By Sarah-Jane Reilly, RD
Protein is a satiating macronutrient which has been shown to reduce the rate of muscle mass decline associated with normal ageing.¹ This supports the practice of offering high protein foods and/or oral nutritional supplements to older adults who are malnourished or clinically at risk of becoming malnourished. However, there are concerns that high protein oral nutritional supplements (ONS) could negatively impact upon appetite and thus compromise oral nutritional intake.² In this article, we explore the latest evidence in this area.
In the UK, it is estimated that approximately 3 million people are malnourished or at risk of malnutrition, with the majority of these individuals (1.3 million) aged over 65 years old.³,⁴ The prevalence of malnutrition is particularly high among care home residents with one study estimating the prevalence to be as high as 85%.⁵ Malnutrition predisposes patients to disease and contributes to decreased immunocompetence, muscle wasting, a higher rate of infection and increased mortality.⁶
NICE Guidelines recommend that dietary intake, through a variety of nutrition support strategies, should be optimised to reduce the risk of malnutrition from occurring. ⁷ Maximising protein and energy intakes alongside the incorporation of weight-bearing exercise is considered the gold standard preventative strategy for limiting the loss in muscle mass and function associated with ageing.⁸,⁹
However, according to SACN, over a quarter (27%) of 65-74 year olds and a third (33%) of people aged over 75 are not meeting the recommended nutrient intake (RNI) for protein. This has led to a focus on optimising protein intake in this population group without compromising total energy intake.¹⁰
Oral nutritional intake is influenced by the ‘satiety cascade’. This describes a series of interactions between our senses, the digestive system, the central nervous system and gut hormones regulating appetite.¹¹ Of note, older adults appear to be more sensitive to the effects of the hormone cholecystokinin (CCK), which is responsible for the short term suppression of appetite and ‘feeling of fullness’ that is experienced following a meal.¹²,¹³ This could help to explain why older adults experience early satiation and tend to prefer to eat smaller portions of food at mealtimes.
Age related differences also affect how individuals regulate their food intake. Food intake decreases by roughly 25% from 40-70 years of age contributing to the phenomenon known as ‘anorexia of ageing’. This adds to the challenge of meeting protein and energy requirements.¹⁴ When compared to younger adults, older adults also experience reduced gastric emptying.¹⁵
Despite the slower rate of gastric emptying observed, older adults demonstrate a blunted response to the satiating effects of protein. In one study, the suppression of energy intake following ingestion of oral whey protein was less in older men compared to younger men (P = 0.008) across a range of protein loads, with total energy intake increasing most in older men on protein drink days compared to control days.¹⁶
A recent systematic review with meta-analysis has revealed that protein supplementation may be a novel means of increasing protein intakes in older adults without compromising total energy intake.¹⁷ The study aimed to examine the impact of protein supplementation on appetite ratings, appetite-related peptides and/or energy intake across a total of 22 studies (9 acute, 13 longitudinal) involving 857 participants. The participants were healthy adults aged 60 years and older, who reported on the effect of protein supplementation (through supplements and whole foods) using questionnaires.
To take into account heterogeneity, subgroup analyses were carried out. The researchers considered protein quantity, type, timing, form and the type of control used in the acute studies. Overall, the review found that in the acute studies, test meal energy intake (EI) was reduced following protein supplementation in the short term. However, when the EI of the protein supplement was considered, the total EI was found to be greater than that of the control (p<0.00001); dispelling the myth that protein supplements are likely to compromise oral nutritional intake at the next mealtime.
While the results of this study are promising, it is important to be aware of the short-comings of the study design. Of note, the study was carried out in healthy adults which limits the generalisability of the results to patients who are acutely unwell. The results of the study are likely to be subjected to a high degree of bias as the inclusion of crossover studies with multiple study arms can lead to ‘double counting.’ Confounding factors as exercise were not accounted for, and the timing of protein supplementation was not considered. Future studies are needed to examine the impact of timing of protein supplementation and its influence on energy intake. Inter-individual factors affecting appetite regulation such as comorbidities and medication side effects must also be considered in future study design.
This recent study suggests that protein supplementation with foods of solid or semi-solid form (such as cheese and yoghurt) appear to promote satiety more than liquid sources of protein such as milk or ONS drinks.¹⁸ The absence of mastication before swallowing could help to explain the lower satiety of liquids compared to solid foods. Liquids empty in the stomach faster than solid foods, inducing weaker signals in the gastrointestinal tract that are responsible for inhibiting nutritional intake.¹⁹ Overall, these findings highlight that protein supplementation can help to meet protein requirements without compromising oral nutritional intake with supplementation >1hr before a meal. ¹⁷ This supports the practice of offering high protein ONS to older adults who are malnourished or clinically at risk of becoming malnourished.