Background Whether decreased physical functioning of patients with mitochondrial disease (MD) is related to altered body composition or low protein intake needs clarification at the background of the nutrition state. Methods In this 2‐site cross‐sectional study, MD patients were age‐, body mass index (BMI)–, and gender‐matched to controls. Body composition was assessed by dual‐energy x‐ray absorptiometry. Physical functioning was measured by handgrip strength, 6‐minute walking test, 30‐second sit‐to‐stand test (30SCT), and 6‐minute mastication test. Total daily protein intake was calculated by 3‐day food records. Malnutrition was assessed by Patient‐Generated Subjective Global Assessment and the Global Leadership Initiative on Malnutrition (GLIM) criteria and sarcopenia by the 2018 consensus. Data were analyzed using independent samples t‐tests, Fisher exact test, and Spearman and Pearson correlation coefficients. Results Thirty‐seven MD patients (42 ± 12 years, BMI: 23 ± 4 kg/m2, 59% females) and 37 matched controls were included. Handgrip strength was moderate, inversely related to fat mass index in both MD patients and controls, whereas it correlated with fat‐free mass index in controls solely. Protein intake was associated with muscle strength (handgrip strength and 30SCT) in MD patients but not in controls. Twenty‐seven MD patients (73%) were malnourished, and 5 (14%) were classified as sarcopenic. Conclusions Muscle strength is related to body composition and protein intake in MD patients. This, in combination with the high incidence of both malnutrition and sarcopenia, warrants individual nutrition assessment in MD patients.

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