Milk Volume

The volume of milk produced varies over the duration of lactation from the first few weeks to 6 months and beyond but is remarkably predictable except during extreme malnutrition or severe dehydration. In periods of acute water deprivation, manifested in a healthy mother by an acute bout of vomiting and diarrhea, the volume of milk will diminish only after the maternal urine output has been significantly compromised (10% dehydration).

Malnutrition, however, is complex, and single-nutrient deficiencies are rare. Malnutrition does seem to have an effect on the total volume of milk produced. In the extreme, when famine occurs, the milk supply dwindles and ceases, with ultimate starvation of the infant. The classic study is the report of Smith on the effects of maternal undernutrition on the newborn infant in the Hunger Winter in Holland in 1944 to 1945. It was reported that the volume of milk was slightly diminished, but the duration of lactation was not affected. The latter is a testimony to courage rather than diet. Analysis of milk produced showed no significant deviations from normal chemical structure. Milk was produced at the expense of maternal tissue.

These data from the Dutch famine in the 1940s during World War II were reexamined by Stein et al., who pointed out that women who conceived during the famine did develop some maternal stores in anticipation of lactation that were not accounted for by the fetus, placenta, or amniotic fluid, even though the fetus was a pound lighter at birth. They reported fetal weight down by 10% but maternal weight down by only 4%. This demonstrates the maternal body’s strong biologic commitment to preparing for lactation during pregnancy.

There is a wide range of volume of milk intake among healthy breastfed infants, averaging 750 to 800 g/day and ranging from 450 to 1200 g/day. Any factor that influences the frequency, intensity, or duration of suckling by an infant influences the volume. In a study of wet nurses in the 1920s, Macy et al. reported human capacity at 3500 mL/day. Compared with the 800 mL from mothers with singletons, studies of mothers producing for multiples, done by Saint et al., confirmed production of 2 to 3 L/day for twins and triplets. At 3 months of age for all populations, the volume averages 770 g/day (range 500 to 1200 g/day). ^, The self-regulation of milk supply by the infant has been confirmed by a study by Dewey et al. in which additional milk was pumped after each feeding for 2 weeks, thus increasing the milk supply. The infants, however, remained at baseline consumption during the pumping. The residual milk supply of healthy women (i.e., that which can be extracted after a full feeding) is about 100 g/day, even when an infant consumes comparatively low volumes of milk. ^{, ,}

Topographic computer imaging has been used to study breast production and storage capacities in the laboratory of Hartmann. Using moiré patterns projected onto the breast, it has been possible to calculate the volume of milk produced. As the breast expands with increasing milk, the moiré patterns change. By correlating the maternal weights before and after a feeding and the imagery patterns, data were converted to accurate milk volumes. This technique has remarkable potential for clinical use. Hartmann reports the normal range of milk production from 1 to 6 months postpartum to be between 440 and 1220 g/day for mothers who gave birth at full term.

Prentice and Prentice described “energy sparing adaptations” that were associated with normal lactation when energy intake is limited. These were decreases in basal metabolic rate, thermogenesis, and physical activity.

When well-nourished mothers reduced their intake by 32% for 1 week, consuming no less than 1500 kcal/day, no reduction in milk volume occurred, although plasma prolactin levels increased. Mothers who consumed less than 1500 kcal/day for a week did experience decreased milk volumes compared with those of the control group and the group consuming more than 1500 kcal.

Exercise, manual labor, and losing weight do not usually alter an established milk volume. Milk production will increase with infant demand, but infant demand will only increase with growth, which depends on sufficient nourishment. Having the mother take supplements could improve production and stimulate the infant’s appetite.

Energy Supplementation and Lactation Performance

When women received supplements during the last trimester of pregnancy, no effect was noted in their milk production. This suggests that short-term supplementation may be ineffective. Other studies that provided supplementation of a maximum of 900 kcal/day for 2 weeks resulted in an increase in milk production (662 to 787 g/day). No increase in infant weight compared with the control group’s infants was seen in this period of 2 weeks.

