Summary Background & aim Past studies reported that the intake of adequate energy is more important than protein intake; however, the relationship between energy intake and muscle mass loss remains unclear thus far. This study therefore explored the association between energy intake and muscle mass loss in people with type 2 diabetes (T2D). Methods In this prospective cohort study, impedance body composition and a brief-type self-administered diet history questionnaire were used for analyzing body composition and habitual diet intake, respectively. Skeletal muscle mass index (SMI, kg/m2) was defined as appendicular muscle mass (kg) ÷ height-squared (m2). Rate of SMI change (%) was calculated as ([SMI at baseline - SMI at follow-up]/[follow-up duration (years) × baseline SMI (kg/m2)]) × 100, and muscle mass loss was defined as rate of SMI change ≥0.5%. Energy intake was defined as total energy intake (kcal/day) divided by ideal body weight (kg), defined as 22 × patient height-squared (m2). Results Among non-older and older participants, 54.8% (n = 51/93) and 58.9% (n = 116/197) experienced muscle mass loss at 16.3 (6.4) and 18.1 (7.1) months’ follow-up, respectively. Logistic regression analyses showed that energy intake was associated with incident muscle mass loss after adjusting for age, sex, insulin, sodium glucose cotransporter-2 inhibitor, glucagon-like peptide-1 agonist, steroids, smoking, exercise, alcohol intake, body mass index, SMI, presence of renal failure, and protein intake (g/actual body weight/day) in the older people (odds ratio [OR] 0.94 [95% confidence interval [CI] 0.88–0.996], p = 0.037), whereas energy intake was not associated with incident muscle mass loss in the non-older people (OR 0.96 [95% CI 0.86–1.06], p = 0.423). Conclusions Insufficient energy intake is associated with muscle mass loss in older people with T2D. Therefore, it is recommended to consume enough energy for older people with T2D to keep muscle mass.