Rising global temperatures and increasingly frequent heatwaves pose a growing threat to human health. Understanding how humans tolerate environmental heat has therefore become a central challenge in environmental physiology. Building upon the scientific legacy of John R. Sutton, this narrative review examines the physiological determinants of human heat tolerance and their implications for health in a warming climate. We first consider approaches used to characterize thermal strain, highlighting evidence that body temperature measurements provide only a partial representation of whole-body heat storage and physiological burden. We then review advances in whole-body calorimetry that have improved understanding of thermoregulatory capacity and revealed substantial variability in heat dissipation associated with aging, chronic disease, biological sex, fitness, acclimation, and other modifying factors. Foundational concepts of compensable and uncompensable heat stress are discussed alongside recent efforts to define critical environmental limits and human survivability thresholds. Emerging evidence indicates that heat tolerance reflects dynamic interactions between thermal demand and heat dissipation capacity rather than a single physiological or environmental threshold. We further examine findings from prolonged heat exposure and heatwave simulation studies demonstrating that physiological strain may develop under thermally compensable conditions and accumulate across multiple systems even when the body is able to maintain thermal equilibrium. These findings highlight an important distinction between human survivability and liveability during extreme heat events. Collectively, the evidence supports an integrative framework in which human heat tolerance is viewed as a dynamic, multisystem process shaped by environmental conditions, exposure duration, recovery opportunities, mitigation strategies, and individual physiological capacity.
Intermittent normobaric hypoxia is used as a hypoxic-conditioning strategy in sport, exercise, and health settings, but its effects on cardiac autonomic modulation remain unclear. This review synthesised passive and exercise-combined protocols in adults. We searched 4 databases through March 2026 and reported the review according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement. Risk of bias was assessed using RoB 2 and ROBINS-I; findings were synthesised narratively, and certainty was graded for coherent evidence. Of 4,866 records, 18 studies were included, 12 arising from controlled parent designs. Controlled resting evidence for the root mean square of successive differences (RMSSD) comprised 3 comparisons involving 47 analysed participants. Of these, 2 multiweek comparisons (n = 34) showed numerically concordant between-group differences in RMSSD change, whereas 1 single-session crossover comparison (n = 13) did not establish a clear between-condition effect; certainty was very low in both strata. Frequency-domain heart rate variability evidence was sparse and methodologically unstable, baroreflex sensitivity findings were non-convergent, and exercise-combined evidence was insufficient for directional inference. Heterogeneous dosing, limited sham control, and limited respiratory monitoring restricted interpretation. Evidence remains insufficient to establish reproducible cardiac autonomic changes after intermittent normobaric hypoxia. Registration: PROSPERO CRD420261338890.
The Canada Prenatal Nutrition Program (CPNP), funded federally since 1995, aims to improve birth outcomes and breastfeeding among families facing health inequities through tailored, community-based delivery of perinatal health and nutrition services. To explore Ontario CPNP coordinators' perspectives and experiences delivering lactation and food supports, including changes over time, this qualitative study engaged coordinators working at multiple Ontario CPNP sites. A total of 28 individuals participated from 26 CPNP sites, 15 hosted by community agencies, 6 by Indigenous-led agencies and 5 by public health units. Focus group discussions were conducted at five locations across Ontario, audio-recorded, transcribed verbatim, and analyzed using reflexive thematic analysis. Three interpretive themes were generated. The first, overarching theme is that the CPNP delivery model is highly precarious due to limited, static funding, resulting in heavy reliance on external and in-kind resources; this affects all aspects of program delivery, including lactation and food supports. Second, accessible, tailored lactation support includes much more than education, and the CPNP is well-positioned to deliver this with adequate human and financial resourcing. Third, food supports are essential to the CPNP, contributing to both nutrition and social aspects of programming. Overall, lactation and food support delivery within the CPNP is responsive to client needs and the broader context of social determinants of health, but constrained by available resources. This study centres perspectives of community-based implementers of the CPNP, offering insights to inform planning, resourcing and programmatic guidance for community programs seeking to reduce health inequities during the critical perinatal period.
This study evaluated energy availability, menstrual function, bone mineral density (BMD), and female athlete Triad (Triad) risk among Black collegiate female track and field athletes. Study participants were female athletes (n = 18) on the track and field team at a Historically Black College/University. Energy intake and expenditure were estimated using an 8-day doubly labeled water protocol. Body composition was measured using dual-energy X-ray absorptiometry. Energy availability (EA) was calculated as (energy intake − activity energy expenditure)/fat free mass (FFM). Triad risk was assessed using female athlete Triad cumulative risk assessment. Survey responses captured eating behaviors, menstrual history, and food security status. Among these athletes, 56% exhibited EA below 45 kcal/kgFFM/day, and 17% were classified with clinical low energy availability (LEA) (<30 kcal/kgFFM/day). Forty-four percent of athletes met criteria for moderate Triad risk, though none were at high risk and none met criteria for low BMD. EA was inversely associated with body mass index (BMI) (r = –0.48, p = 0.04). Athletes reporting disordered/binge eating had significantly lower EA (p = 0.02). Food insecurity was identified among 22% of the study population but was not significantly associated with LEA or Triad risk. Over half of the Black female collegiate track and field athletes exhibited LEA, with nearly half meeting criteria for moderate Triad risk despite normal BMD and BMI. These findings highlight the need for routine screening and tailored interventions addressing LEA, menstrual dysfunction, and disordered eating in underrepresented athlete populations. ClinicalTrials.gov Identifier: NCT07631312.