The problem of insufficient milk supply for a baby is reported in well-nourished as well as poorly nourished populations, but in cross-cultural studies it appears to be unrelated to maternal nutrition status. The effect of supplementation may be more psychologic than physiologic.

In countries where food supplies vary with the season, milk supplies drop 1 dL/day during periods of progressively greater food shortages. Studies continue on lactation performance of poorly nourished women around the world, including Burma, The Gambia, Papua New Guinea, and Ethiopia as well as among Navajo people. Results continue to reflect an impact on quantity, not quality, of milk. ^{, , ,}

The interrelationship of milk volume, nutrient concentration, and total nutrient intake by the infant must be considered. The reason for low protein content in a given sample may be lack of protein stores, lack of total energy content, or lack of vitamin B ₆ , a requirement of normal protein metabolism.

Of concern, however, is the report of dietary supplementation of Gambian nursing mothers in whom lactational performance was not affected by increased calories (700 kcal/day). The supplement produced a slight initial improvement in maternal body weight and subcutaneous fat but not in milk output. Whether the mothers utilized the increased energy to work harder farming or whether the infants did not stimulate increased milk production is unresolved. Food supplementation of lactating women in areas where malnutrition is prevalent has generally had little, if any, impact on milk volume. Such supplementation improves maternal health and is more likely to benefit the mother than the infant except where milk composition had been affected by specific deficiencies.

Protein Content

Since the work of Hambraeus reestablished the norms for protein in human milk to be 0.8 to 0.9 g/dL in well-nourished mothers, figures from previous studies have been recalculated to consider that all nitrogen in human milk is not protein; 25% of the nitrogen is nonprotein nitrogen (NPN) in human milk, and only 5% of the nitrogen is NPN in bovine milk. The protein content of milk from poorly nourished mothers is surprisingly high, and malnutrition has little effect on protein concentration. An increase in dietary protein increases volume but not overall protein content, given the normal variations seen in healthy, well-nourished women.

Observations made over a 20-month period of continued lactation showed that milk quality did not change, although the quantity decreased slightly, which has been attributed to the decreasing demand of a child who is receiving other nourishment. Therefore, the total protein available with the decreased volume of milk and increased weight of the child decreased from 2.2 g/kg of body weight to 0.45 g/kg. The need for additional protein sources from other foods for the child after 1 year of age becomes obvious.

The composition of human milk is maintained even with less-than-recommended dietary intake of macronutrients. The concentrations of major minerals, including calcium, phosphorus, magnesium, sodium, and potassium, are not affected by diet. Maternal dietary intakes of selenium and iodine, however, are positively affected: an increase in the diet increases the level in the milk. The proportion of different fatty acids in human milk varies with the maternal dietary intake.

In Zaire, lactating mothers with protein malnutrition were given 500 kcal (2093 kilojoules [kJ]) and 18 g of protein as a cow milk supplement for 2 months, after which their nutritional status improved significantly. The volume of milk did not change (607 versus 604 mL). Their breastfed infants, however, did show significant improvement in their mean serum albumin levels, and their growth matched that of healthy infants of the same age.

The effect of very-low-protein (8% of energy) and very-high-protein (20% of energy) diets on the protein and nitrogen composition of breast milk in three healthy Swedish women “in full lactation” was significant. High-protein diets produced higher production and greater concentrations of total nitrogen, true protein, and NPN. The increased NPN was caused by increased urea levels and free amino acids. The 24-hour outputs of lactoferrin, lactalbumin, and serum albumin were not significantly higher.

When marginally nourished women were provided a mixed-protein diet predominantly from plant sources up to 1.2 g/kg/day, equilibrium was achieved at a protein intake of 1.1 g/kg. In a study of healthy women given marginal protein intakes, Motil et al. reported that maternal milk production and the protein nitrogen, but not the NPN, fraction of human milk were relatively well preserved in the short term.

The practical significance, except as related to fad diets, of these results is limited because the diets were extreme and were maintained for only 4 days. The impact on human nutritional physiology, however, was significant.

