A comparison between bioelectrical impedance analysis and air-displacement plethysmography in assessing fat-free mass in patients with motor neurone diseases: a cross-sectional study

Abstract

Aim

To determine the validity of bioelectrical impedance analysis (BIA) in quantifying fat-free mass (FFM) compared to air-displacement plethysmography (ADP) in patients with a motor neurone disease (MND).

Methods

FFM of 140 patients diagnosed with MND was determined by ADP using the BodPod (i.e. the gold standard), and by BIA using the whole-body Bodystat. FFM values were translated to predicted resting energy expenditure (REE); the actual REE was measured using indirect calorimetry, resulting in a metabolic index. Validity of the BIA compared to the ADP was assessed using Bland-Altman analysis and Pearson’s r. To assess the clinical relevance of differences, we evaluated changes in metabolic index and in individualized protein demand.

Results

Despite the high correlation between ADP and BIA (r = 0.93), averaged across patients, the assessed mean fat-free mass was 51.7 kg (± 0.9) using ADP and 54.2 kg (± 1.0) using BIA. Hence, BIA overestimated fat-free mass by 2.5 kg (95% CI 1.8–3.2, p < 0.001). Clinically, an increased metabolic index would be more often underdiagnosed in patients with MND using BIA (31.4% according to BIA versus 44.2% according to ADP, p = 0.048). A clinically relevant overestimation of ≥ 15 g in protein demand was observed for 4 (2.9%) patients using BIA.

Conclusions

BIA systematically overestimates FFM in patients with MND. Although the differences are limited with ADP, underscoring the utility of BIA for research, overestimation of fat-free mass may have consequences for clinical decision-making, especially when interest lies in determining the metabolic index.

Keywords:

• Motor neurone disease
• fat-free mass
• bioelectrical impedance
• air-displacement plethysmography
• gallagher formula

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.

Data availability statement

All protocol, analyses, and anonymized data will be shared on request from any qualified investigator. We take full responsibility for the data, the analyses and interpretation, and the conduct of the research.

Additional information

Funding

Mark R. Janse van Mantgem, Maaike L. Soors d’Ancona, Myrte Meyjes, Elles Steenhagen, Annemieke Kok and Ruben P.A. van Eijk reports no disclosures. Leonard. H. van den Berg reports grants from Netherlands ALS Foundation (Stichting ALS Nederland), The Netherlands Organization for Health Research and Development (vici schema; and funded through the EU Joint Program – Neurodegenerative Disease Research, JPND (SOPHIA, STRENGTH, ALS-CarE projects)), personal fees from Shire, Biogen, Cytokinetics, and Treeway, outside the submitted work.