We hypothesized that competition diets would lead to changes in serum hormones, BMD, performance, and mood in both sexes. As expected, significant changes in serum hormone levels were observed, indicating energy deficiency, and these changes were in part sex-specific. While mood and strength changes were observed, no changes in BMD were detected. A primary finding was the reduction in IGF-1 and its binding protein IGFBP-3 among both sexes during the competition preparation, suggesting that these may serve as potential biomarkers for monitoring physiological stress (Angelidi et al. 2024).

Insulin-like growth factor 1, a crucial hormone for muscle development (Yoshida and Delafontaine 2020), significantly decreased in the female and male COMP groups. Similarly, IGFBP-3, which modulates IGF-1 bioavailability, also decreased. This observation aligns with Mitchell et al. (2018), who reported similar reductions in IGF-1 among male physique athletes during competition preparation. Nutrition plays a crucial role in regulating circulating IGF-1 levels, with energy restriction profoundly impacting serum concentrations (Henning et al. 2014). In this study, the reduction in IGF-1 and IGFBP-3 may be attributed to reduced EI and LEA during the competition preparation. We previously reported an association between IGF-1 levels and EA at baseline before the competition preparation among a larger sample of physique athletes (Mursu et al. 2023).

We observed correlations between IGF-1 levels and changes in FFM in male and female COMP. This supports the argument that IGF-1 may be a potential biomarker to monitor physiological stress during the intense training period and nutritional changes that alter body composition, such as physique competition preparation. It also aligns with previous studies in soldiers; Nindl et al. (2007) demonstrated that IGF-1 and IGFBP-3 align with energy deficits and body composition changes, with free IGF-1 positively correlating with changes in body mass and FFM. Additionally, IGF-1 has a role in facilitating protein synthesis and muscle growth, and low levels of IGF-1 are associated with reduced BMD (Yan et al. 2016).

There was LEA during competition preparation in both female and male COMP groups with no significant sex differences. Despite LEA, no significant changes were observed in BMD, aligning with findings by Schoenfeld et al. (2023) in their systematic review of competitors during preparation for physique competitions. However, a study on elite endurance athletes revealed that LEA can reduce BMD (Heikura et al. 2018). Furthermore, Ihle and Loucks (2004) conducted a 5-day intervention manipulating EI in young females, resulting in significant effects on bone formation and resorption. Our results suggest that a 21-week weight loss period may not, on average, negatively impact bone health in physique athletes despite low EI and reduced IGF-1 levels. One explanation for this may be that physique athletes engage in extensive strength training, which is known to protect bone mineral content and density by stimulating new bone formation in areas subjected to mechanical strain (Pal et al. 2023).

We observed a significant reduction in insulin in male COMP, with a similar but not statistically significant trend in females. This aligns with Mäestu et al. (2010), who documented significant insulin declines in male bodybuilders during competition preparation. On the other hand, this was not observed in the study by Mitchell et al. (2018) who found no significant changes in insulin levels. The decrease in insulin was accompanied by reduced blood glucose as reported by us earlier from this cohort. (Jouhki et al. 2024).

Sex hormone changes among physique athletes appear to be driven by sex-specific physiological responses to energy deficits. In male COMP, T and FT decreased, consistent with hormonal adaptation patterns observed by Mäestu et al. (2010). These decreases are likely due to hypothalamic–pituitary–gonadal (HPG) axis disruptions. At the same time, males often experience more pronounced disruptions in this axis under energy deficits than females, leading to greater reductions in T (Pasiakos et al. 2019). No significant changes in T and FT were observed in female COMP, highlighting the different physiological responses to competition preparation. This finding contrasts Hulmi et al. (2017) and Mero et al. (2010), who reported decreases in T among females under weight loss conditions. One explanation for the potential sex differences might be that males exhibit reductions in T and FT due to their higher initial levels (and possibly greater reliance on these hormones for muscle maintenance), while females might experience changes in menstrual cycle regularity and function due to changes in estrogen levels rather than T during the weight-loss period (Williams et al. 2015). T and FT levels returned to baseline in male COMP after recovery.

SHBG increased in male and female COMP during competition preparation. Significant weight loss achieved through reduced caloric intake has previously led to increased SHBG in both female (Mero et al. 2010) and male athletes (Karila et al. 2008). Elevated SHBG levels suggest that less FT is available to tissues because more T is bound to SHBG, with approximately 65% of T being bound to SHBG (Allen et al. 2002). When high SHBG concentrations are combined with lower T levels, it likely indicates a lower availability of FT. An earlier study conducted in our laboratory reported that during high-volume strength training periods, decreases occurred in serum T and in the T/SHBG ratio in male weightlifters, with individual changes in the T/SHBG ratio correlating with changes in weightlifting performance (Häkkinen et al. 1987). Additionally, baseline serum basal T concentrations in males (Ahtiainen et al. 2003) and the serum testosterone/SHBG ratio and FT in females have been shown to correlate with individual changes in maximal strength and/or muscle cross-sectional area both in the lower and upper body during the strength training period (Häkkinen et al. 1992, 2022). These findings highlight the importance of monitoring serum SHBG and T levels in physique athletes, as they may play a critical role in maintaining muscle mass and optimizing performance during training periods and competition preparation. Ensuring adequate energy availability, modifying training intensity and volume to reduce physiological stress during competition preparation, and incorporating strategies to enhance recovery could mitigate these hormonal disruptions.

Our analysis of cortisol in female athletes suggested an upward trend, although not statistically significant. This observation is nuanced, given mixed literature results. Hulmi et al. (2017) found basal cortisol was not significantly changed in female physique athletes, while Mitchell et al. (2018) also reported no significant change in C in male physique athletes. Elevation of cortisol is typically linked to LEA, suggesting an adaptive response to energy stress (Mountjoy et al. 2023).

