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?
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.