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1. National Academy of Sciences. Nutrition during lactation. Washington, DC: National Academy Press, 1991.
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34. Harzer G, Haug M, Bindels JG. Biochemistry of human milk in early lactation. Z Ernahrungswiss 1986;25:77-90.
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Dr. Garza
Dr. Prentice, I also am intrigued by the calcium data. If you look at casein levels, are they comparable between the Gambia and Britain, or is the casein just not saturated? (I know that is not an appropriate term.)
Dr. Prentice
Clemens Kunz of the University of Dortmund in Germany measured the casein levels for us not long ago, and they seem to be very similar in American and Gambian mothers. In human milk, however, the main vehicle for calcium is citrate, not casein, and we are concentrating on the citrate levels first before going for the casein, as we are not expecting to see any major differences there.
Dr. McNeilly
Dr. Prentice, I have learned a lot. Could you tell me if nutrition might affect the rate of milk synthesis, or is that not shiftable?
Dr. Prentice
Dr. Hartmann could probably answer that question better than I. The only information we have is from our Gambian supplementation study, in which both unsupplemented and supplemented women produced the same amount of milk per day, but the supplemented women fed fewer times per day. So we think that it quite possibly has something to do with the mother's nutritional state, not necessarily with substrate supply affecting the storage capacity, as Dr. Hartmann said. Perhaps the unsupplemented women had smaller storage capacity and had to feed much more frequently during the day to produce the same volume. This is speculation, because we have not done any work on it. Have you any comment, Dr. Hartmann?
Dr. Hartmann
Yes, I think that when substrate supply is limited, the babies have to feed very frequently, regardless of the storage capacity, to keep the rate of synthesis at the maximum. Providing the mothers with more energy from the diet somewhat increases their ability to produce milk. Then the baby might not suckle as frequently to get the volume of milk that it requires, so I don't think it is inconsistent.
Dr. Rasmussen
I would like to contribute the results from a supplementation trial in Guatemala that have been presented at meetings but haven't yet been published. We selected women at the end of pregnancy and the beginning of lactation based on mid-calf circumference, not on body-mass index (BMI), because BMI has the properties that you mentioned. In that supplementation trial, we gave mothers either a low-energy or a high-energy supplement; we didn't have a no-treatment group, because we didn't consider it ethical, given the state of the population. The women in the high-energy group produced more milk, and-of particular importance to this meeting-they exclusively breastfed their babies for a longer period. We were not in the field long enough to ask if there was an effect on fertility in these women (in other words, on the length of their amenorrhoea), but I think there were clearly changes in the way the women breastfed as a result of changes in their milk volume. So I think there may be important effects of supplementation that we may not have seen in the trials that we have had today.
Dr. Prentice
I don't want to detract from that at all, but I still feel that we may be talking about two different things: the biological limits on lactation and the whole area of the mother's well-being-the social factors that affect the way she and the baby interact to increase milk volume. It seems from the work we have been doing that most women can produce reasonable volumes of milk, and if they don't, there are other factors that are probably affecting it other than the capacity of the breast in most instances. I wanted to ask you if you had any macronutrient content data from the breastmilk-that was my brief this morning. Does it affect the composition of the milk?
Dr. Rasmussen
The only component we have looked at is energy. There are compensatory changes in energy, such that women who produce more milk have less energy-dense milk The effects on infant growth are expected to be minimal. But our statistical power was not calculated on measuring infant growth.
Dr. Prentice
I think I would have predicted that. In our supplementation studies we found that if we gave women essentially a higherfat diet, there was more fat in the breastmilk, but lactose was reduced. This has been seen in quite a lot of other studies as well. Thus, the mother's breastmilk fat concentrations may well be responsive either to her own diet or to her adipose tissue stores. We have seen it, other people have seen it, but some other studies have not, and so it is still relatively controversial. But whatever happens, total milk energy seems to stay remarkably similar.
Dr. Hartmann
Could I add that we looked at some immunologic factors in the same material, and lactoferrin remained the same in both groups.
Dr. Garza
What was the content of fat in the Gambian diet?
Dr. Prentice
It's about 20% to 25%.
Dr. Garza
In studies done with Salvador Villalpando and Nancy Butte in Mexican women consuming diets of lower fat content, we found much lower levels of fat in human milk that were not fully compensated for by changes in lactose.
Dr. Prentice
That were nor compensated for?
Dr. Garza
That's right. We saw the same directional changes you did, but the reduction in fat was much greater than the increase in lactose, so that in populations with very low levels of fat (in the 10% to 12% range), the breastmilk fat content may be influenced dramatically.
Dr. Prentice
I certainly could believe that fat intakes might be limiting at that very low level. Did you find a difference in the shift between the manufactured fatty acids and the dietary fatty acids?
Dr. Garza
We didn't look at fat composition. Dr. Villalpando, has fat composition been examined since those earlier results were available?
Dr. Villalpando
We are looking at those data now. We see an increase in the medium-chain-length fatty acids, but the milk energy increase was not that impressive.
Dr. Prentice
Mike Crawford long ago suggested the same thing from Tanzanian studies, but there the total fat tended to stay the same but with larger amounts of medium-chain-length fatty acids.
Professor Howie
I apologize that this question is a little lateral to what you have been talking about, but I want to ask about the concept that breastfeeding mothers may not have to increase their own diet in proportion to the amount of milk they are giving the baby. In other words, if a mother is breastfeeding, the total energy cost to the community is less than if she were feeding herself and her baby by artificial means. That concept was discussed quite a bit a year or two ago, and I wonder if you have any comments on that and on its potential importance for nutrient provision.
Dr. Prentice
I think I would still say the same thing. There do appear to be women who are not able to increase their food intake, that is, their energy intake, and there do seem to be relatively simple compensations that can occur: a reduction in activity, for example. We are still not really sure that in lactation there is much in the way of changes in costs like basal metabolic rate so on, although there may well be in pregnancy, but fairly minor and adjustments may be satisfactory to enhance efficiency. These include accessing energy from fat stores; many, but not all, women in that situation will lose some of their fat stores while they are lactating. Certainly, in the Gambia some women put on weight during lactation at the time of year when food in good supply. But they are obviously eating enough is for both weight gain and milk production. So, yes, I think we would still be saying the same, that there are compensations for the energy needs of lactation.