7. ENERGY REQUIREMENTS OF LACTATION

Exclusive breastfeeding is recommended during the six months after delivery, with introduction of complementary foods and continued breastfeeding thereafter (WHO, 2001). The energy requirement of a lactating woman is defined as the level of energy intake from food that will balance the energy expenditure needed to maintain a body weight and body composition, a level of physical activity and breastmilk production that are consistent with good health for the woman and her child, and that will allow economically necessary and socially desirable activities to be performed. To operationalize this definition, the energy needed to produce an appropriate volume of milk must be added to the woman’s habitual energy requirement, assuming that she resumes her usual level of physical activity soon after giving birth.

The mean amount of breastmilk produced daily is similar among population groups with different cultural and socio-economic settings (Prentice et al., 1986; Butte, Lopez-Alarcon and Garza, 2002) (Table 7.1). There may be some variation in milk composition related to maternal nutrition, but the main factors that influence the energy needs of lactating women are the duration of breastfeeding practices and the extent of exclusive breastfeeding. As these vary significantly in different societies, dietary energy recommendations for lactating women should be population-specific. Regardless of the cultural and social environment, the ideal situation is that women be well nourished from the beginning of pregnancy and that they maintain adequate nutritional intake with appropriate weight gain throughout gestation. This will allow them to attain body fat reserves that may act as an energy substrate to cover part of the additional energy needs in preparation for and during lactation.

TABLE 7.1
Average milk production rates (g/d)

Source: Butte, Lopez-Alarcon and Garza, 2002.

7.1 Determinants of the energy cost of lactation

The energy cost of lactation is determined by the amount of milk that is produced and secreted, its energy content, and the efficiency with which dietary energy is converted to milk energy.

7.1.1 Human milk production

The mean amount of milk ingested by exclusively breastfed infants is similar in industrialized and more traditional societies, according to a WHO-sponsored comprehensive review (Butte, Lopez-Alarcon and Garza, 2002). After six months, variation among individuals and populations increases, owing to the nature and amount of complementary foods provided to the growing infant. From the age of six months onwards, when infants are partially breastfed, milk production is estimated at 550 g/day.

7.1.2 Energy content of human milk

The energy content of human milk depends primarily on milk fat concentration, which shows complex diurnal, within-feed and between-breast fluctuations. Twenty-four-hour milk sampling schemes have been developed that interfere minimally with the secretion of milk flow and capture the diurnal and within-feed variation (Garza and Butte, 1986). Measurements of the gross energy content of representative 24-hour milk samples determined by adiabatic bomb calorimetry or macronutrient analysis in a number of studies of well-nourished women gave a mean value of 2.8 kJ/g (0.67 kcal/g) from 1 to 24 months of lactation (Garza and Butte, 1986; Prentice and Prentice, 1988; Butte and King, 2002; WHO, 1985; Institute of Medicine, 1991; Goldberg et al., 1991; Panter-Brick, 1993).

7.1.3 Efficiency of energy conversion

The efficiency with which food energy and body energy reserves are converted into milk energy has been calculated from theoretical estimates of the biochemical efficiency associated with the synthesis of milk lactose, protein and fat, and from metabolic balance studies (Prentice and Prentice, 1988). Taking into account the energy costs of digestion, absorption, conversion and transport, biochemical efficiency has been estimated at 80 to 85 percent (Butte and King, 2002). Based on that estimate, on the theoretical efficiency used in the 1985 FAO/WHO/UNU report and on the suggestion of the United States Institute of Medicine (1991), an efficiency factor of 80 percent was applied to calculate the energy cost of human milk production.

7.1.4 Energy cost of milk production

Table 7.2 shows the energy cost to produce the mean amounts of milk needed for exclusively breastfed infants. Monthly milk volumes are those reported for well-nourished women with healthy babies in the WHO-sponsored review (Butte, Lopez-Alarcon and Garza, 2002), and gross energy contents are those described in section 7.1.2.

