Population aging is associated with a rising prevalence of obesity-related chronic diseases, including type 2 diabetes, cardiometabolic disease and sarcopenia, significantly impacting quality of life and healthcare costs. Many species, including humans, consume more energy when dietary protein is diluted by fats and carbohydrates, a phenomenon known as protein leverage, potentially increasing the risk of weight gain and adverse health outcomes. Branched-chain amino acids (BCAA) are essential amino acids involved in protein metabolism and are well recognised for their role in muscle preservation. Preclinical research suggests that elevated BCAA in the diet, relative to other amino acids including tryptophan, is associated with hyperphagia, obesity and reduced lifespan. Selectively increasing dietary tryptophan relative to BCAA, but not methionine, was shown to prevent hyperphagia under high-BCAA intake. This randomised controlled trial aimed to translate this work to humans, by examining the effects of BCAA per se and in combination with tryptophan and methionine on appetite and body composition in a cohort of older participants. We hypothesised that BCAA and BCAA with methionine would increase appetite and energy intake, whereas BCAA with tryptophan would not. Participants were randomised to one of four 4-week interventions: control (CON; no supplementation), BCAA (1.8 g/day), BCAA with tryptophan (BCAA+Trp; 1.8 g/day BCAA + 2 g/day Trp), or BCAA with methionine (BCAA+Met; 1.8 g/day BCAA + 1.5 g/day Met). All participants received ad libitum access to a standard diet containing 18–19% energy from protein, with intervention groups additionally receiving amino acid capsules to be taken with main meals. Primary outcomes were self-reported appetite and fibroblast growth factor 21 (FGF21); secondary outcomes included body composition, cardiometabolic health, gut microbiota, sleep and physical performance. Appetite was assessed weekly, and body composition was measured pre- and post-intervention using air-displacement plethysmography. Of 110 participants enrolled (mean age 71.1 ± 4.2 years; 29 males, 71 females), 100 completed the study. Preliminary observations showed the highest median [IQR] hunger in BCAA (36.2 [15.8–42.2]) and lowest in BCAA+Trp (21.5 [7.2–35.4]), with intermediate values in CON (30.0 [16.0–37.0]) and BCAA+Met (31.0 [15.8–42.5]). Across all groups, body weight (−2.06 kg), fat mass (−1.80 kg), fat percentage (−1.45%), waist circumference (−1.92 cm) and hip circumference (−1.73 cm) decreased over the intervention. Preliminary hunger observations showed patterns partially consistent with the hypothesis derived from preclinical studies, with higher hunger in BCAA and lower hunger in BCAA+Trp. Ongoing analysis incorporating dietary intake, FGF21 and other metabolic outcomes will support whether these patterns reflect intervention effects and further characterise the role of amino acid balance in older adults.