Our findings indicated that FFM loss, along with PhA reduction was the predominant changes during the weight loss experienced by MND patients. These results were similar to a large study with 74 MND patients [5], which reported statistically significant losses in weight, FFM and PhA from diagnosis to pre-death. Although the increase we observed was not statistically significant, FM percentage increased from 30.7% to 33.4%, in line with previous literature [5].

Several possible hypotheses have been proposed to explain the loss of FFM and relatively preserved FM in MND. Firstly, neurodegeneration of motor neurons accelerate muscle loss [5]. Secondly, FFM loss occurs due to reduced caloric intake from dysphagia and loss of appetite [9]. As typical caloric restriction leads to change of both FFM and FM stores, the selective depletion of FFM suggests that metabolic processes in MND may preferentially target lean tissue, pointing to altered energy metabolism beyond simple undernutrition. Lastly, MND induces a hypermetabolic state [10], hypothesised to stem from increased respiratory muscle expenditure [10]. Future research should investigate whether neuromuscular changes drive shifts in substrate utilisation and metabolic flexibility with nutritional intake, to better inform targeted nutritional strategies.

While not all FFM loss is modifiable, some may be and this deserves clinical attention. In light of the inevitable denervation-induced skeletal muscle atrophy in MND, a delay in nutritional intervention may exacerbate the loss of muscle mass. As a pragmatic diagnostic approach to detecting malnutrition in MND patients, it has been previously advised to monitor BMI and employ body composition assessment concurrently [8]. Our observations support the rationale for considering interventions aimed at preserving body weight and composition, particularly FFM, prior to gastrostomy placement.

We acknowledge our study has some limitations. Our study involved a small sample size. Further, the interval between BIA measurements varied among participants. Future research should be directed towards identifying exercise and nutrition interventions that minimise FFM losses in the context of MND, prior to gastrostomy placement, to enhance its utility in clinical practice.

In conclusion, our study provided longitudinal data on the changes in the body composition and phase angle of MND patients from diagnosis to gastrostomy insertion and identified that weight loss is largely characterised by FFM loss. Clinicians are recommended to monitor body composition and future research should explore interventions aimed at preserving body weight and composition, particularly FFM, prior to gastrostomy placement.