The results were compared with the energy requirements of exclusively breastfed infants from one to six months of age, calculated as described in chapter 3. To do so, human milk intakes that are measured by the test-weighing technique must be corrected for insensible water loss during the course of feeding (5 percent correction factor) and for the digestibility of human milk. The metabolizable energy in human milk was assumed to be 5.3 percent lower than its gross energy content based on proximate analyses and energy factors of 23.6 kJ (5.65 kcal) per gram of protein and free amino acids, 38.7 kJ (9.25 kcal) per gram of fat and 16.5 kJ (3.95 kcal) per gram of lactose. From months one to six the figures are on average within 5 percent, which is remarkable considering that energy requirements of infants were calculated from quite different information (i.e. from predictive equations based on DLW measurements of TEE, plus estimates of growth accretion based on growth velocity and body composition).

TABLE 7.2
Energy cost of human milk production by women who practise exclusive breastfeeding

^a From Butte, Lopez-Alarcon and Garza, 2002.
^b Insensible water losses assumed to be equal to 5 percent milk intake.
^c Gross energy content measured by adiabatic bomb calorimetry or macronutrient analysis.
^d Based on energetic efficiency of 80 percent.

TABLE 7.3
Comparison of the energy cost of human milk production and energy requirements of exclusively breastfed infants

^a From Butte, Lopez-Alarcon and Garza, 2002.
^b Insensible water losses assumed to be equal to 5 percent milk intake.
^c Gross energy content measured by adiabatic bomb calorimetry or macronutrient analysis.
^d Metabolizable energy values based on proximate analysis of milk are 5.3 percent lower than bomb calorimetry values.
^e Mean values of boys and girls, calculated as described in chapter 3 of this report.

7.2 Energy requirements for lactation

Compared with non-pregnant, non-lactating women, during lactation there are no significant changes in BMR, efficiency of work performance, or TEE (Butte and King, 2002), and in most societies women resume their usual level of physical activity in the first month postpartum or shortly thereafter (Goldberg et al., 1991; Panter-Brick, 1993; Roberts et al., 1982; Tuazon et al., 1987; van Raaij et al., 1990). It could be argued that where exclusive breastfeeding is prevalent, lactating mothers may have a lower TEE than non-pregnant, non-lactating women owing to the frequency of breastfeeding, which involves periods of little maternal activity. On the other hand, lactating women often carry their infants while moving around, and this additional workload might balance the lower physical activity associated with breastfeeding. Thus, total energy requirements during lactation are equal to those of the pre-pregnancy period, plus the additional demands imposed by the need for adequate milk production and secretion.

These additional demands correspond to the energy cost of milk production. For women who feed their infants exclusively with breastmilk during the first six months of life, the mean energy cost over the six-month period is: 807 g milk/day × 2.8 kJ/g/0.80 efficiency = 2.8 MJ/day (675 kcal/day) (Table 7.2). From the age of six months onwards, when infants are partially breastfed and milk production is on average 550 g/day (Table 7.1), the energy cost imposed by lactation is 1.925 MJ/day (460kcal/day).

Fat stores accumulated during pregnancy may cover part of the additional energy needs in the first few months of lactation. Postpartum loss of body weight is usually highest in the first three months, and generally greater among women who practise exclusive breastfeeding, but the extent to which the energy mobilized supports lactation depends on the gestational weight gain and the nutritional status of the mother. A review of 17 studies indicated that, on average, well-nourished women lost 0.8 kg/month, whereas undernourished mothers lost only an average of 0.1 kg/month (Butte and Hopkinson, 1998). Assuming an energy factor of 27.2 MJ/kg (Butte and King, 2002; Butte and Hopkinson, 1998), the rate of weight loss in well-nourished women would correspond to the mobilization of 27.2 × 0.8 kg/month = 21.8 MJ/month, or 0.72 MJ/day (170 kcal/day) from body energy stores. This amount of energy can be deducted from the 2.8 MJ/day (675 kcal)/day needed during the first six months of lactation. The result, 2.1 MJ/day (505 kcal/day), is similar to the additional energy required when infants are partially breastfed after six months of lactation.

