InTable1, each of the stages of gestation at which measurements were made are shown, together with the relevant data. The difference in body weight between week 11 and week 16 is 1.6 kg. Using the measured value for body fat of the mother (from densitometry and skinfold thicknesses), this is converted into the appropriate mass of fat, i.e.,0.43 kg with an energy equivalent of 4,730 kcal ( the energy require for the deposition of 1 kg of fat, not adipose tissue, being taken as approximately 11,000 kcal). The energy equivalents for the relevant increases in the mass of the fetus, placenta and uterus, with values for the energy needed to deposit the protein and fat in these tissues, are 325 and 55kcal. Thus the overall total for the energy needed for these processes is 5,110 kcal. Similar calculations for the other gestational periods are shown in the other rows of the table, with an overall total for the whole pregnancy of 66,850 kcal.
Table 1 also shows the measured energy intakes (E1) and expenditures (EE) the differences between these in kcal/day, and the calculated total this energy difference for each period(EI-EE multiplied by number of days in the period) with also the total for the complete pregnancy in the bottom row, i.e., 49,840 kcal. The overall totals in columns 7 and 11 ought to be approximately of minus 17,010 kcal, inTable 2 of + 32,212 kcal,inTable 3 of minus 47,588 kcal and in Table 4 of minus 105,372 kcal.Apart from Table 1 most of these disagreements are considerable and would markedly influence the calculation of the supposed energy need of prehnancy. These 4 tables show result which are more or less proportionately typical of the group of 22 women measured in Glasgow.
There are various possible reasons for these large discrepancies, of which the most likely are errors in a) the measurement and estimated energy cost of the increased body mass, and b) the energy intake and energy expenditure measurements.
From a long experience of all these techniques in this laboratory, it seems unlikely that the measurements of the body mass, by repeated estimations using both body density and skinfold thicknesses, will have been in error by more than one to two kg at the most. The total amount of fat gained is, usually, of the order of 4-5kg so an error of 1-2kg represents+- 20-40% - a large and unlikely error indeed!
There seems little scope for error in the energy cost of laying down the fat mass.The energy value of fat is 9 kcal(38 kg)per g , depositing this in the body has been taken to be 11,000 kcal(46.0 MJ)/kg (which includes the energy cintent of the fat). It is almost impossible that than an error of more than about+-1,000kcal!(4.2MJ)can 0ccur here.
We are therefore almost compelled to assume that by far the largest source of error is either in the measurement of food intake or of energy expenditure.Food intake seems an improbable source of consistent or large bias in assessing energy intake, as has been briefly discussed above. Energy expenditure, on the other hand, because of the notorious difficulty in applying the rather primitive methodology in applying conditions, is the most likely source of a consistent and important error. For example, if energy expenditure were over or under extimated by as little as 200 kcal(840 KJ) per day – which is about a 10% error and would be acceptable normally, as a tolerable error- this is equivalent to 56,000 kcal (234 MJ) over the 280 days of pregnancy.
The conclusion would appear to be that energy expenditure measurements, as they are at present caeeied out, are incapable of being of much assistance to us in assessing energy balance in free-living conditions. This os not wide-ranging uses; they give information on the energy cost of single activities which may be of much help in determining the stress of work situations; they are of interest and perhaps of medical importance in sport; they are useful in giving advice about the prevention or treatment of obesity; they can be utilized in measuring mechanical efficency of different body movements, or of differing designs of work tools. Valid assessment of physical activity can give us critically important information in marginally nourished or flagrantly malnourished populations. However, the errors are of such magnitude when these measurements are applied to the assessment of total energy expenditure over large parts of a day, or as the average over several complete days, that the interpreyation of such information must have very serious limitation.
There are several reasons for the potential errors, one of the largest being uncertainty about the validity of the measured value of energy expenditure in any activity when this is applied in real life situations where the actual energy expended could vary considerably. For example, the average of even several measurements of sitting or walking may not represent the average energy expended in such highly variable activities.
One the other hand, the estimate of total energy expenditure is hardly ever really essental in the context of energy balance. A general indication of the level of physical activity and of its fluctuations in normal everyday life is often useful in situations where energy balance is being investigated; e.g. in rural populations whose nutritional status may be marginally adequate. However, reasonably satisfactory information can usually be obtained by a few measurements of the energy expended in some specific activities coupled with an approximate breakdown of the duration of the varying activities for an average day. There is no need- and it is probable invalid in any case – to attempt an accurate estimation of the average total 24 – hour energy expenditure.
