages  and of both sexes, there is  a large variation  in energy intake but the reasons for this wide range of  nutri requirements are not known. The  concept of nutritional  individuality needs to be stressed, and its neglect may result in the overfeeding  of some  whose needs happen to be less than the " average  standard  requirement". 

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