NB may exert a more significant influence on human physiology than previously recognized. While its role in optimizing respiration and cardiovascular function is well-established [1,2,3,4,5,6,7,8], the potential influence of NB on metabolic function remains largely unexplored. To address this knowledge gap, this study investigated the relationship between NB and RMR, while assessing the impact of nasal surgery on RMR.

Key findings and interpretation

Higher RMR in SRP Group: Patients undergoing SRP exhibited significantly higher post-operative RMR than those undergoing ST alone. This difference persisted after adjusting for potential confounders such as age, sex, and height. Nevertheless, the association between SRP and RMR disappeared when PNIF was included in the analysis. This key finding indicates that the difference in RMR between the groups was influenced by nasal airflow, with SRP patients showing lower PNIF values post-operatively compared to other groups. This observation may translate nasal airflow interference in RMR, with lower PNIF relating to higher RMR.

PNIF as a Predictor of RMR: Pre-operative and post-operative PNIF values were significantly correlated with RMR. This correlation was negative, meaning higher PNIF predicted lower RMR values and vice-versa. After confounder adjustment methods, post-operative PNIF emerged as an independent predictor of post-operative RMR. This observation aligns with earlier research by Singhal and Vishwakarma (1987), who highlighted the role of NB in energy expenditure, demonstrating that individuals with nasal obstruction exhibited increased metabolic activity compared to those with normal NB [9].

Pre to post-operative changes in RMR and PNIF: While surgical intervention significantly improved PNIF across all groups, a corresponding change in RMR was not observed. Changes in PNIF (ΔPNIF) did not correlate with RMR (ΔRMR) changes. This dissociation suggests that the relationship between nasal airflow and metabolic function is more nuanced than initially anticipated. It is plausible that the impact of surgical modifications on RMR extends beyond simple alterations in nasal airway resistance. Factors such as changes in nasal geometry, mucosal function, or even autonomic nervous system activity may mediate the metabolic response to surgical intervention.

Increase in muscle mass: surprisingly, there was a statistically significant increase in muscle mass after surgery. This finding may relate to a potential anabolic effect of improved nasal breathing. Enhanced oxygen delivery and utilization due to improved nasal breathing could contribute to muscle protein synthesis. Studies have shown that hypoxia can impair muscle protein synthesis and promote muscle atrophy [21, 22]. Conversely, improved oxygenation may create a more favorable environment for muscle growth. On the other hand, nasal surgery can improve olfactory and gustatory perception by increasing airflow and reducing nasal congestion [23, 24]. This enhanced sensory experience may lead to increased enjoyment of food, potentially promoting greater food intake and contributing to the observed increase in muscle mass [25, 26]. Nevertheless, it is essential to consider the potential influence of peri-operative factors, such as fluid shifts, inflammation, and medication use after surgery, on body composition changes.

Metabolic response to other surgical procedures

It is worth noting that studies investigating other surgical procedures have also reported that RMR remains stable after surgery. For instance, research on patients undergoing cardiac surgery with moderate hypothermic cardiopulmonary bypass has shown that no significant alteration in RMR was observed on the first 6 postoperative days [27]. Similarly, patients who underwent major abdominal surgery showed stable RMR values in postoperative days 3 and 5 [28]. This aligns with the findings of the present study, which demonstrated no significant RMR differences 3 months after nasal surgery, although favoring a correlation between RMR and PNIF at each time point.

While this study observed an anabolic effect with increased weight and muscle mass after nasal surgery, research on other head and neck interventions, such as orthognathic procedures, has shown a contrasting trend towards weight loss [29]. This discrepancy may be attributed to post-operative pain interfering with feeding, highlighting the diverse metabolic responses across different surgical procedures.

It is important to acknowledge that the majority of research on surgical effects on RMR focuses on bariatric surgery, where the primary goal is to induce metabolic alterations [30]. Therefore, findings from bariatric surgery studies may not be directly comparable to the metabolic responses observed in other surgical specialties, including nasal surgery.

Potential mechanisms of association: nasal breathing and RMR

Several mechanisms could explain the observed association between nasal breathing and RMR.

Respiratory effort: It is plausible that alterations in respiratory mechanics and energy expenditure mediate the observed association between PNIF and RMR. Increased nasal airway resistance can lead to higher intrathoracic pressure during inspiration, necessitating greater activation and muscular tone of the diaphragm and intercostal muscles to achieve adequate ventilation [31]. This heightened respiratory effort increases energy expenditure, which could contribute to a higher RMR [32, 33].

Autonomic Nervous System Activity: Sensory receptors in the nasal cavity can detect changes in airflow and temperature, relaying this information to the brainstem and influencing autonomic outflow [34, 35]. NB can stimulate parasympathetic branches of the autonomic nervous system, depending on airflow patterns [9]. Increased parasympathetic activity may decrease heart rate and blood pressure [25]. Therefore, NB patterns favoring parasympathetic activation could reduce RMR and promote an anabolic response.

Hormonal Regulation: Nasal breathing might modulate the production and release of hormones involved in energy balance, such as leptin [36]. Primarily produced by adipose tissue, leptin acts as a satiety signal, reducing appetite and increasing energy expenditure [36, 37]. Studies described lowering of leptin levels in patients with upper airway obstruction after treatment [38, 39]. This raises the possibility that improved NB after surgery could impact circulating leptin levels. In that case, increased appetite and reduced energy expenditure could contribute to observed significant weight gain.

Implications for metabolic health

The potential impact of NB on RMR has significant implications for metabolic health. Given the rising prevalence of obesity and metabolic disorders worldwide, understanding the factors influencing energy expenditure and metabolic regulation is crucial.

If NB can relate to RMR, optimizing nasal airflow through medical or surgical interventions may benefit individuals with upper airway-related breathing disorders.

Limitations and future directions

This study has several limitations. The sample size was relatively small, potentially limiting the generalizability of the findings. Furthermore, the absence of a control group makes it difficult to take illations about the observed changes with surgery. The inclusion of a control group in future studies, such as patients undergoing unrelated surgical procedures or individuals with nasal obstruction who are not undergoing surgery, would help to isolate the specific effects of nasal surgery on metabolic function. The short follow-up period also restricts the ability to assess long-term trends and the sustainability of any observed effects. It is possible that the observed changes in weight and muscle mass may be influenced by peri-operative factors and may not persist in the long term. Future research with longer follow-up periods (e.g., 1 year after treatment) would be valuable to assess the durability of these changes. Furthermore, it is important to consider the potential iatrogenic effects of post-operative treatment on metabolic function. Medications such as analgesics and anti-inflammatories could potentially influence metabolic parameters and body composition. To mitigate the potential confounding effects of general anesthesia and allow healing, post-operative measurements were conducted at 3 months after surgery. This decision was based on research indicating that RMR typically returns to preoperative levels shortly after surgery (usually within 24 h) [40]. Regarding the impact of surgery on muscle mass, there is limited objective data on the direct effects of anesthesia. Existing studies primarily focus on critically ill patients who experience prolonged periods of sedation and muscle relaxants in the intensive care setting [41]. These circumstances differ significantly from the relatively short duration of anesthesia and post-operative recovery in this study´s population. Finally, this work did not fully account for confounding factors such as diet, exercise, medications, stress, and sleep, which could independently influence RMR measurements and body composition. Future research should address these limitations by including larger populations, incorporating a control group, extending the follow-up period, and further controlling for potential confounding factors.