Taurine, an amino acid found only in animal products, is the second most abundant free amino acid in human milk. Even milk of women who have no animal foods in their diet contains some taurine at levels (35 mg/dL) that are lower than those in women who consume animal products (54 mg/dL). Taurine is singularly important as the principal protein in the human brain. Cow milk does not contain taurine.

Of practical significance for counseling healthy women in the industrialized world is the work of Butte et al. investigating the effect of maternal diet and body composition on lactational performance; 45 healthy lactating women were followed for 4 months from delivery with detailed measurements of milk production, dietary intake, and maternal body composition. The overall mean energy intake was 2186 ± 463 kcal/day. Milk production averaged 751, 725, 723, and 740 g/day for months 1, 2, 3, and 4. Average maternal weight reduction was from 64.6 to 59.3 kg. Energy was calculated to be sufficient for maintenance and activity, yet the mothers achieved gradual weight reduction. The authors conclude that energy intakes of approximately 15% less than those currently recommended are compatible with full lactation, full activity, and gradual weight reduction to prepregnant weight ( Tables 9-1 and 9-2 ). Diets otherwise contained recommended daily allowances for lactation. Other investigators studying the impact of weight loss noted that the rate of postpregnancy weight loss affected the level of elaidic acid in milk and of trans fatty acid level. This is explained by the mobilization of fatty acids from maternal adipose tissue.

Fat, Cholesterol, and Omega-3 Fatty Acids

Mature human milk contains about 50% of its energy as fat. This fat is necessary for the tremendous growth of the newborn and is essential to the structural development of the brain, retina, and other tissues. Both n -6 and n -3 fatty acids are essential components of the phospholipids of cell membranes. They are critical to the fluidity, permeability, and activity of membrane-bound enzymes and receptors. During the first 4 to 6 months of life, an infant accumulates 1300 to 1600 g of lipids.

Considerable attention has been focused on the impact of dietary fat and cholesterol on the composition of human milk. Fat is the main source of kilocalories in human milk for the infant. The fatty acid composition of the triglycerides, which make up more than 98% of the lipid component of human milk, can be affected by maternal diet. Diets with different lipid composition, caloric content, proportion of calories from fat, and fatty acid composition have been studied.

In a classic work that was carefully controlled, Insull et al. fed lactating women in a metabolic ward diets that differed in caloric content, proportion of calories from fat, and fatty acid composition. Neither milk volume nor total milk fat was affected by diet. When the high-calorie, no-fat diet was fed, milk triglycerides were higher in fatty acids 12:0 and 14:0 and lower in 18:0 and 18:1, which indicated that when fatty acids were synthesized from carbohydrate, more intermediate-chain fatty acids were produced. With the low-calorie, no-fat diet, the fatty acid composition of the milk resembled the maintenance diet and the depot fat. When corn oil was the fat source, milk levels of 18:2 and 18:3 were higher, with a major increase in linoleic acid, than when lard or butter was used. Multiple studies have shown that medium-chain fatty acids, lauric and myristic acid (12:0 and 14:0), are not affected by diet, indicating synthesis by the mammary gland.

Trans fatty acids are produced in hydrogenation reactions and appear in human milk as a reflection of dietary intake, so that women who eat margarine rather than butter have high levels in their milk. Elaidic acid (18:1 trans) is found in margarine, for instance. Because of the high level of trans fatty acids in hydrogenated vegetable oils such as margarine, the milk of women in the United States is high in trans fatty acids, whereas the milk of women in West Germany who do not use margarines is low in trans fatty acids. Considerable controversy surrounds the biologic effects. The recommendations for substituting margarines were reversed in 1997. In mammals, trans isomers have been noted to alter permeability and fluidity of membranes, inhibit a number of enzyme reactions of lipid metabolism, and impair synthesis of arachidonic acid (AA) and prostaglandins.

