4.1 Sex differences in body composition changes

Following typical sex-distribution in human body composition (Wells, 2007), we found that females on average had significantly lower body mass, lower lean muscle mass, and higher body fat compared to males at all time points on the course. Participants of both sexes experienced overall declines in body fat as well as rebounds in body mass across the course. Given the energetically demanding conditions of the expedition-setting, it might be expected that there would be dramatic losses in body mass, body fat, and lean muscle mass, particularly if at least some individuals failed to consume enough rations to meet the daily energetic costs on the course. However, the relatively stable patterns we observed suggest that individuals may be avoiding a substantial negative energy balance and successfully acclimatizing to these conditions over the course duration.

At the end of the course, we found that there was no significant difference in participants’ body mass or lean muscle mass compared to before they started, but that they did experience a significant loss in body fat. These data showed that after initial losses in body mass (~0.45 kg) and body fat (2%), individuals were able to partially rebound over the length of their course, regaining any lost body mass and some body fat, though body fat was lost once more during the last section of the course. Lean muscle mass remained consistent throughout. The observed decreases in both body mass and body fat as individuals acclimatized to the course stressors in the first section (precourse to checkpoint 1) potentially reflect a negative energy balance where individuals were likely drawing on internal energy stores. The later patterns suggest that individuals eventually attained a positive energy balance to regain these lost stores. The lower body fat postcourse, commensurate to the initial loss in body fat, suggests that fat stores were spent once more to accommodate expeditionary energetic stresses in the final section. However, loss of lean muscle mass was prevented and (nonsignificantly) increased on average 2.5%, likely contributing to the overall body mass increase. When adjusting for activity difficulty in addition to sex and age, there was a steady increase in body mass and lean muscle in the middle of the course after the first section, but no significant change in body fat throughout the course. This suggests that the difficulty of the activities during each section of the course played a role in explaining at least losses in body fat. After the first section of the course, the middle sections may have provided a respite where individuals were back in an energy surplus and could regain lost body fat. Contrary to our prediction, however, we did not find that sex moderated changes in body mass, body fat, or lean muscle mass.

Most prior work assessing changes to body composition in similar but non-NOLS settings (i.e., mountaineering, backpacking, etc.) are limited to shorter-term expeditions, that last a couple weeks (Hamad & Travis, 2006; Zaccagni et al., 2014). However, across the course of ~3 months, if a population has acclimatized to their surrounding environment and has ample nutrition, they would likely return to their starting body mass and potentially exhibit readjusted body fat and lean muscle, as we see here. The rebounded body mass and redistribution of body fat and lean muscle suggests that study participants may have avoided some of the negative health and performance outcomes related to loss of strength and stamina that can accompany rapid and sustained weight loss in such settings. While past research in expedition settings have demonstrated that lean muscle mass tends to decline, often at substantial rates, and is exacerbated by concurrent intensive loss in body fat (Hoppeler et al., 1990; Tschöp & Morrison, 2001; Zaccagni et al., 2014). Our results instead show that across all individuals, lean muscle maintenance and even a transient gain when adjusting for section difficulty. These findings differ from earlier research with NOLS students on similar 90-day courses, where individuals experienced substantial and significant losses in body mass and body fat, particularly in ways that differed by sex (i.e., men lost substantial lean mass when in similar conditions; Ocobock, 2017). These differences were possibly due to higher energy availability during the course in the present study as well as a key difference in the populations’ essential fat profiles.

