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32. Nicholas KR, Hartmann PE. Progesterone control of the initiation of lactose synthesis in the rat. Aust J Biol Sci 1981;34:435-43.

33. Hartmann PE, Trevethan P. Shelton JN. Progesterone and oestrogen and the initiation of lactation in ewes. J Endocrinol 1973;59:249-59.

34. Hartmann PE, Whitely JL, Willcox DL. Lactose in plasma during lactogenesis, established lactation and weaning in sows. J Physiol 1984;347:453-63.

35. Whitely JL, Hartmann PE, Willcox DL, Bryant-Greenwood GD, Greenwood FC. Initiation of parturition and lactation in the sow: effects of delaying parturition with medroxyprogesterone acetate. J Endocrinol 1990;124: 475-84.

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Discussion of paper by Hartmann et al.

Dr. McLaren

The two women you showed us both produced more milk from the right breast than the left. Is this usual, and is it related to which breast the baby is put to first at a feeding?

Dr. Hartmann

I think that perhaps two-thirds of women are uneven in the milk production between their left and right breasts, but it doesn't necessarily mean they are consistently higher in the right breast. We found that some women changed from one lactation to the next, so most milk production in one lactation is in the right breast, and in the next it's in the left breast, so there is a fairly random distribution between breasts. Some people think it has to do with the handedness of mothers. We haven't looked at that, but certainly it can change during subsequent lactations. It seems to me to relate to a behavioural characteristic in animals, which is teat order. There is evidence from Sweden, I believe, that indicates that when babies are put on their mother's abdomen, they make their way up to the breast. They also tend to associate with the breast that they attach to first. This observation would be consistent with observations of the establishment of teat order in other mammals.

Dr. McLaren

I was particularly interested in your observation that fat content increases as the breast is emptied. Does that mean it is better for a woman to empty one breast before transferring the baby to the other?

Dr. Hartmann

This is an interesting situation, and one has to be very careful in responding, because how it is applied in practice can be quite misleading. The breast has regular mechanisms that can cope with most patterns of feeding. So far, our results indicate that it doesn't much matter how the breast is emptied; the baby will get the same amount of energy over a 24-hour period. One-sided feeding has only been recommended for women with oversupply problems. For normal feeding it is reasonable to finish one breast before offering the other.

Dr. McNeilly

Back to milk production. Presumably, in the case of twins, both breasts can operate to full capacity if they are emptied at the same rate?

Dr. Hartmann

That is correct. Our studies suggest there is additional capacity. I think mothers in Western societies have to down-regulate their physiological capacity to produce milk to their babies' requirements. We can look at the endocrine system as setting the upper limits of the mother's milk production capacity and the autocrine mechanism as then cutting it down to the actual needs of the baby. With twins the mother sustains a higher level of production that perhaps approximates the breasts' maximum capacity. We studied a woman who fully breastfed triplets for two and a half months, so the breast has enormous capacity.

Dr. McNeilly

You didn't mention the relationship of oxytocin to storage capacity. It seems that much of the milk won't come out without oxytocin release. Is that true? There is very little information on that.

Dr. Hartmann

Residual milk, in my opinion, has done more to confuse our understanding of lactation than just about any other concept. This is because it is based on the very unnatural situation of the dairy cow. The suckling situation is totally different. Oxytocin is obviously required for milk ejection, but the amount of milk that is taken depends on the baby's appetite, not what the mother has available. So in mothers with high storage capacity, there is enormous variation between the storage capacity and the amount of milk that the baby might take at any one time.

Dr. McNeilly

But if the mother didn't release oxytocin at all, would that dramatically reduce the amount of milk the baby could get?

Dr. Hartmann

Yes, it certainly can be a problem. We don't notice it particularly in women who are breastfeeding. It is often a problem for mothers who are expressing milk. If they don't get a let-down, they can have a great decrease in milk production.

It is very important to look at milk production from individual breasts, which relates to Dr. McLaren's first question. We did an initial study in Oxford on six mothers to determine if the interval between feedings was correlated with the amount of milk the baby took. If you combine the production of both breasts, there's no relationship. If you split the intake from the left and the right breasts, there is a very tight coupling. That is, the gap between feeds determines how much the baby takes from a breast. For the other breast, there is no such relationship. It was clear women showed "breastedness" in the same way they show handedness, but it wasn't linked to handedness, because we had five "left breasted" mothers and one "right-breasted" mother, but they were all right-handed.

So it's not clear what the relationship is. It was almost as if the control mechanisms were located in one side, as in true handedness. When we moved to Bristol, we studied 20 more mothers and found no such relationship. We found the relationship was with the first breast to be offered, which confused us somewhat. Therefore, we didn't publish the results.

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