The concern about fat composition in terms of the polyunsaturated fatty acid (PUFA) to saturated fatty acid ratio (P/S ratio) and the high level of cholesterol normally found in breast milk have led to monitoring mothers on altered lipid intakes. Lactating women were placed on one of two experimental diets after a period of a study of their normal Australian diet, which included 400 to 600 mg of cholesterol per day and fat that was rich in saturated fatty acids. After this baseline study, the mothers were given either diet A, with 580 mg cholesterol and a high level of saturated fats, or diet B, with 110 mg cholesterol and a higher level of polyunsaturated fats from vegetable oils. A second study was carried out with the two diets high in either saturated or unsaturated fats, but the cholesterol remained the same, 345 to 380 mg/day.

The low-cholesterol diets lowered the maternal blood cholesterol but not the triglyceride levels. The cholesterol level of the milk, however, was unaffected in any diet combination. The increase in PUFA in the diet rapidly increased the levels of linoleate in the milk to twice the previous level at the expense of myristate and palmitate. Protein levels remained the same in the milk throughout the study. Infant plasma cholesterol levels decreased in response to an increase in the concentration of linoleate in the milk. The significant dietary change seemed to depend on the consumption of high PUFA and low cholesterol to alter the levels in the milk and thus in the infant’s plasma ( Table 9-3 ).

Cholesterol levels remain relatively stable throughout at least 16 weeks of lactation. The presence or absence of phytosterols influences both the accuracy of analysis (i.e., overestimated level of cholesterol) and the physiologic significance of cholesterol. Phytosterols are those sterols derived from plant sources. They are distinguishable from cholesterol, which is of animal origin. During a given feeding, the concentration of cholesterol in the milk may increase more than 60%, although the total for the feeding is constant. The effect of maternal diet on cholesterol and phytosterol levels in human milk was measured by Mellies et al., who reported no change in cholesterol but a dramatic increase in phytosterols on high-cholesterol and phytosterol diets. The level of phytosterol in infant plasma did not change, however. These observations further confirm that cholesterol is synthesized at least in part in the mammary gland, whereas phytosterol is not.

Thus, no evidence is available that concentrations of cholesterol and phospholipids can be changed by diet. Milk cholesterol is stable at 100 to 150 mg/L even in hypercholesterolemic women and increases only in severe cases of pathologic hypercholesterolemia, according to Jensen. The fat globule membrane contains both cholesterol and phospholipids, and their secretion rates are related to the total quantity and are not influenced by diet. This supports the conclusion that cholesterol is essential to the diet of the infant.

Where maternal undernutrition is commonplace, the percentage of maternal body fat may influence the concentration of fat in the milk. Milk fat concentrations in Gambian women were positively correlated with maternal skinfold thickness and decreased over the course of lactation. Women with parity of 10 and above appear to have a decreased capacity to synthesize milk fat and thus have lower milk fat concentrations in their milk.

The synthesis of fatty acids up to the carbon number of 16, as well as the direct desaturation of stearic acid into oleic acid, can take place in the mammary gland, whereas longer-chain fatty acids come directly from plasma triglycerides ^, (see Chapter 4 ). The intake of both carbohydrate and fat must be taken into account when evaluating maternal diet because high-carbohydrate diets increase lauric acid and myristic acid and moderate levels of carbohydrate influence linoleic acid.

When serum lipids are measured in African women accustomed to a low-fat intake, the levels are relatively low and the women are virtually free of coronary heart disease. ^, Among long-lactating (1 to 2 years minimum) African mothers, the amount of fat in their daily milk is of the same order as that ingested in their habitual diet. Despite this, they are not significantly hypolipidemic when compared with nonlactators.

