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4.1. Estimation of protein needs
4.2. Influence of protein intake on milk composition
4.3. Studies of whole-body protein turnover
4.4. Effects of protein intake on milk production
4.5. Summary of protein needs during lactation
The factorial method is
used most often to estimate the protein costs of lactation, i.e.,
the amount of protein 'lost' through milk is added to baseline
requirements of non-pregnant women. The most recent Recommended
Dietary Allowance (National Research Council, 1989) for protein
during lactation promotes the addition of 15 g/protein per day to
baseline needs for the first 6 months of full lactation and 12
g/d thereafter. These estimates are based on milk volumes of 750
mg/d for the first 6 months and 600 mg/d thereafter; the
secretion of 1.1 g of protein/de throughout lactation, an
efficiency of 0.7 in the utilization of dietary protein for milk
protein production; and a factor of 0.3 to account for
interindividual variability.
There are factors in this calculation which either are not correct or have not been measured in humans. While milk volume is fairly constant at approximately 750 mL for the first 4 to 6 months of full lactation among populations whose milk lipid concentrations are in the range of 3.5 g/dL, higher volumes may be produced by populations whose milk lipid concentrations are closer to 2.5 g/dL. The higher volumes are expected, if infants are to meet energy requirements and maintain expected growth trajectories in early infancy. Protein concentrations in human milk are not constant; they fall from approximately 1.2 g/dL during the first month to approximately 0.8 to 0.9 g/dL by the third or fourth month.
Furthermore, the efficiency of nitrogen utilization by lactating women may be significantly below 0.7 at levels of protein intake required to maintain N balance. MOTIL et al. (1989a; 1990b) measured nitrogen balances in lactating and non-lactating women at intakes of 1.0 g or 1.5 g protein/kg body weight. The apparent efficiency of nitrogen utilization among the non-lactating groups was 0.6, a value sufficiently close to the expected 0.7 given the small number of subjects (n= 8) who were studied, the availability of balance data at only two levels of dietary protein, and the recruitment of two distinct groups for the studies at each level of protein intake. The apparent efficiency of nitrogen utilization among the lactating women (n= 24), however, was 0.25.
This level of efficiency is close to that reported by CALLOWAY (1974) for pregnant women. However, unlike the data obtained in pregnant women who were in definite positive nitrogen balance, mean nitrogen balances for lactating women were negative or very close to zero. It is, therefore, unlikely that the slope of the line relating nitrogen intake to nitrogen balance would have been dampened by levels of protein intake which were very high relative to requirements. Furthermore, urinary N losses were 7 to 8% greater among the lactating group than in control women at both levels of intake; surprisingly, therefore, 'losses' of nitrogen in milk were not compensated by decreased losses of urinary nitrogen. Similarly, at both levels of intake, fecal nitrogen losses also were greater in the lactating groups.
Higher
nitrogen losses in urine of lactating women were not explained by
energy intakes, since energy levels were 50% greater in the
lactating group than in the corresponding non-lactating controls.
Nonetheless, it is important to keep in mind that only two levels
of protein were studied, individual values were highly variable,
periods of study were relatively short, and the groups that were
studied were small. Furthermore, while there was no evidence that
the various experimental groups had not adapted to the respective
dietary protein levels during balance periods, the direction of
dietary changes from usual intakes to experimental levels acts as
a potential confounding variable and further complicates
interpretation of those results.
The influence of dietary
protein on milk composition is uncertain (GARZA and BUTTE, 1986;
Subcommittee on Nutrition During Lactation, 1991). Total protein
concentrations in milk, produced by populations expected to have
widely different protein intakes, do not differ. There are
problems, however, in the interpretation of these data, because
total nitrogen often was used as a measure of total protein. In
milk from well-nourished mothers, approximately 25% of the total
nitrogen is not bound to protein. Much of the non-protein
nitrogen is bound to urea. Unfortunately, it is not possible to
estimate protein nitrogen from measurements of total nitrogen,
because urea concentrations in milk may depend on serum urea
concentrations, which in turn depend on dietary protein and
energy intakes. It is, therefore, not unreasonable to expect that
the proportion of non-protein nitrogen in milk will be dependent
on diet.
The effects
of maternal nutrient status on milk immunological proteins have
received significant attention. Unfortunately, most studies have
either not documented maternal nutritional status adequately
and/or not measured total milk volume. Various studies have
indicated either no differences between well-nourished women and
those at risk of malnutrition or lower concentrations of selected
immunological factors among malnourished women (Subcommittee on
Nutrition During Lactation, 1991).
No significant differences
in most of the usual indices of protein turnover have been
reported between non-lactating women and women at various stages
of lactation MOTIL et al., 1989a). The possible exception
is the urinary excretion of 3-methylhistidine, a measure of
muscle protein breakdown (MOTIL et al., 1990b). Lactating
women who consumed approximately 1 g of protein/kg had
significantly lower levels of 3-methylhistidine excretion in
urine than did groups of non-lactating post partum and
nulliparous controls who consumed similar amounts of protein
under metabolic study conditions. This latter observation
suggests a compensatory response in muscle catabolism that may
promote more efficient use of absorbed protein during lactation.
Alternatively, the lowered level of 3-methylhistidine excretion
may have been a response to the negative nitrogen balances
observed in this group of lactating women. Unfortunately,
3-methylhistidine output in milk was not measured in those
studies.