On the other hand, undernourished women and those who did not gain adequate body weight during pregnancy must conserve as much energy as possible for their own well-being and that of their infants. Hence, in these women the full energy demands of lactation must be provided by an increment in dietary intake.

In conclusion, well-nourished women with adequate gestational weight gain should increase their food intake by 2.1 MJ/day (505 kcal/day) for the first six months of lactation, while undernourished women and those with insufficient gestational weight gain should add to their personal energy demands 2.8 MJ/day (675 kcal/day) during the first semester of lactation. Energy requirements for milk production in the second six months are dependent on rates of milk production, which are highly variable among women and populations.

References

Brown, K., Dewey, K.G. & Allen, L. 1998. Complementary feeding of young children in developing countries: A review of current scientific knowledge. Geneva, WHO.

Butte, N.F. & Hopkinson, J.M. 1998. Body composition changes during lactation are highly variable among women. J. Nutr., 128: 381S-385S.

Butte, N. & King, J.C. 2002. Energy requirements during pregnancy and lactation. Energy background paper prepared for the joint FAO/WHO/UNU Consultation on Energy in Human Nutrition.

Butte, N.F., Lopez-Alarcon, M.D. & Garza, C. 2002. Nutrient adequacy of exclusive breastfeeding for the term infant during the first six months of life. Geneva, WHO.

Garza, C. & Butte, N.F. 1986. Energy concentration of human milk estimated from 24-h pools and various abbreviated sampling schemes. J. Pediatr. Gastroenterol. Nutr., 5: 943-948.

Goldberg, G.R., Prentice, A.M., Coward, W.A., Davies, H.L., Murgatroyd, P.R., Sawyer, M.B., Ashford, J. & Black, A.E. 1991. Longitudinal assessment of the components of energy balance in well-nourished lactating women. Am. J. Clin. Nutr., 54: 788-798.

Institute of Medicine. 1991. Nutrition during lactation. Washington, DC, National Academy Press.

Panter-Brick, C. 1993. Seasonality of energy expenditure during pregnancy and lactation for rural Nepali women. Am. J. Clin. Nutr., 57: 620-628.

Prentice, A.M. & Prentice, A. 1988. Energy costs of lactation. Ann. Rev. Nutr., 8: 63-79.

Prentice, A., Paul, A., Black, A., Cole, T. & Whitehead, R. 1986. Cross-cultural differences in lactational performance. In M. Hamosh and A.S. Goldman, eds. Human lactation 2: Maternal and environmental factors, pp. 13-44. New York, Plenum Press.

Roberts, S.B., Paul, A.A., Cole, T.J. & Whitehead, R.G. 1982. Seasonal changes in activity, birth weight and lactational performance in rural Gambian women. Trans. R. Soc. Trop. Med. Hyg., 76: 668-678.

Tuazon, M.A., van Raaij, J.M., Hautvast, J.G. & Barba, C.V. 1987. Energy requirements of pregnancy in the Philippines. Lancet, 2: 1129-1131.

van Raaij, J.M.A., Schonk, C.M., Vermaat-Miedema, S.H., Peek, M.E.M. & Hautvast, J.G.A.J. 1990. Energy cost of walking at a fixed pace and self-selected pace before, during and after pregnancy. Am. J. Clin. Nutr., 51: 158-161.

WHO. 1985. Energy and protein requirements: Report of a joint FAO/WHO/UNU expert consultation. WHO Technical Report Series No. 724. Geneva.

WHO. 2001. Expert consultation on the optimal duration of exclusive breastfeeding. Conclusions and recommendations. Geneva.