Human milk samples obtained from women living in five different regions of China showed the great diversity of milk fatty acids. The docosahexaenoic acid (DHA) concentrations in women from the marine region were twice as high as those from rural areas. The milk concentrations of DHA varied greatly (0.44 ± 0.29 to 2.78 ± 1.20 g/100 g total fat), with pastoral regions being lowest and the marine region highest. Seafood consumption was high in the marine group. Similarly, AA, when stated as a ratio (AA/DHA, g/g), was 2.77 in pastoral areas and 0.42 in the marine region. AA has been associated with infant growth and DHA with brain and retinal growth. Similar findings are reported in Alaskan Inuit people who have a diet high in fish and fish oil. When women’s diets were supplemented with fish and fish oils, the blood concentrations of DHA in the maternal plasma and red blood cells (RBCs) were increased. Infants showed a 35% DHA increase in RBCs and 45% increase in plasma, which supports the concept that maternal diet can influence the DHA levels in newborns. The fatty acid patterns of human milk correlate with the current American diet, which has a high P/S ratio; there is a shift toward higher levels of C18:2 fatty acids, linoleic acid, and C18:3 linolenic acid. ^{, ,} Depot fat reflects dietary fatty acid patterns and thus the pool for mammary gland synthesis of milk fats. The mammary gland can dehydrogenate saturated and monosaturated fatty acids.

Diet composition affects milk fat synthesis. When a woman is in energy balance, the fatty acids from the diet account for about 30% of the total fatty acids in her milk.

The habitual diet of healthy primiparas in Finland was associated with breast milk containing 3.8% fat. Their diet was 16% protein, 39% fat, and 45% carbohydrate. Half the fatty acids of the diet and the milk were saturated, and one third were monoenoic. PUFAs were 15% of the diet and 13% of the breast milk, with a P/S ratio of 0.3 for both. The maternal diet had no effect on total fat content of the milk except for the low level of oleic acid, which is apparently peculiar to Finnish breast milk.

DHA, a long-chain fatty acid (22:6, omega-3), has attracted attention because deficiency has been associated with visual impairment in offspring of rhesus monkeys. Essential n -3 fatty acids in pregnant women have been linked to visual acuity and neural development in their term infants. Some pregnant women in the United States have been found to be deficient in DHA. A descriptive meta-analysis of 106 studies worldwide was culled to 65 to include only those utilizing modern analysis methods to obtain fatty acid profiles. The highest DHA concentrations were found in coastal populations and associated with consumption of fish. DHA was 0.32% + 0.22% and 0.47% + 0.13% for AA, representing the mean concentrations worldwide. Omega-3 DHA is important to the fetus and to the offspring through breastfeeding, and emerging science suggests it may protect against preterm delivery, and postpartum depression as well.

Fish Consumption during Lactation

Maternal fish consumption during pregnancy has been correlated with cognitive and visual abilities in offspring. Maternal omega-3( n -3) LCPUFA supplementation during pregnancy was evaluated comparing early childhood cognitive and visual development in mother’s with and without supplementation. A systematic review and meta-analysis of randomized controlled trials failed to prove or disprove that omega-3 LCPUFA supplementation in pregnancy improves cognitive and visual development of the children.

Fish oil is an excellent dietary source of DHA, and women who consistently eat fish have higher levels in their milk. In a study, Finley et al. found that vegetarians have higher DHA levels in their milk than omnivore control subjects. Many formulas have been supplemented with synthetically derived DHA in an effort to mimic human milk. They do not, however, contain cholesterol, and no data support the concept that synthetically derived DHA is as effective as natural DHA in human milk.

A strong association exists between the body fat of the mother and lipid in her milk. Lovelady et al. found that the best predictor of milk lipids was overall “fatness” rather than the distribution of that fat. Dietary fat was not associated with milk fat in the “fat” women (27% or more body fat) but was positively correlated with diet in lean women (less than 27% body fat).

When healthy pregnant women are supplemented with fish oil capsules from the thirtieth week of gestation, the fatty acid compositions of the phospholipids isolated from umbilical plasma and umbilical vessel walls differ from those of unsupplemented mothers, with more n -3 and less n -6 fatty acids. This suggests that DHA status can be altered at birth.

