Atwater factors indicate the average amount of energy yielded by
one gram of ingested carbohydrate, fat or protein; they are used in the
calculation of the metabolizable energy content of foods, for instance in food
composition tables and in infant formulas. Atwater (as well as Durnin and
Southgate after him) derived them from the heat of combustion, corrected for
energy losses in the form of unabsorbed nutrients in feces and urine of adults.
The question was raised whether the same factors were also applicable to
infants. The answer to this question does not affect energy requirements per
se but becomes important in a discussion of recommended dietary intakes.
Several factors may influence the metabolizable energy derived from food: (1)
the chemical form of the macronutrient in the food, (2) the coefficient of
digestibility; (3) the extent to which the nutrients are not completely
oxidized, but stored in the body; (4) gut maturation and (5) age. In growing
infants nitrogen retention will be higher. Preterm infants absorb less fat than
term infants, and fat is generally less well absorbed by newborn infants than by
older infants. Fat digestibility is also highly dependent upon the fat source
and its processing, e.g. butterfat is poorly absorbed, whereas a mixture of
vegetable oils is absorbed nearly to the same extent as human milk. In a study
of 10 breast-fed infants fed unpasteurized milk, Southgate found that
metabolizable energy averaged 92%. Application of the Atwater factors to human
milk components indicated 96% metabolizable energy. Using Atwater factors in
normal infants, therefore, does not seem to entail great errors. Application of
the Atwater factors in preterm or sick infants may overestimate energy
In young infants the energy content of human milk is of particular
importance. Since it is very variable throughout days and feeds and there is no
generally agreed upon, standard method for obtaining representative milk samples
and for estimating their energy density, published figures vary considerably.
Butte et al, using different methods, obtained values between 0.65 and
0.67, whereas values from Sweden (0.72) and a WHO study in Hungary are
considerably higher (Waterlow). In the first two figures of her paper, Butte
used energy intakes as reported. Dewey pointed out that differences in fat
secretion in breast milk between groups of women had been observed, even when
exactly the same methods were used. Maternal body fat can affect milk fat
(Prentice), as can fat intake in lean women (Dewey). Since pasteurization alters
the fat, it is important to note whether pasteurized or non pasteurized milk is
used. In the end, the prevailing opinion was that Dewey and Butte had made the
most rigorous assessments and that their values should therefore be relied upon
Several participants were intrigued by the low level of the first
two data points in the line representing energy requirements derived from TEE
and growth in Butte's figures 7 and 8. Most likely this is an artifact due to an
underestimate of the cost of growth in these first two time periods.
Should recommendations be the same or different for breast- and
bottle-fed infants? Reeds argued that requirements and intakes should not be
confused. Requirements are to be seen as a function of the organism and not of
the diet, whereas recommended dietary allowances are a function of the diet and
the degree to which it meets requirements. Dewey pointed out that in practice
the picture was less clear and the feeding mode seemed to affect physiology.
Energy expenditure is lower in breast-fed infants or, in other words,
formula-fed infants appear to require more energy than breast-fed ones. These
differences are most marked between 3 and 6 months of age; then they gradually
disappear, probably as a consequence of the phasing out of pure breast-feeding.
Butte tried to derive energy requirements from data of a mixed group of infants,
50% breast- and 50% formula-fed. Dewey advocated separate recommendations for
the two feeding groups in order to avoid the :impression that breast-fed infants
do not get enough energy and ought to be supplemented or the risk that formula
fed infants will not get enough energy to cover their needs. Giving a wide range
of requirements does not appear to be a satisfactory solution either.
Butte et al tried to determine how much of a difference in
diet-induced thermogenesis (DIT) there was between breast- and formula-fed
infants. During the first 4h after the meal, DIT appeared slightly lower in
breast-fed infants, but the difference was not statistically significant.
Waterlow queried the validity of 42% for the energetic efficiency
of protein synthesis (Table 5, footnote d), and suggested that a figure of 75%
would be more in accordance with the evidence.
Do infants growing up in the more stressful environment of
developing countries or urban slums have the same or higher energy requirements
than infants in industrialized countries? The little information that exists on
this issue shows smaller differences than expected. Total energy expenditure
(TEE), expressed as kcal/kg, was for instance very similar in infants from The
Gambia and the UK (Prentice). Butte compared TEE of small groups (n = 20)
of 4-month-old infants from Mexico and Houston. In. Mexico it was 74 kcal/kg, in
Houston 64 and 73 kcal/kg for breast- and bottle-fed infants, respectively.
Several participants felt that more information was needed to decide the extent
to which frequent infections and desirable catch-up growth add to energy
requirements in poor