Up to 30% of the total group studied in Ocobock’s earlier study were at a critically low fat-mass nearing essential fat levels at some point during the expedition period. Essential fat, required for everyday function, is located in bone marrow, lungs, liver, kidneys, intestines, muscles, and central nervous system, and compromises around 3% of body mass in men and 9% in women (Wells, 2007). Comparatively, in the present study, only 2.7% of this group were at a critically low fat-mass at any point during the expedition. Further, in response to Ocobock’s work in 2013, NOLS altered the food rations for all participants to include macronutrient-dense snack foods that students preferred to eat and initiated an education campaign to teach instructors how to explain complete proteins and coach students to eat all their rations (particularly the less popular protein-rich foods like powdered milk and eggs). This likely had an impact on overall energy balance and increased individual energy intake even in light of the potential appetite suppressant effects of expeditionary activity. The changes in expeditionary diet and education instituted by NOLS following Ocobock’s study may have led to a higher energy availability during the course for all individuals. Since extensive fat mass loss to the point of reaching essential fat levels puts muscle mass at risk, the minimal fat loss in this sample allowed for rebounds in body mass and fat as well as muscle maintenance and even gain despite section difficulty. Our findings suggest that with minimized negative energy balance across the acclimatization period, deleterious body mass and muscle mass loss can be protected against, even under intensive expedition conditions. They also highlight the importance of adequate energy availability, which can inform preparing individuals for similar energetically demanding conditions, such as in high altitude/cold environments or climate-devastated regions, with the aim to protect health and performance. If additional energy availability can buffer against substantial and sustained body mass and lean muscle mass loss, it can potentially prevent consequences associated thereof, from decreased capacity of major activity and reduced bone density due to muscle deterioration to deleterious effects on immune system and cognitive function associated with excessive body mass and fat loss (Kell et al., 2001; Khodaee et al., 2015; Vermeiren et al., 2016; Yamaner et al., 2015).

4.2 Physical activity intensity and energy expenditure during the course

Physical activity intensity. There was no significant difference in individual’s average %MVPA between early and late in the course. Though the ecological conditions may have changed across the different sections and the final section was consistently ranked as more physically intensive and difficult, our findings suggest that the intensity of activity did not change in any significant way (as discernable via our measures). The lack of difference in %MVPA is surprising due to anecdotal reports about the intensity of the final sections (i.e., winter camping sections) from students participating in NOLS courses. Participants reported that they perceived these final sections as harder and more activity intensive than earlier sections given the additional work needed, including setting up camp in the snow or cross-country skiing from site to site. These findings may indicate the early sections of the courses, including hiking and mountaineering sections, could potentially have been equally physically intensive as later canyoneering or winter camping sections. Further, the relatively consistent %MVPA during the first and last sections and the energy status rebounds in the middle sections (i.e., increases in body mass and fat) support the notion that the first and last sections of the courses were likely more physically intense than the middle sections of the courses. Future studies in this setting would benefit from routine measurement of %MVPA across the entire expedition to capture these potential differences. Alternatively, because we measured physical activity using the Actigraph via tri-axial motion, these findings may demonstrate that there is not enough variation in motion across all axes during the different conditions (terrains/activities/etc.) to distinguish significantly different activity intensity at the beginning and end of the course.

Energy expenditure. We found that across all individuals, there was no significant difference in TEE (kcal day⁻¹) from early to later in the course. Recent research by Ocobock (2016a, 2020) in similar expedition settings suggests that as relative “newcomers” to this kind of highly challenging environment, the continuous exposure to high levels of physical activity while undergoing novel and intense ecological stress, leads to sustained total energy expenditure at very high levels, well over ~4000 kcal day⁻¹, over weeks and months without any tapering of energetic costs (Ocobock, 2016a, 2020). Averaging across, subjects in our study maintained estimated TEE at ~4000–6000 kcal day⁻¹ from early to late in the course. While our estimates cannot take into account changes in physical fitness, such as reduced energetic cost per hour or cost per mile while hiking, elevated TEE at these levels at the beginning and end of the course seen across all individuals is comparable to other intensive expedition settings (Hattersley et al., 2020; Ocobock, 2016a). While possible that the unmeasured middle sections of the course may have had declines in TEE, particularly given the rebounds in body mass and body fat, and even gains in lean muscle mass when adjusted for activity difficulty, these data show that over ~3 months, TEE levels still reach elevated levels, even well after initial acclimatization. These sustained levels of TEE are much higher than what would be estimated given assumptions of human energy expenditure. For example, the Constrained Energy Expenditure model predicts that energy expenditure eventually attenuates over time to a range of ~2500 kcal day⁻¹ as the body brings itself to an evolved, relatively restricted range of total energy expenditure, even as physical activity demands persist (Pontzer, 2015a, 2015b; Pontzer et al., 2016). However, given that individuals were regularly changing activities, it is not unreasonable for TEE estimates to remain high. In addition, the regular change in ecological conditions/temperature that could not be controlled for in this study, and the associated changes in individual thermoregulation thereof, could have also contributed to the relatively high TEE observed in participants. In contrast, research on trained individuals doing the same intensive activity (i.e., running) every day without substantial environmental change demonstrated comparably high TEE followed by an eventual attenuation in expenditure (Thurber et al., 2019). Our findings potentially shed light on the differences in TEE between habitual activity over a long period of time by trained individuals versus habitually high physical activity levels and TEE but doing nonhabitual activity by untrained individuals.