A group of lactating women were given supplements of different doses of fish oil concentrates rich in omega-3 fatty acids, including DHA. Receiving 5 g/day for 28 days, 10 g/day for 14 days, and 47 g/day for 8 days, each experienced significant dose-dependent increases in DHA in their milk and plasma. Baseline levels in milk were 0.1% of total fatty acids, and levels rose from 0.8% to as high as 4.8% on the 47 g/day diet. This suggests that relatively small supplements of DHA can enhance levels in the milk. Preformed dietary DHA is known to be better synthesized into nervous tissue than that synthesized from linolenic acid, and other essential fatty acids can inhibit this transformation to DHA. The consumption of fish during pregnancy and lactation is an important dietary consideration in preference to fish oil capsules. The concern rests with possible mercury contamination. Fish, however, provides lean protein, and an abundance of vitamins B, zinc, iodine and selenium as well as naturally rich sources of long-chain omega-3 fatty acids and vitamin D. It has been recorded that women who do not eat fish during pregnancy put their infants at risk for suboptimal visual, cognitive, motor, and behavior skill outcomes. International studies have shown the value of fish in pregnancy and lactation. The most thorough was a 15-year follow-up of infants breastfed on the Seychelles Islands by mothers with a high intake of fish, measurable mercury levels, and developmental growth scores that were higher with greater consumption of fish and greater levels of breastfeeding. The Food and Drug Administration (FDA) has stated that, while fish oil supplements are beneficial for those who cannot eat fish, fish has the full range of nutrients. The FDA recommends a minimum of two meals of fish per week (up to 12 ounces) during lactation.

Studies of linoleic acid supplementation from 20 weeks’ gestation in normal women showed that levels increased in those with low linoleic acid levels to match those with high levels. The neonatal linoleic acid status did not change. Linoleic acid supplementation did result in slightly but significantly higher total amounts of n -6 long-chain polyenes in umbilical plasma. Linoleic acid (18:2, n -6) is essential to the maintenance of the epidermal water barrier and is the ultimate dietary precursor of eicosanoids, which include leukotrienes, prostaglandins, and thromboxanes. Linoleic acid is not synthesized by humans and must be supplied by diet.

A diet deficient in omega-3 fatty acids leads to a triad of signs in the rhesus monkey: visual impairment, electroretinographic abnormalities, and polydipsia. Profound biochemical changes in fatty acid composition of the membranes of the retina, brain, and other organs are seen experimentally. Low concentrations of omega-3 fatty acids occur at birth in the placenta, RBCs, and neural tissues when the mothers are fed deficient diets. Studies in monkeys confirm that the most critical period of life for providing omega-3 fatty acids is during pregnancy and during lactation in early infancy. In humans, supplementation of the maternal diet with fish and fish oils has increased the levels of omega-3 fatty acids, especially DHA. Humans can synthesize DHA from linolenic acid, but this is limited in both infants and adults. Supplementing with linolenic acid does not significantly increase DHA in the blood. No evidence suggests that supplements in normal women with good diets are beneficial. Excesses of DHA affect AA levels and interfere with the AA/DHA ratio. The content of conjugated linoleic isomer and trans-vaccenic acid in human milk was found to be higher in women who consumed organic dairy and organic meat products. The health effects of conjugated linoleic isomer and trans-vaccenic acid on human newborns are pending but the effects in animals and in human adults show immunomodulating properties such as on influenza and other viruses. The recommendations of the Committee on Nutrition of the American Academy of Pediatrics (AAP) regarding cardiovascular health in childhood have no comment about infants until they are weaned from formula. They do not mention the value of being breastfed. For children 12 months to 2 years who are obese or have a family history of obesity, dyslipidemia, or cardiovascular disease, skim milk is suggested ( Figure 9-1 ).

ChooseMyPlate (A) and MyPyramid (B) are tools for ensuring adequate nutrition in both children and adults. See ChooseMyPlate.gov and Nutrition.gov for more information.

(From the U.S. Department of Agriculture.)

Lactose

In human milk the principal carbohydrate is lactose, present at approximately 70 g/L and second only to water as a major constituent. The milk of all species is isotonic with maternal plasma, and 60% to 70% of the osmotic pressure is created by lactose. Lactose provides twice the energy value per molecule or unit of osmotic pressure. Because milk volume is driven by available lactose, its concentration is stable. Changes in the carbohydrate levels in the diet have been studied. Comparison of mothers on diets with three different levels of carbohydrate shows that the amounts of protein, fat, and carbohydrate in their milk are similar. No evidence indicates that dietary manipulations affect lactose.

