Human milk is, on average, approximately 85-90% water (Hinde
and Milligan, 2011), reflecting our long evolutionary history as primates with
dilute, high sugar milks. The reference composition data for milk describes
milk as approximately 4.0% fat, 1.2% protein, and 7.2% sugar, although both
individuals and populations will vary in the distribution of these
macronutrients.
Human milk is considered dilute when compared to that of
other mammals. The best interpretation of this comes from the work of Ben Shaul
(1962) who described humans as a high contact, high frequency nursing species.
Other primates, with a few exceptions, also have dilute milks (Hinde and
Milligan, 2011). As with humans, the
dilute composition of primate milk is thought to be driven by distinct patterns
of infant care – high levels of contact, high nursing frequency, and low volume
transfer per bout. Environmental factors, such as dry and arid environments and
elevated rates of whole body water turnover, also predict dilute milks (an
example would be camels).
However, within human populations, much less is known about
the ways in which the environment may influence the composition of milk,
particularly the amount of water in the milk. For many species, geographical
ranges are quite limited, and there is little variation in ecological stressors
such as temperature or aridity. Humans
are among a small number of primates with a wide geographical distribution, and
occupy a diversity of environments. These environments present any number of
ecological stressors (temperature, aridity, altitude, humidity, pathogens) that
may influence breastfeeding behaviors or milk composition.
Human infants are at increased risk of dehydration compared
to older children and adults. Infants have higher whole body water turnover
rates and immature kidney function, limiting their capacity to reserve water
should intake decrease or water loss (often associated with diarrhea)
increase. However, the majority of
discussions on milk composition and breastfeeding behaviors have largely
ignored maintaining infant hydration as a possible selective pressure (Bentley
1998). Is it possible that hydration has been an important factor in shaping
breastfeeding behaviors, and maybe even some aspects of milk variation, in
humans?
“We may conjecture that at different climatic extremes
different nutritional priorities are placed upon breastfeeding. So in tropical
or arid conditions, one might presume that high evaporative water loss would
place the emphasis on maximizing water throughput to prevent dehydration.” –
Michael Woodridge (1995).
Looking at population diversity in nursing frequency among
traditional foraging societies, Woolridge (1995) reports increased nursing
frequency among mothers living in arid or semi-arid environments. One of the
best examples of this are the !Kung, a population of foragers living in the
Kalahari desert (Figure 1). The !Kung have very high nursing frequencies – on
the order of 4 times an hour! Other foraging populations living in arid or semi-arid
conditions also show increased nursing frequency, although there appears to be
variation within these climate extremes.
Infants who feed with a high frequency but short duration
may be biologically altering the composition of the milk. Daly et al., (1993)
and more recently Kent et al., (2006) have shown that the amount of fat in milk
(they did not measure milk lactose) changes based on the frequency of feeding.
More frequent feeds predict lower overall change from the end to the beginning
of the next feed, and closely spaced feeds may have an overall effect of
maximizing water transfer and reducing the amount of fat in the milk. The
infant must nurse more frequently to meet energy and fat needs and may, over
the course of the day, take a large quantity of milk.
Biologically, this might be incredibly important in thinking
about the ways in which differences in milk composition are often interpreted
as differences in milk quality, especially in older published studies. Discussions
of human milk quality are often synonymous with energy and fat, completely
ignoring the fact that in certain environments where dehydration is a risk and
total body water turnover rates high, low fat milk with a high water content might
be adaptive. This milk will keep the infant drinking regularly, and may be
important for maintaining physiological well-being and preventing dehydration. As
discussed above, total daily fat intake by the infant should be equivalent to that
reported in other populations because of the overall increase in milk volume
consumed by the infant. However, if we were to simply look at the milk
composition without the added context of nursing frequency or total volume of
milk consumed over the course of the day (and these are separate issues), it might
appear that the infant was not getting enough to eat.
 |
| Figure 1: A Bedouin mother struggles with a tent and a baby. Image:AFP, by way of http://womennewsnetwork.net/2012/07/03/rising-number-bedouin-women-enter-work-force/. |
One other interesting piece of support for this dehydration hypothesis comes from
studies looking at milk composition across seasons, comparing summer and
winter, or rainy and dry seasons for example. Few studies are available, and
observations are confounded by the fact that in many tropical and sub-tropical
populations, seasonal workloads and pathogen risk will vary greatly. Yagil
(1986), in a sample of Bedouin mothers, reported a modest increase (+4%) in the
percentage of water in milk during the summer compared to the winter (and a
decrease in milk fat), although the collection methodology was somewhat
problematic.
While the case is far from solved – and really, the
population level data on aridity and breastfeeding frequency may not exist – it
remains an interesting hypothesis for some of the possible variation in milk
composition and breastfeeding behaviors. Moreover, it may be important to think
about changes to milk composition across seasons – is milk more dilute during
hot or arid months, promoting increased nursing to maintain hydration? Is some
of the regional variation in human milk composition and nursing frequency
driven not by hunger but by thirst?
References
Bentley, G.R. (1998), Hydration as a limiting factor in
lactation. Am. J. Hum. Biol., 10: 151–161.
doi: 10.1002/(SICI)1520-6300(1998)10:2<151::AID-AJHB2>3.0.CO;2-O
Daly SE, Di Rosso A, Owens RA, Hartmann PE. (1993) Degree of
breast emptying explains changes in the fat content, but not fatty acid
composition, of human milk. Exp Physiol. 78(6):741-55.
Hinde, K. and Milligan, L. A. (2011), Primate milk:
Proximate mechanisms and ultimate perspectives. Evol. Anthropol.,
20: 9–23. doi: 10.1002/evan.20289
Kent JC. (2007) How breastfeeding works.J Midwifery Womens
Health. 52(6):564-70.
WoolridgeM(1995) Baby-controlled breastfeeding: Biocultural
implications. In P Stuart-Macadam and KA Dettwyler (eds.): Breastfeeding:
Biocultural Perspectives. New York: de Gruyter, pp. 217–242.
Yagil R, Amir H, Abu-Rabiya Y, Etzion Z (1986) Dilution of
milk: A physiological adaptation of
mammals to water stress. J. Arid Environ.
11:243–247.
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