used oral contraceptives; this proportion was much higher among blacks (26.9%) than among whites (11.7%) (Ford and Labbok, 1987).
In providing guidance to women planning to use oral contraceptives, it is important to consider the composition and dosage of the pill and the intended duration of exclusive breastfeeding. In most studies conducted on the subject, the use of combined estrogen and progestin pills has been associated with reduced milk volume and duration of breastfeeding (Koetsawang, 1987; Lönnerdal, 1986). A recent multi-center, randomized double-blind trial in Hungary and Thailand demonstrated that even low-dose combined oral contraceptives (150 µg of levonorgestrel and 30 µg of ethinyl estradiol) have this effect: between 6 and 24 weeks post partum, the rate of milk volume decrease in women taking these pills was about twice the rate observed in control women (WHO Task Force on Oral Contraceptives, 1988). The nitrogen content of milk also was lower in those taking the combined pills, but there was no consistent effect on lactose or fat concentrations.
In contrast, no effect on milk volume or composition has been associated with progestin-only pills (Koestsawang, 1987; Lönnerdal, 1986; WHO Task Force on Oral Contraceptives, 1988). Although progesterone is known to inhibit lactogenesis, once lactation has been established it has no known inhibitory effect on milk production, possibly because progesterone binding sites are apparently not present in lactating tissues (Neville and Neifert, 1983). Further, there are substantial chemical differences between natural progesterone and synthetic progestins. Progestin-only pills have been found to be slightly less effective contraceptives than combined pills in studies of nonlactating women (Winikoff et al., 1988), but it is not known if this difference in effectiveness applies to lactating women as well. Progestin-only pills are also associated with altered menstrual cycles in nonlactating women, but the prevalence of this dysfunction is unknown in lactating women, who are likely to have a longer period of postpartum amenorrhea. For lactating women who wish to use oral contraceptives and maintain milk production, the World Health Organization states that progestin-only pills are the preferred choice (WHO Task Force on Oral Contraceptives, 1988).
This section begins with consideration of maternal energy balance during lactation; this is followed by discussions of protein and fluid intakes. Studies on the influence of other nutrients have dealt primarily with milk composition, rather than volume, and are discussed in Chapter 6.
In its review, the subcommittee gave greatest weight to evidence with the greatest relevance to making causal inferences to human populations. Causal relationships can be most definitively demonstrated in intervention studies with