The effects of inadequate
protein intakes on milk production have not been well documented
in humans. Results of protein supplementation field trials during
lactation (GOPALAN, 1958; EDOZIEN et al., 1976) suggest
that protein supplementation increases milk volume; however, the
small number of women who were studied and design features which
may have biased results in favor of increased volumes limit the
usefulness of those observations (Subcommittee on Nutrition
During Lactation, 1991). Nonetheless, MOTIL et al. (1989b)
demonstrated a similar finding in women when milk production was
expressed as g/kg/d over a relatively narrow range of dietary
nitrogen (225 mg/kg/d to 260 mg/kg/d) in controlled metabolic
studies. Various studies of swine and rats (MAHAN, 1977; NAISMITH
et al., 1982; SAMPSON and JANSEN, 1985) also indicate that
increased protein consumption or improvements in dietary protein
quality are associated with increased milk volumes.
Factorial methods have been
used to estimate the protein needs of lactating women; however,
the data available are insufficient to validate the utility of
this approach. Available data illustrate the need to document
efficiencies of protein utilization at varying levels of energy
intakes. These studies should focus on abroad range of outcomes
which include maternal protein status, milk volume, milk
composition, and the responsiveness of related physiological
variables to changing demands of distinct lactational stages.
Investigators have not examined the effect of expected energy
deficits during lactation on the efficiency of protein
utilization despite the anticipated use of stored fat to meet
energy needs.
Milk volume, milk composition and the efficiency of milk production are the major determinants of energy loss/expenditure during lactation. Changes in basal or resting metabolic rate during lactation are not well documented. Various reports indicate that BMR falls, increases, or remains stable compared with measurements in non-pregnant, non-lactating women (MOTIL et al., 1990a). The reported disparities may be due to methodological differences or variations in the nutritional status of subjects. For example, RMR is reported to fall below prepregnant levels in Gambian women at 10 to 25 weeks gestation, and the reduced RMR persists through lactation, if the usual diet is maintained (LAWRENCE et al., 1984). If the diet is supplemented, reductions in RMR appear to reverse. Thermic response to food also has been measured in lactating women and conflicting reports have been published. Thermic responses to food are reported to be decreased (ILLINGWORTH et al., 1986) or increased (MOTIL et al., 1990a) during lactation.
The energy cost of milk production has been estimated to be approximately 110 to 125% of the energy output in milk (THOMSON et al., 1970). This efficiency further was estimated indirectly from differences between total daily energy intakes and estimated energy expenditures of lactating and non-lactating women. MOTIL et al. (1990b) also estimated the efficiency at approximately 80% from measurements of the thermic response to food in lactating women. Both estimates, derived by distinct approaches, are in approximately the same range.
If a daily output of 750 mL of milk with a gross energy content of 0.67 kcal/mL is assumed, the energy cost of milk production is approximately 630 kcal/d at an efficiency of 80%. The conventional view is that energy costs of lactation are met by increments in dietary intake and the mobilization of energy stored during pregnancy. Well-nourished lactating women appear to lose 0.6 to 0.8 kg body weight per month during the first 4 months of lactation and lesser amounts beyond that period (BUTTE et al., 1984; Subcommittee on Nutrition During Lactation, 1991), but is is important to point out that weight changes are highly variable. If one assumes that all weight lost is adipose tissue and that the energy density of adipose tissue is 9.1 kcal/g, approximately 180 to 240 kcal/d are supplied from energy stores, and the remaining 390 to 450 kcal/d are supplied by the diet.
Tables
7-9, which summarize energy costs of pregnancy, suggest that
women with BMIs < 19.8 and whose weight gain is at the 85th
percentile are able to mobilize adipose tissue during lactation
for approximately 6 to 9 months, if energy deficits of 180 to 240
kcal/d are maintained. Women of similar BMI who experience
comparable deficits of energy, but who gained weight at the 15th
percentile, have sufficient fat stores for only 3 or 4 months.
The role of gestational weight gains in determining dietary
energy needs during lactation is evident from that comparison.
Dietary energy requirements
during lactation are determined by the volume and composition of
milk that is produced, the efficiency of energy utilization for
milk production, the amount of fat accumulated during pregnancy,
and the rate of fat mobilization during lactation. As was the
case for estimates of energy needs during pregnancy, energy needs
during lactation are expected to be influenced strongly by
gestational weight gains. Nonetheless, in comparison to
pregnancy, lactation is a much costlier process from an energy
perspective. Exclusion of the 35000 kcal presumably stored during
pregnancy for lactation sets the actual cost of pregnancy at
approximately 45000 kcal for a woman of moderate BMI and a
gestational weight gain at approximately the 50th percentile.
That amount of energy is equivalent to only about 2.5 months of
full lactation. If, however, one sets the cost of pregnancy at
80000 kcal, the cost of one pregnancy is equivalent to
approximately 4 months of lactation. Two years of lactation
characterized by 6 months of exclusive breastfeeding (with milk
volume of 750 mL/d) and 18 months of partial breastfeeding (with
milk volume of 600 mL/d) therefore are equivalent to
approximately five pregnancies assuming a 'conservative' cost of
80000 kcal per pregnancy.
Dietary protein and energy
needs, and therefore also protein/energy ratios, during pregnancy
and lactation are influenced significantly by gestational weight
gains. Furthermore, relationships between functional outcomes and
gestational weight gains are modulated by prepregnancy body mass
indices. Protein and energy needs appear to be substantially
higher than present recommendations under conditions where
gestational weight gains > 12.5 kg are acceptable; however, it
is not clear that higher weight gains are desirable in all
populations. Improved estimates of the composition of gestational
weight gains are needed for women representative of a
population's expected range of prepregnancy body mass indices and
gestational weight gains. Similarly, improved estimates of the
efficiency of protein and energy utilization during pregnancy and
lactation are crucial to reliable estimates of desirable
protein/energy ratios. Ideally, these data needs will be met by
studies which focus on functional outcomes relevant to both
mother and infant.
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