Protein-rich supplements are used widely for the management of malnutrition in young and older people. Protein is the most satiating of the macronutrients in young. It is not known how the effects of oral protein ingestion on energy intake, appetite, and gastric emptying are modified by age. The aim of the study was to determine the suppression of energy intake by protein compared with control and underlying gastric-emptying and appetite responses of oral whey protein drinks in eight healthy older men (69-80 yr) compared with eight young male controls (18-34 yr). Subjects were studied on three occasions to determine the effects of protein loads of 30 g/120 kcal and 70 g/280 kcal compared with a flavored water control-drink (0 g whey protein) on energy intake (ad libitum buffet-style meal), and gastric emptying (three-dimensional-ultrasonography) and appetite (0-180 min) in a randomized, double-blind, cross-over design. Energy intake was suppressed by the protein compared with control (P = 0.034). Suppression of energy intake by protein was less in older men (1 ± 5%) than in young controls (15 ± 2%; P = 0.008). Cumulative energy intake (meal+drink) on the protein drink days compared with the control day increased more in older (18 ± 6%) men than young (1 ± 3%) controls (P = 0.008). Gastric emptying of all three drinks was slower in older men (50% gastric-emptying time: 68 ± 5 min) than young controls (36 ± 5 min; P = 0.007). Appetite decreased in young, while it increased in older (P < 0.05). In summary, despite having slower gastric emptying, elderly men exhibited blunted protein-induced suppression of energy intake by whey protein compared with young controls, so that in the elderly men, protein ingestion increased overall energy intake more than in the young men.

It is not well recognized that in the elderly weight loss is more common than weight gain. The aim of this analysis was to determine the effect of ageing on appetite (hunger/fullness) and energy intake, after overnight fasting and in a postprandial state, by meta-analyses of trials that included at least two age groups (>18 years). We hypothesized that appetite and energy intake would be less in healthy older compared with younger adults. Following a PubMed-database systematic search up to 30 June 2015, 59 studies were included in the random-effects-model meta-analyses. Energy intake was 16%–20% lower in older (n = 3574/~70 years/~71 kg/~25 kg/m2) than younger (n = 4111/~26 years/~69 kg/~23 kg/m2) adults (standardized mean difference: −0.77 (95% confidence interval −0.90 to −0.64)). Hunger was 25% (after overnight fasting; weighted mean difference (WMD): −17 (−22 to −13) mm) to 39% (in a postprandial state; WMD: −14 (−19 to −9) mm) lower, and fullness 37% (after overnight fasting; WMD: 6 mm (95% CI: 1 to 11 mm)) greater in older than younger adults. In conclusion, appetite and energy intake are less in healthy older than younger adults, suggesting that ageing per se affects food intake.

Intraduodenal protein suppresses appetite and energy intake less in healthy older than in young adults. In older subjects, intraduodenal protein at low doses increased overall energy intake, which supports the use of protein supplements in undernourished older people. This trial was registered at www.anzctr.org.au as 12612000906853.