Differences in appetite, food intake, and gastric emptying responses to protein intake by older adults varying in level of physical activity: A randomised controlled trial

Older adults are encouraged to increase their protein intake and engage in more physical activity to preserve muscle mass. However, since protein is considered the most satiating macronutrient, this advice might lead to a decrease in overall energy consumption. Physical activity is also recommended to older adults to enhance appetite, as it has been shown to help regulate appetite in younger adults, yet there is limited evidence to support this in older populations. The objective of this study was to investigate the impact of physical activity and protein on food intake, perceived appetite, and gastric emptying in older adults. Nineteen active and 19 less active older adults completed a single-blind, randomised, crossover trial involving two test days at home. Participants received a standard breakfast, followed by an isovolumetric (250 ml) and isocaloric (∼300 kcal) high- or low-protein preload milkshake (57% versus 17% energy as protein) matched for sensory properties. Three hours after the preload, participants were offered an ad libitum meal. Food intake was weighed, perceived appetite was measured by 100 mm visual analogue scales, and gastric emptying via the ¹³C-octanoic acid breath test. Higher protein intake did not affect subsequent energy intake or appetite ratings in both active and less active groups. Gastric emptying half time was longer following the high-protein milkshake compared to the low-protein milkshake. The active group had a lower perceived appetite, but faster gastric emptying time compared to the less active group. In conclusion, while higher protein intake slows gastric emptying, it did not reduce appetite or subsequent food intake in older adults, regardless of physical activity level. Additionally, being physically active suppresses perceived appetite and accelerates gastric emptying without affecting food intake.