Water

No data support the assumption that increasing fluid intake will increase milk volume, and restricting fluids has not been shown to decrease milk volume. Forcing fluids, however, has been shown to affect milk production negatively in a controlled crossover-design study of breastfeeding mothers. Thus, women taking excessive fluids produced less milk, suggesting that drinking to thirst and heeding body cues is more physiologic than prescribing a specific amount of fluid per day. This observation was first demonstrated in a 1939 study that concluded, “Forced, excessive drinking is therefore neither necessary nor beneficial as far as the nursing is concerned and may even be harmful.” Hypogalactia cannot be arrested by forced drinking beyond the natural dictates of thirst. Urine output in these studies was proportional to intake. A similar study of 210 postpartum mothers, half of whom drank ad lib, taking an average of 69 oz daily, while the other half were forced to take 6 pt and averaged 107 oz daily, showed that the mothers forced to drink beyond thirst produced less milk, and their babies gained less well.

From a practical standpoint, mothers have increased thirst. When fluids are restricted, mothers will experience a decrease in urine output, not in milk. Sharply decreasing fluids to prevent engorgement in the mother who is not lactating is ineffectual, however, and only adds another inconvenience and discomfort.

Kilocalories

The caloric content, sample by sample, of milk from well-nourished mothers does vary but averages 75 kcal/dL. Because fat is the chief source of kilocalories, the fat content has the greatest impact on total kilocalories, with lactose and protein also contributing to the total. Thus, in malnourished mothers with low fat stores the caloric content may be reduced.

Body fat increases during pregnancy and decreases during lactation. Changes in the adipose depot primarily result from a change in fat cell size, not number. Adipose tissue fatty acid synthesis remains low throughout pregnancy, as does lipoprotein lipase activity. Conversely, mammary lipoprotein lipase activity increases and remains high during lactation.

How does this correlate with the caloric needs of the mother to produce milk? The calculations for energy requirements have been made by comparing the energy intakes of nursing mothers and nonnursing mothers who were matched for other variables. Nursing mothers consumed 2460 kcal daily and nonnursing mothers consumed 1880 kcal, a net difference of 580 kcal.

Lactation will not produce a net drain on the mother if the amount of energy available and the requirement of any given nutrient are replaced in the diet. There is only a small energy cost of milk production because the breasts work at remarkable efficiency. During pregnancy, fat and other nutrients are stored for the fetus and in preparation for lactation. Lactation is subsidized, as is fetal growth, by maternal stores, even though the diet on any given day may be relatively deficient in a specific nutrient. This can be clarified by Figure 9-2 , which shows that diet and stores are available for milk, as well as for maintenance of the mother.

Energy utilization during lactation, showing availability of body stores and dietary sources.

A study of 26 healthy, normotensive, nonsmoking, euthyroid women—12 of whom were breastfeeding, 7 bottle-feeding, and 7 nonpregnant, nonlactating control subjects—was reported by Illingworth et al. Energy expenditure at rest and in response to a meal and to an infusion of noradrenaline was measured. During lactation, the resting metabolic rate was unaltered, but a reduced response to infusion of noradrenaline and to a meal was observed. These responses returned to normal control values in these women postlactation. Women who bottle fed were similar to control subjects. The woman’s metabolic efficiency is greatly enhanced during lactation and results in a reduction in the nonlactational component of maternal energy expenditure ( Figure 9-3 ).

Metabolic response to test meal while breastfeeding compared with response of women who bottle feed and nonpregnant control subjects.

(From Illingworth PJ, Jung RT, Howie PW, et al: Diminution in energy expenditure during lactation, Br Med J 292:437, 1986.)