Notably, our data did show that sex moderated the change in TEE. Females demonstrated a modestly negative slope whereas males exhibited comparatively increasing TEE from the beginning to the end of course. In the pairwise-comparison following the significant interaction of sex and time point and adjusting for relevant covariates, we found that males, on average, expended significantly more energy (+1085.83 kcal day⁻¹, ~13.10% increase) later in the course than they did early in the course. In contrast, females, on average, expended less energy, spending 361.07 kcal day⁻¹ less late in the course compared to earlier in the course, though this decline was not significant.

Particularly when adjusted for total body mass, these findings introduce a potential sex-driven nuance in how average energy is spent across the length of a course. Given the baseline differences in BMR and body composition, understanding how sex may influence energy expenditure during and after acclimatization is of particular interest. Males and females differ in starting stored energy reserves and metabolic mechanisms and these differences are then potentially exacerbated through the acclimatization period and thereafter. These findings support the need to generate a series of new hypotheses to further explore and test how sex may moderate energy expenditure in extreme and energetically demanding conditions. While these differences could possibly hint to behavioral rather than physiological differences between males and females, where males may spend more time in intensive activity at the end of course and thus spend more energy per hour, this is unlikely. Given that there was no significant difference in activity intensity between males and females at any timepoint, this would suggest that differences in TEE were not due to physical activity, notwithstanding any limitations in our measurements.

Our data demonstrate a slight decline in TEE in females when comparing early to late in the course. This could be a reflection of energy expenditure levels reaching a constrained energy asymptote as theorized by Pontzer (Pontzer et al., 2016). However, female TEE levels later in the course are still relatively more elevated than what would be expected if constrained, averaging well over ~2500 kcal day⁻¹. This alternatively reflects a training or acclimatization effect in females resulting in greater efficiency by the end of the course.

Compared to earlier studies in this setting, these data indicate a population that is in energetic flux rather than in a negative energy balance. This additional energy availability likely changes the observed energy status and expenditure in this group since it protects against substantial body fat and lean muscle breakdown. In the previous population, where males were reaching essential fat levels and losing body fat and muscle, TEE was comparable to female TEE. Here, we potentially see that with enough energy availability and fat to protect against muscle catabolism, males exhibit even higher TEE. In comparison, possibly after initial losses in energy stores, female bodies may buffer against future losses by not ramping up TEE even as energetic resources are available. This could support a possible buffering capacity female bodies have in response to energetically demanding environments. Prior research has posited the benefits of higher body fat percentage of female bodies as protective under energetically demanding conditions like famine or cold stress (Castellani & Young, 2016; McArdle et al., 1984; Norgan, 1996; Toner et al., 1986; Wells, 2007), but our data may point to differences in how energy is allocated and expended as well. This buffering mechanism seen in female bodies in a challenging environment is likely the result of evolutionary processes. Maintaining energetic stores, is necessary for fertility and beneficial to ongoing fetal growth and lactation (Ellison, 2017; Jasienska, 2009; Vitzthum, 2008; Wells & Stock, 2007). This has in turn shaped human sexual dimorphism in body composition, where females tend to have greater body fat on average (Wells, 2007). Though it is important to note that while these reproductive needs may have shaped our species through evolution, in these kinds of energy intensive settings in particular, female bodies do more than just reproduce—it is worthwhile thinking of how these buffering capacities may play a role in everyday function when states like pregnancy and lactation are unlikely and even unwanted. This also speaks to the seemingly contradictory evidence between male and female energy expenditure in expedition settings—in intensive, yet energy replete settings, untrained male bodies may continue to spend energy as resources are available, while untrained female bodies may continue to be more conservative in energy expenditure to prepare and adjust for potential future energy deficiencies.

