Sex differences in body composition and serum metabolome responses to sustained, physical training suggest enhanced fat oxidation in women compared with men

Abstract

Sex differences in energy metabolism during acute, submaximal exercise are well documented. Whether these sex differences influence metabolic and physiological responses to sustained, physically demanding activities is not well characterized. This study aimed to identify sex differences within changes in the serum metabolome in relation to changes in body composition, physical performance, and circulating markers of endocrine and metabolic status during a 17-day military training exercise. Blood was collected, and body composition and lower body power were measured before and after the training on 72 cadets (18 women). Total daily energy expenditure (TDEE) was assessed using doubly labeled water in a subset throughout. TDEE was greater in men (4,085 ± 482 kcal/d) than in women (2,982 ± 472 kcal/d, P < 0.001), but not after adjustment for dry lean mass (DLM). Men tended to lose more DLM than women (mean change [95% CI]: -0.2[-0.3, -0.1] vs. -0.0[-0.0, 0.0] kg, P = 0.063, Cohen's d = 0.50) and have greater reductions in lower body power (-244[-314, -174] vs. -130[-209, -51] W, P = 0.085, d = 0.49). Reductions in DLM and lower body power were correlated (r = 0.325, P = 0.006). Women demonstrated greater fat oxidation than men (Δfat mass/DLM: -0.20[-0.24, -0.17] vs. -0.15[-0.17, -0.13] kg, P = 0.012, d = 0.64). Metabolites within pathways of fatty acid, endocannabinoid, lysophospholipid, phosphatidylcholine, phosphatidylethanolamine, and plasmalogen metabolism increased in women relative to men. Independent of sex, changes in metabolites related to lipid metabolism were inversely associated with changes in body mass and positively associated with changes in endocrine and metabolic status. These data suggest that during sustained military training, women preferentially mobilize fat stores compared with men, which may be beneficial for mitigating loss of lean mass and lower body power.NEW & NOTEWORTHY Women preferentially mobilize fat stores compared with men in response to sustained, physically demanding military training, as evidenced by increased lipid metabolites and enhanced fat oxidation, which may be beneficial for mitigating loss of lean mass and lower body power.

Keywords: endurance exercise; energy expenditure; lipolysis; metabolism; metabolomics.

Figure 1.

Experimental design. The 17-day military training was composed of 7 days of military skills training (Skills), including an abbreviated 2-day field training exercise (Mini-FTX), followed by 1 day of preparation (Prep), and culminated with a 9-day field training exercise (Full-FTX). Urine samples were collected throughout training in a subset of participants to assess total daily energy expenditure using doubly labeled water. Blood collection, bioelectrical impedance, and vertical jump measurements were conducted before (PRE) and upon completion (POST) of training. [Image created with BioRender.com and published with permission].

Figure 2.

Total daily energy expenditure (TDEE) during Cadet Leader Development Training (CLDT). TDEE was assessed via doubly labeled water in men (n = 37) and women (n = 18). Independent sample t test was used to compare energy expenditure between men and women. Bars represent mean and 95% confidence interval. Men had a significantly (*P < 0.001) greater TDEE compared with women (A). TDEE relative to initial dry lean mass (DLM) did not differ between men and women (B).

Figure 3.

Changes in body composition from pre- to post-Cadet Leader Development Training (CLDT). Body composition was assessed in men (n = 54) and women (n = 18) pre- and post-CLDT. Independent sample t test was used to compare changes in body composition between men and women. Mann-Whitney U test was used if assumptions of normality were not met. Lines and error bars represent mean change and 95% confidence interval. Individual data points are presented in gray. *Significantly different between men and women (P < 0.05). Men lost significantly more total body mass (A) and demonstrated a tendency to lose more dry lean mass (DLM) (B) than women. Changes in fat mass did not differ between sexes (C), though fat oxidation per kilogram (kg) of DLM, calculated as Δ fat mass/initial DLM, was greater in women than in men (D). Men and women exhibited minimal changes in total body water (E).

Figure 4.

Associations between declines in dry lean mass (DLM) and lower body power were assessed using Pearson’s correlation. Greater decreases in DLM were significantly associated (P < 0.05) with greater declines in peak lower body power (A) and average lower body power (B) following Cadet Leader Development Training (CLDT). Individual data points represent a single individual (n = 70). The solid green line represents line of best fit based on simple linear regression and shaded green region represents the 95% confidence interval.