In our study, the significant decrease in the T/C ratio in male COMP during competition preparation likely indicates increased physiological stress and an imbalance between anabolic and catabolic processes. The T/C ratio may be a valuable marker for assessing this balance and optimizing training loads and recovery strategies, especially during prolonged training periods lasting several weeks to months, such as competition preparation (De Luccia 2016; Häkkinen et al. 1985). However, the interaction between the hypothalamic–pituitary–adrenal (HPA) axis and the HPG axis may vary between sexes, influencing hormonal responses and adaptations during periods of energy deficit and stress (Pasiakos et al. 2019). Our observed T/C ratio decrease may indicate greater psychological stress in male COMP, characterized by increased fatigue, which was consistent with a case study of a male bodybuilder by Rossow et al. (2013). This increased psychological stress and fatigue in male COMP may partly be explained by achieving very lower body fat and energy intake by the end of the competition preparation (Isola et al. 2023). Supporting this, low energy intake can be particularly challenging for individuals aiming for very low body fat levels through dieting, as observed in bodybuilders during competition preparation (Newton et al. 1993). During the approximately 5-month recovery phase from competition with increased EA, the T/C ratio and fatigue seemed to recover. Monitoring psychological well-being during competition preparation is crucial to mitigate the adverse effects of low energy availability (LEA) and prevent extreme symptoms of relative energy deficiency in sport (RED-S). Supporting mental well-being can reduce mood disturbances, enhance resilience, and improve performance (Angelidi et al. 2024).

We observed the decreases in isometric and concentric lower body strength among male and female COMP during competition preparation. These findings align with previous research on male bodybuilders whose maximal isometric deadlift decreased during competition preparation (Bamman et al. 1993). We previously observed a decrease in upper-body strength among female physique athletes during the competition preparation, while no changes were observed in isometric maximal and explosive lower-body strength during competition preparation (Hulmi et al. 2017). Importantly, isometric and concentric strength returned back to baseline after the recovery phase in the present study and in our earlier study with females (Hulmi et al. 2017). Interestingly, the relationship between the force/CSA ratio was not significantly different from CTRL or between sexes during the competition preparation. Previous studies align with our findings, suggesting that the F/CSA ratio is similar between sexes among similarly trained individuals (Jones et al. 2008), although a firm consensus has not yet been formed. It can be speculated that the stability in the F/CSA ratio, may support lean mass retention and gains sometimes observed during competition preparation (Isola et al. 2023), emphasizing the crucial role of resistance training. Maintaining muscle strength during competition preparation may be an essential goal for preserving FFM and achieving the best possible physique outcomes for the stage (Robinson et al. 2015) as mechanical stimuli are important for muscle hypertrophy (Wackerhage et al. 2019).

Despite the strengths of our study, several limitations must be acknowledged. The follow-up study design does not allow a reliable assessment of causalities. One challenge is that we used MET to estimate EEE, which may not fully capture individual physiological responses. Accurate estimation of EA under real-life conditions is difficult. As noted by (Areta et al. 2021), the quantification of EI and EEE can be inaccurate, affecting our EA estimates. However, the nutritional data is likely to be accurate, considering that physique athletes are known for their strict adherence to diet plans and their expertise in accurately weighing their food portions (Sarin et al. 2022). Additionally, we did not account for potential menstrual cycle variability in female participants. This variability impacts serum estradiol concentrations, complicating the interpretation of hormonal data. Furthermore, in some of the present hormone analyses, the sample size was lower because participants using hormonal contraceptives were excluded, adding to the variability in our hormone analyses for females (ACTH, FSH, estradiol, SHGB). While this variability reflects real-life situations where athletes cannot control the timing of their menstrual cycles during the competition, it nonetheless presents a challenge for data interpretation.

Our results revealed sex-specific physiological responses to energy deficits and established IGF-1 and its binding protein as key, sex-neutral biomarkers for assessing competitive stress. Although many effects of LEA and symptoms of RED-S are observable, our study, along with our previous findings (Isola et al. 2023), indicates that a single 21-week weight loss period does not produce long-term detrimental effects on performance or skeletal health. Additionally, changes induced by competition preparation appear to reverse during recovery when EA is increased pre-competition levels. These insights underscore the need for a holistic approach to training and nutrition that considers physique athletes’ unique physiological and psychological demands. For coaches, athletes, and sports nutritionists, integrating individualized nutrition plans that address specific energy and protein needs and provide mental or psychological health support can mitigate the possible adverse effects of competition preparation, ultimately enhancing athlete performance and well-being.

The findings of this study can inform training and nutritional strategies for physique athletes aiming to optimize performance while minimizing adverse effects. Coaches and sports nutritionists should consider closely monitoring biomarkers such as IGF-1 and IGFBP-3 to gauge physiological stress. Ensuring adequate energy availability and avoiding severe energy deficits can help maintain hormonal balance, muscle mass, and performance. Implementing structured and individualized nutrition plans that focus on energy and protein requirements, alongside mental health support, can mitigate the adverse effects of competition preparation. Additionally, modifying training intensity and volume during competition preparation and incorporating sufficient recovery strategies can help maintain muscle strength and overall well-being.

Future research should focus on longitudinal studies to better understand the long-term effects of competition preparation on hormonal profiles and overall health. It is crucial to investigate the impact of repeated competition cycles on hormonal balance, bone mineral density, muscle mass, and psychological well-being. Studies exploring the effectiveness of various recovery strategies post-competition and their influence on restoring hormonal balance and performance are warranted.