When comparing dietary intake during lactation at 6 weeks postpartum to the intake of a comparable group of nonpregnant women and a group of nonlactating but postpartum women using a 7-day food diary and questionnaire, total daily intakes and meal patterns were not different between body weight-matched lactators and nonlactators. The lactating women, however, consumed a significantly smaller percentage of the recommended dietary allowances (RDAs) per day and were much more calm both before and after meals. The lactating women did not increase their daily intake over their prepregnancy diet.

The total amount of nutrients that the lactating mother secretes in her milk is directly related to the extent and duration of lactation. Furthermore, lactating women who consume a well-balanced diet with adequate calories (2700 kcal/day) meet the RDAs for all nutrients with the exception of calcium and zinc, according to assessments of the average American diet for young women. This is based on nutrient density (nutrient intake per 1000 kcal) of the average woman’s diet in the United States. Nutrient densities for proteins, minerals, and vitamins have been determined from nationally representative samples of women aged 19 to 50 years of age. The nutrient values at three different levels of energy are calculated (nutrient density × kcal of energy = total intake). The levels of energy used are 2700 kcal, the recommendation for lactating women; 2200 kcal, as reported by lactating women as actual consumption; and 1800 kcal, the minimal level a lactating woman should consider in a restricted diet. The relative nutrient deficiencies are identified next.

For the lactating woman, a 2700-kcal diet may be deficient in calcium and zinc; the 2200-kcal diet is deficient in calcium, magnesium, zinc, thiamin, vitamin B ₆ , and vitamin E; and the 1800-kcal diet is deficient in all the previously mentioned nutrient levels plus riboflavin, folate, phosphorus, and iron unless special attention is paid to intake of these nutrients ( Tables 9-4 through 9-7 ).

Table 9-7

Dietary Reference Intakes: Estimated Average Requirements for Groups (Food and Nutrition Board, Institute of Medicine, National Academies)

Life Stage Group	CHO (g/day)	Protein (g/day)	Vit A ( μ g/day) a	Vit C (mg/day)	Vit E (mg/day) b	Thiamin (mg/day)	Riboflavin (mg/day)	Niacin (mg/day) c	Vit B 6 (mg/day)	Folate ( μ g/day) d	Vit B 12 ( μ g/day)	Copper ( μ g/day)	Iodine ( μ g/day)	Iron (mg/day)	Magnesium (mg/day)	Molybdenum ( μ g/day)	Phosphorus (mg/day)	Selenium ( μ g/day)	inc (mg/day)
Females
9-13 yr	100	28	420	39	9	0.7	0.8	9	0.8	250	1.5	540	73	5.7	200	26	1055	35	7.0
14-16 yr	100	38	485	56	12	0.9	0.9	11	1.0	330	2.0	685	95	7.9	300	33	1055	45	7.3
19-30 yr	100	38	500	60	12	0.9	0.9	11	1.1	320	2.0	700	95	8.1	255	34	580	45	6.8
31-50 yr	100	38	500	60	12	0.9	0.9	11	1.1	320	2.0	700	95	8.1	265	34	580	45	6.8
51-70 yr	100	38	500	60	12	0.9	0.9	11	1.3	320	2.0	700	95	5	265	34	580	45	6.8
> 70 yr	100	38	500	60	12	0.9	0.9	11	1.3	320	2.0	700	95	5	265	34	580	45	6.8
Pregnancy
14-18 yr	135	50	530	66	12	1.2	1.2	14	1.6			520	2.2	785	160	23	335	40	1055
19-30 yr	135	50	550	70	12	1.2	1.2	14	1.6			520	2.2	800	160	22	290	40	580
31-50 yr	135	50	550	70	12	1.2	1.2	14	1.6			520	2.2	800	160	22	300	40	580
Lactation
14-18 yr	160	60	885	96	16	1.2	1.3	13	1.7			450	2.4	985	209	7	300	35	1055
19-30 yr	160	60	900	100	16	1.2	1.3	13	1.7			450	2.4	1000	209	6.5	255	36	580
31-50 yr	160	60	900	100	16	1.2	1.3	13	1.7			450	2.4	1000	209	6.5	265	36	580

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