Figure 5.

Changes in circulating sex hormones, free fatty acids (FFAs), glycerol, and leptin from pre- to post-Cadet Leader Development Training (CLDT). Hormones were assessed in men (n = 54) and women (n = 17) pre- and post-CLDT. Independent sample t test was used to compare changes in hormones between men and women. Mann-Whitney U test was used if assumptions of normality were not met. Lines and error bars represent mean change and 95% confidence interval. Individual data points are presented in gray. *Significantly different between men and women (P < 0.05). Men and women demonstrated no differences in changes of circulating estradiol (A) or progesterone (B). Men exhibited an increase in testosterone (T) (C), but not free testosterone (D), compared with women. No differences were observed in changes in glycerol (E) or free fatty acids (FFAs) (F). A greater decrease in leptin (G) was observed in women compared with men.

Figure 6.

Changes in plasma metabolites during military training. Metabolites were assessed in men (n = 54) and women (n = 17) before (pre) and after (post) Cadet Leader Development Training (CLDT). A: principal component (PC) analysis of all metabolites. B: PC plots of the log₂ fold change from pre- to post-CLDT for all metabolites. C: PC plot of lipid metabolites. D: PC plots of the log₂ fold change from pre- to post-CLDT for lipid metabolites. Individual data points represent the metabolome or changes in the metabolome within a single individual. Data points in closer proximity to one another are more similar.

Figure 7.

Changes in plasma metabolite profiles during Cadet Leader Development Training (CLDT). Orthogonal Projections to Latent Structures Squares Discriminant Analysis (OPLS-DA) (n = 71) (A) was used to identify the top 22 metabolites ranked based on the variable importance in projection (VIP) score (B). Arrows indicate direction of change from pre- to post-CLDT. Volcano plot (C) of significantly different metabolites having a fold change threshold of ≥ 2 (x axis) and t test Q-value threshold of ≤ 0.2 (y axis). Of the 860 metabolites identified, 160 metabolites significantly increased (green) and 114 significantly decreased (purple) from pre- to post-CLDT. ¹Compound that has not been confirmed based on a standard but identified with high confidence.

Figure 8.

Changes in circulating hormones correlate with changes in metabolites, changes in body composition (Body Comp), and energy expenditure (EE). Changes (Δ = Post – Pre) in circulating hormone concentrations were correlated with significant log₂ fold changes in metabolites of amino acid, carbohydrate, energy, lipid, and peptide pathways, as well as changes in body composition and energy expenditure during CLDT (n = 71) using Spearman’s correlation (ρ). P values were adjusted using the Benjamini–Hochberg correction (Q). Data presented are statistically significant correlations (P < 0.05, Q < 0.2). Inverse correlations indicate that a decrease in circulating hormones during CLDT was associated with a greater log₂ fold change in metabolites. No significant correlations were identified for ΔEstradiol (data not included in the figure). ¹Compound that has not been confirmed based on a standard but identified with high confidence. BF%, body fat percentage; DLM, dry lean mass; FFA, free fatty acid; FM, fat mass; FM/DLM, fat mass relative to initial dry lean mass; PRG, progesterone; T, testosterone; TBM, total body mass; TDEE, total daily energy expenditure; TDEE/TBM, total daily energy expenditure adjusted for initial total body mass.

Figure 9.

Changes in body composition correlate with changes in metabolites. Changes in body composition (Δ = Post – Pre) were correlated with significant log₂ fold changes in metabolites of amino acid, carbohydrate, energy, lipid, and peptide pathways during CLDT (n = 71) using Spearman’s correlation (ρ). P values were adjusted using the Benjamini–Hochberg correction (Q). Data presented are statistically significant correlations (P < 0.05, Q < 0.2). Inverse correlations indicate that a decrease in total body mass (TBM), fat mass (FM), fat mass relative to initial dry lean mass (FM/DLM), or body fat percentage (BF%) during CLDT was associated with a greater log₂ fold change in metabolites. No significant correlations were identified for ΔDLM, TDEE, TDEE adjusted for initial DLM, or TDEE adjusted for initial total body mass; data for these variables are not included in the figure. ¹Compound that has not been confirmed based on a standard but identified with high confidence. ²Compound that has not been confirmed based on a standard but identified with moderate confidence.