The anorexia of aging, which affects 20% of older adults, has been well documented within the current body of evidence as a primary cause of reduced daily intake in this population. This condition is characterized by reductions in appetite in combination with early satiation and sustained increases in satiety.
Circulating concentrations of the hunger-stimulating hormone ghrelin are lower, whereas the satiety hormones cholecystokinin, peptide YY, and GLP-1 are elevated in older adults compared with their younger counterparts. Gastric emptying also tends to be slower in older adults than in younger adults, which leads to early satiation. These responses occur concomitantly with an increase in perceived fullness and decreased hunger, desire to eat, and prospective food consumption.
Am J Clin Nutr
Am J Clinical Nutrition
The United States is experiencing a demographic shift, with an estimated doubling of adults aged ≥65 y by 2050, creating a need to target nutrition strategies to promote health as we get older. Aging leads to a substantial loss in muscle mass of ∼1%/y. The loss in skeletal muscle with aging is typically accompanied by reduced muscle strength and function, which increases fall risks, reduces the ability to complete day-to-day activities and tasks, and subsequently reduces quality of life. Progressive declines in both daily energy and protein consumption are primary contributors to the weight and muscle mass losses observed in older men and women. Several reasons exist for the inability to maintain energy balance and protein needs with aging.
The anorexia of aging, which affects 20% of older adults, has been well documented within the current body of evidence as a primary cause of reduced daily intake in this population. This condition is characterized by reductions in appetite in combination with early satiation and sustained increases in satiety. Furthermore, underlying ingestive behavior mechanisms have been identified, which include deleterious alterations in gut-derived hormonal responses and gastric motility. Specifically, circulating concentrations of the hunger-stimulating hormone ghrelin are lower, whereas the satiety hormones cholecystokinin, peptide YY, and GLP-1 are elevated in older adults compared with their younger counterparts. Gastric emptying also tends to be slower in older adults than in younger adults, which leads to early satiation. These responses occur concomitantly with an increase in perceived fullness and decreased hunger, desire to eat, and prospective food consumption.
In addition to the altered appetite and satiety responses, older individuals also display the inability to appropriately regulate food intake. Providing additional (or fewer) calories to older adults does not lead to subsequent compensatory reductions (or increases) in subsequent food intake to re-establish energy balance conditions. These findings suggest that older adults are unable to physiologically respond in a compensatory manner to prevent obesity-related overconsumption or chronic undereating and malnutrition.
With respect to whether older adults are meeting protein needs, consistent evidence has shown daily quantities of between 1-1.5 g protein/kg/day to prevent losses in muscle mass. Unfortunately, most older adults consume nowhere near this quantity, as only 30% of adults aged >50 y meet the Recommended Dietary Allowance (0.8 g protein/kg/day) for daily protein intake, with older women consuming less protein than men. More recently, the importance of meal-specific quantities of protein has been highlighted, with recommendations of ∼25–30 g protein for young to middle-aged adults and 35–45 g protein for older adults for muscle-related outcomes. These larger quantities create challenges for most adults, but especially for older individuals due to several factors discussed below.
Dietary protein has been reported as the most satiating macronutrient. Higher-protein meals elicit greater perceived fullness and peptide YY concentrations and reduced ad libitum food intake than do lower-protein versions. Although these findings have been replicated in numerous studies, the majority were conducted in young to middle-aged adults, raising the question as to whether older adults elicit greater protein-related satiety than do younger adults due to alterations in ingestive behavior mechanisms.
A previous study by the same authors from the currently highlighted study in this issue of the Journal compared the protein-related satiety responses in young and older men. The younger group showed increased fullness after the protein preloads, whereas the older group did not. In addition, the combined preload plus lunch intake was greater after the 70-g protein/280-kcal preload than after the 30-g protein/120-kcal and 0-g/0-kcal preloads in older men compared with their younger counterparts. Thus, it is possible that the lack of protein-related satiety in the older group contributed to the increased energy intake and lack of dietary compensation. The current study published in this issue of the Journal extends the findings to assess whether similar results are observed in older men compared with older women.
In addition to the satiety challenges, older adults experience chewing difficulties, increased cost, and reduced convenience due to cooking requirements as primary reasons for their decision to eat less protein, particularly animal proteins. Because beverages are convenient, easier to consume, and elicit weaker satiety responses and reduced dietary compensation than do solid foods in older adults, the incorporation of higher-protein beverages may be one strategy to improve energy status and protein consumption in older men and women.
The current study includes protein beverages at 2 quantities, 1 containing 30 g protein, which is the amount proposed to elicit a protein-satiety threshold in younger and middle-aged adults, and 1 containing 2.5 times the amount (i.e., 70 g protein), which continues to elicit increased satiety in younger adults. The current study includes practical outcomes to assess compensatory food intake as well as potential ingestive behavior mechanisms such as gut-derived hormonal measures in combination with ultrasound to measure gastric emptying in an understudied population: older women compared with older men.
Older men and women responded similarly to the beverage treatments in the current study. Although satiety increased and gastric emptying slowed in a protein dose–dependent manner after the protein preloads compared with the control, the additional protein (and energy) did not suppress subsequent food intake at the next eating occasion.
Collectively, these data suggest that the consumption of protein-rich beverages may assist in the re-establishment of energy balance in older men and women by increasing overall energy intake.
Future directions include the following: 1) longer-term intervention designs of ≥12 wk, 2) optimal protein quantities between 35-45 g protein/beverage preload, and 3) larger cohorts of older adults who show a range of disturbances, including, but not limited to, malnutrition, frailty, sarcopenia, and osteoporosis. The completion of these studies is needed to fully establish whether the habitual consumption of protein beverages elicits sustained improvements in daily energy and protein intake to affect muscle mass and health outcomes over the longer term in a broader aging population.