Fat and Energy Contents of Expressed Human Breast Milk in Prolonged Lactation
ABSTRACT
- Dror Mandel, MD, MHA*‡,
- Ronit Lubetzky, MD*‡§,
- Shaul Dollberg, MD, FACN*‡,
- Shimon Barak, MD*,
- Francis B. Mimouni, MD, FAAP, FACN*‡
Objective. To estimate fat and energy contents of human milk during prolonged lactation.
Methods. Thirty-four mothers, of term, healthy, growing children, who had been lactating for >1 year (12–39 months) were recruited. Control subjects were 27 mothers, of term infants, who had been lactating for 2 to 6 months. Fat contents of the milk samples were estimated as creamatocrit (CMT) levels. Energy contents of the milk were measured with a bomb calorimeter.
Results. The groups did not differ in terms of maternal height and diet, infant birth weight, gestational age, or breastfeeding frequency. They differed significantly in terms of maternal age, maternal weight, and BMI. The mean CMT levels were 7.36 ± 2.65% in the short-duration group and 10.65 ± 5.07% in the long-lactation group. The mean energy contents were 3103.7 ± 863.2 kJ/L in the short-duration group and 3683.2 ± 1032.2 kJ/L in the long-duration group. The mean CMT levels and mean energy contents were correlated significantly with the duration of lactation (R2 = 0.22 and R2 = 0.23, respectively). In multivariate regression analysis, CMT levels (or energy contents) were not influenced by maternal age, diet, BMI, or number of daily feedings but remained significantly influenced by the duration of lactation.
Conclusions. Human milk expressed by mothers who have been lactating for >1 year has significantly increased fat and energy contents, compared with milk expressed by women who have been lactating for shorter periods. During prolonged lactation, the fat energy contribution of breast milk to the infant diet might be significant.
The optimal duration of breastfeeding is unknown. The American Academy of Pediatrics recommends exclusive breastfeeding for 6 months and a total duration of ≥1 year to obtain the “full benefits of breastfeeding.”1 Among frequently recognized long-term benefits of breastfeeding are reductions in cardiovascular risks in adulthood.1, 2 These reductions were challenged by a retrospective epidemiologic study of men born in 1920 to 1930 in Hertfordshire, England, which suggested that the beneficial effects of breastfeeding on cardiovascular risks existed as long as weaning was performed before 1 year of age; after that time, continued breastfeeding was associated paradoxically with increased cardiovascular risks.2 Moreover, a study by Leeson et al3 suggested that prolonged breastfeeding might lead to unwelcome outcomes and might even increase cardiovascular risks in adulthood.
In developed countries, a minority of women continue to lactate for >1 year; in one study from Italy, 17% of mothers were still breastfeeding at 12 months after delivery.4 The energy contribution of human milk (HM) to the diet of partially breastfed children beyond the first year of life is unknown, because there are no data on the amounts of HM consumed by these children and the nutritional content of HM after prolonged breastfeeding is little known. In particular, the fat and energy contents of HM after prolonged breastfeeding have not been analyzed systematically. We therefore conducted this cross-sectional study of 61 women who had been lactating for periods of 2 to 6 months (short lactation duration) or >1 year (long lactation duration), to estimate the fat and energy contents of HM.
Therefore, it seems theoretically that, during prolonged lactation, the contribution of breast milk to the infant diet might be significant, from an energy intake standpoint. Indeed, a reduction in the volume of milk consumed by a child who is also eating solid foods might well be counterbalanced by the increase in energy concentration. A limitation of this statement is that, similar to the study by Dewey et al,22 we were not able to measure accurately the volumes consumed at the breast by each infant, for obvious technical reasons. Mothers in our study were not instructed to double-weigh the infant before and after every feeding, because of the cumbersome aspects. Nevertheless, it seemed that some women in the long-duration lactation group were able to reach extraordinarily high CMT values, such as a woman whose expressed breast milk had a CMT level of 28%.
The long-term effects of such high fat intake have not been studied. We did not analyze the fat qualitatively in this study, but it is known that, compared with infant formula, breast milk is much richer in cholesterol and saturated fat.24 The effects of high intake of saturated fat and cholesterol on infant cholesterol metabolism are consequential. We showed that, in the first few months of life, higher cholesterol intake among breastfed infants led to downregulation of endogenous cholesterol production.25 Whether continued high saturated fat and cholesterol intake through breastfeeding beyond the first year of life is beneficial is unknown. As quoted earlier, prolonged breastfeeding has been “accused” of possibly inducing long-term endothelial damage and of decreasing arterial distensibility,2, 3, 26–28 but many objections have been raised against this accusation.29–31 The type of fats present in the milk of mothers lactating for >1 year needs to be measured before suggestions of the role in adult heart disease can be mentioned. Also, because of the changes in diets from childhood to adult years, it may not be possible to determine the influence of prolonged breastfeeding on cardiovascular disease.
We must point out that, at the present time, the official policy of the American Academy of Pediatrics is not to put any limit on the duration of lactation.1Moreover, a recent review of biological versus cultural aspects of weaning suggested that, from an anthropologic standpoint based on primates studies, “breastfeeding a child for 2.5 to 7 years is normal for our species.”
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