For all animals, reproduction entails an investment of time and energy. Among humans, lactation (i.e., breastfeeding) is, perhaps surprisingly, the most metabolically expensive phase of the entire reproductive cycle. That is, mothers need more energy (i.e., calories) per day to produce breast milk at peak lactation than they do during the pregnancy itself, even the third trimester. Nevertheless, it is still unclear how women in different environments and socioeconomic circumstances are able to meet this additional energy demand.
There are several potential (and not mutually-exclusive) ways that humans can meet the energy costs of lactation. First, mothers could take in more energy, either by consuming more food or food of a higher caloric density. However, this is unlikely to be a realistic option for women in many societies, where access to food may require cash and/or physical labor, or where food supplies may be scarce, to begin with. In other words, most mothers are probably unable to increase their energy intake on demand.
Second, women could reduce their energy expenditure in physical activity by moving around less, and spend more time sitting or resting. Again, however, this may not be a viable option for many women around the world, whose physical activity levels may be constrained by the demands of wage labor, childcare, and other domestic tasks. Most mothers probably do not have the luxury of voluntarily reducing their activity level during lactation.
Third, women could mobilize their own bodily energy stores to support the metabolic demands of lactation. In other words, women may sacrifice their own body tissues, breaking down fat and/or muscle to provide the fuel necessary for generating breastmilk. Overall, given what we currently know about the energetics of lactation worldwide, these potential strategies don’t quite add up. Although all of these strategies have been observed among lactating women in various populations, the sum total of energy savings from these strategies, to the extent that they can be assessed in field research, does not seem sufficient to fully meet the energy demands of lactation. This suggests that an additional energetic mechanism may be involved.
Another possibility is that women undergo a metabolic shift during lactation, resulting in increased energy efficiency during physical activity. Energy efficiency refers to the ratio of work output to energy expenditure. A more energy-efficient system – such as a car engine or a living organism – can accomplish more work for a given level of energy expenditure (i.e., fuel consumption). There is evidence, primarily from the field of exercise physiology, that individual humans differ in energy efficiency during different levels of physical activity. There is also some evidence that individual energy efficiency can change over time, due to athletic training, for example. This opens up the possibility that an increase in energy efficiency could help women meet the increased energy demands of lactation.
In 1998, a cross-sectional study conducted in the city of Cali, Colombia, showed that energy efficiency was significantly greater in a sample of 29 lactating mothers compared to 109 non-lactating women. Following up on this work, we conducted a longitudinal experiment in Boulder, Colorado to assess the effect of lactation on women’s energy efficiency. To accomplish this, we recruited a sample of 62 healthy women aged 20-38 years: 33 lactating mothers and a control group of 29 non-lactating women.
We brought the lactating women into the laboratory twice for measurements, once at peak lactation (about 3.5 months postpartum) and again at post-lactation (one year later). This enabled us to observe changes within individual women over time. The non-lactating women in the control group were also measured twice, at a baseline visit and at a follow-up visit an average of 8 months later. Our experimental design of longitudinal measurements with a concurrent control group provided us with a statistically-powerful way of evaluating changes in energy efficiency due to lactation, rather than to incidental random differences between sample groups.
First, we measured women’s energy expenditure during various levels of exercise using a method called gas-exchange indirect calorimetry: by simultaneously measuring a person’s total inhalation of oxygen and total exhalation of carbon dioxide over a set time period, one can calculate that person’s caloric expenditure over that time period. Participants began the protocol by pedaling at a constant cadence on a stationary exercise bike with no resistance, i.e., zero workload. While the participant maintained her constant pedaling cadence, the resistance was increased to a workload of 15 watts, then successfully increased at 15-watt increments every 3 minutes up to 75 watts. By comparing how women’s caloric expenditure rose in relation to increasing work output, we were able to calculate that individual’s energy efficiency. Just as a fuel-efficient vehicle requires less gasoline to travel a given distance, so too does a more energy-efficient human require less caloric expenditure to accomplish a given workload.
Contrary to the previous Colombia study, we found no difference in efficiency between lactating women and the non-lactating control group. We also found no change in efficiency within the lactating group from peak lactation to post-lactation. However, the lactating women did show greater variability in efficiency between laboratory visits than did the non-lactating control group. Additionally, we found that 79% of the lactating women lost weight over the course of lactation, with an average weight loss of 3.6 kg. This was consistent with the idea that women mobilize their own bodily energy stores to meet the metabolic costs of lactation, even in this healthy and well-nourished sample from Boulder, Colorado.
In sum, we found no evidence of an increase in women’s energy efficiency to support the costs of lactation. We note that if we had in fact found such evidence, we would have been faced with an additional question. If energy efficiency is physiologically malleable throughout an individual’s lifetime – as demonstrated in trained athletes, for example – then why not stay maximally efficient all the time? Presumably, there must be a trade-off between increased energy efficiency on the one hand, and a detriment to some other physiological function on the other. Unfortunately, we are not in a position to determine what such a trade-off might be since this issue has received little attention in the scientific literature and we lack data to suggest a specific mechanism in our present research. So, theoretical questions remain as to how increases in energy efficiency would be physiologically compensated for.
Overall, we found that women employ multiple strategies to meet the energy costs of lactation. Some women showed an increase in energy efficiency during lactation, while others showed a decrease. Most women lost weight during lactation, while others gained weight. From an evolutionary standpoint, these findings are not surprising. Every stage of reproduction – from initial fertility to pregnancy, to birth, and through lactation – entails an investment of time and energy. Each of these investments is more or less sensitive to energy availability. Unlike other reproductive stages, lactation is relatively buffered against energy stress. When faced with insufficient energy, mothers generally do not cease producing breastmilk. Instead, they employ other strategies, including sacrificing their own body tissues and maintenance functions, to maintain the costly investment in lactation to support infant growth. It appears that once the initial investment in reproduction is made, mothers’ physiologies will tenaciously devote resources to sustain their offspring – including support after birth via costly lactation.
In any case, it is clear that more research is needed to understand the varied and complex ways that women meet the substantial energy costs of lactation, particularly in the context of undernourished women, women experiencing socioeconomic stress, or women who are balancing lactation with heavy physical demands.
These findings are described in the article No change in energy efficiency in lactation: Insights from a longitudinal study, published in the American Journal of Human Biology. This work was led by Darna Dufour from the University of Colorado Boulder.