These modest sex differences may have important implications for strategizing how to best situate mixed-sex groups in extreme environment settings, particular in what energy budgets should be anticipated and costs thereof as individuals acclimatize over an extended period of time. In addition, here, sex is categorized dichotomously into two groups based on participant response. The variable of sex in humans is far more varied than two discrete categories (DuBois & Shattuck-Heidorn, 2021; Fausto-Sterling, 2012) and while we report the averages of the two groups, we currently do not know if the acclimatization process would further differ across the spectrum of sex. In addition, while male and female are important categories, training and background also shape performance and acclimatization, for example, highly trained elite females cannot be compared to males that are expedition novices (Bassami et al., 2007; Trapp et al., 2007; Westerterp, 2017). Our work here, however, demonstrates some important variation between sex in untrained individuals. This may begin to expand on the similarities and differences across individual bodies that are salient and relevant to potential functional outcomes.

Limitations. Because of field-based time restrictions, we were unable to collect exact energy intake and there was only a rough estimate of the foods consumed (energy) all participants had access to. All participants took a standard amount of food with them on each section of the expedition, within the cook groups (usually groups of 3–4) and there was general variation in what percentage of total food as well as individual preferences in the content of what people ate. This data was collected but not reported reliably, which is a known problem for self-reported energy intake data (Schoeller, 1995; Snodgass et al., 2006). However, through consistent measurements of body composition and the changes throughout the course, here we infer energy status (Ellison, 2003; Ocobock, 2020; Thurber et al., 2019). Future work testing these changes in similar settings or assessing the specific impact of energy intake could benefit from measuring specific caloric and macronutrient consumption.

Our study design also captured actigraphy data, including physical activity intensity and total energy expenditure data from a subset of the entire population at the beginning and ending of the course to specifically look at acclimatization at the beginning and end of exposure to such an environment. However, given that our findings suggest that the middle sections may have been less physically and energetically intense, it would be worthwhile to capture these sections in future studies. The activities during the first and last sections were programmed into the NOLS curriculum to facilitate specific skill-development, and thus the activity intensity and TEE may have been greater at the beginning and end of the course based on the requirements needed for course logistics. However, during the sections there was between-individual variation that we were able to capture to estimate physical activity intensity and TEE. Since estimates of physical activity intensity are based on motion, our measurements do not account for factors like increased fitness gains. We adjusted for the kinds of activities and how strenuous they are by accounting for their difficulty in the models. However, since our measures were estimates of TEE and heart rate data were not calibrated to a treadmill test, we recommend more research is required to parse out the specific relationship between physical activity intensity and TEE when individuals are acclimatizing. We recommend further research is needed to measure activity and energy expenditure at multiple points throughout an exposure/expedition period, across a greater number of individuals, as well as more robust measures of individual changes in fitness in order to ascertain specific and precise changes in physical fitness, strength, and fatigue. Finally, this study would have benefited from collecting energetic data via the Doubly-Labeled Water (DLW) method, largely accepted as the gold-standard for all TEE measurements (Dugas et al., 2011), though our work here shows a potential for using a relatively less cumbersome method to estimate TEE. However, it is important to note that while the average error for the flex-HR method is relatively small, it can overestimate individual TEE up to 17% particularly at higher levels of energy expenditure (i.e., >3000 kcal day⁻¹) (Leonard, 2003; Ocobock, 2016b). If our flex-HR estimates were inflated for higher values, it would suggest that TEE on average across the whole group would be slightly lower than seen in other shorter-term expedition settings, though still higher than the constrained level ~ 2500 kcal day⁻¹ (Hattersley et al., 2020; Pontzer et al., 2016). Post-hoc analysis suggests that this overestimation does not seem to impact the patterns of difference between male and female TEE observed in this data set. Regardless, future research would benefit in measuring a larger percentage of the total sample size and include DLW measurements in order to increase statistical power, model accuracy, and measurement precision.