The global rise in obesity affects all age groups, with a significant increase observed in older populations. In younger obese adults, increasing dietary protein relative to other macronutrients is a common weight management strategy. However, its effectiveness in older obese individuals remains uncertain, particularly considering potential age-related changes in nutrient sensitivity. Previous research by the authors indicated that a 30g whey protein drink slows gastric emptying more in older men but leads to less energy intake suppression compared to younger, non-obese men. This study aimed to investigate the effects of a 30g whey protein drink on energy intake, gastric emptying (GE), and glycaemia in obese men of both younger and older age groups. The study's importance lies in clarifying whether protein supplementation, a common weight-loss strategy, is equally effective across age groups when dealing with obesity, a significant health concern affecting an increasing elderly population. Understanding the effect of age and obesity on the body's response to protein intake can guide more effective weight management strategies for older adults, potentially minimizing negative health outcomes associated with obesity and age-related muscle loss.

The literature supports the notion that protein is more satiating than carbohydrates or fats, and whey protein, rich in essential amino acids, is rapidly digested, which stimulates muscle protein accretion more effectively than casein or casein hydrolysate, particularly in older men. Previous studies have shown that whey protein suppresses energy intake in young, non-obese individuals but to a lesser extent or not at all in healthy older men. This suppression in young individuals is possibly due to increased pyloric and reduced antral and duodenal motility affecting gastric emptying. Obesity has minor, inconsistent effects on gastric emptying, while gastric emptying is slower in older adults. The relationship between gastric emptying, energy load, and subsequent food intake is complex, depending on the timing of subsequent eating, gastric distension, and small intestinal mechanisms. This study builds on this existing research by focusing specifically on obese older and younger men, aiming to address the limited information on whey protein's effect on energy intake in this population. Understanding the impact of obesity on the satiating effects of whey protein is crucial to effectively inform dietary strategies for managing obesity and preserving muscle mass in older people.

A randomized, double-blind, crossover study design was employed with twenty obese men (ten younger and ten older) recruited via advertisement. Younger participants (27 ± 2 years; BMI: 36 ± 2 kg/m²) and older participants (72 ± 1 years; BMI: 33 ± 1 kg/m²) were studied twice, separated by at least three days. On each visit, participants consumed either a 30g whey protein drink (120kcal) or a control drink (2kcal) after an overnight fast. Postprandial gastric emptying was assessed using 2D ultrasonography (measuring antral area) every 15 minutes for 180 minutes. Blood glucose concentrations were also measured during this period. A buffet meal (2457kcal) was provided at 180 minutes, and ad libitum energy intake was recorded. Inclusion criteria excluded subjects with smoking, alcohol abuse, diabetes (HbA1C > 6%), significant gastrointestinal surgery, gastrointestinal symptoms, medications affecting energy intake or gastrointestinal function, impaired cognitive function (Mini Mental State <25), or depression (Geriatric Depression Questionnaire ≥11) in the case of older participants. Statistical analysis using SPSS version 25 was performed to determine effects of age, treatment, and their interaction using repeated measures mixed-effects model. Energy intake was calculated from the buffet meal and cumulatively, with absolute and percentage suppression calculated. Gastric emptying was calculated from ultrasound measurements of antral area, with T50 (time for 50% emptying) determined. Appetite and gastrointestinal symptoms were assessed using Visual Analogue Scales (VAS), and area under the curve (AUC) was calculated for appetite ratings and blood glucose concentrations.

The study protocol was well tolerated. Older participants consumed approximately 20% less energy than younger participants (p=0.16), but this difference was not statistically significant. Whey protein did not significantly suppress energy intake at the buffet meal compared to the control in either age group. There was no significant difference in energy intake suppression between the younger (-3.0 ± 7.7%) and older (-2.3 ± 8.4%) groups (p=0.95). Cumulative energy intake (buffet meal and drink) also showed no significant difference based on age or treatment. Whey protein slowed gastric emptying to a similar degree in both age groups (T50: younger men – control 25 ± 5 min, protein 40 ± 7 min; older men – control 16 ± 5 min, protein 50 ± 8 min; p=0.001 for protein effect). Baseline antral areas were not significantly different between age groups or treatments. Fasting blood glucose was higher in older men (p=0.003), but whey protein did not affect blood glucose concentrations in either group. Baseline appetite perceptions (hunger, fullness, nausea, bloating) were similar between age groups and treatments. However, a significant age x protein interaction effect was observed for prospective food consumption and desire to eat at baseline. In younger men, prospective food consumption was higher on the control day while in older men, it was higher on the protein day. A similar pattern was observed for desire to eat, but not as strongly. There were no significant age x protein interaction effects in post-drink appetite ratings.

This study's findings show that a 30g whey protein drink did not suppress ad libitum energy intake in either younger or older obese men, contrasting with previous research in lean men. This suggests that obesity may blunt the satiating effects of whey protein, possibly mimicking the effects of aging. The lack of appetite suppression despite slower gastric emptying indicates that gastric distension alone may not be the primary mechanism of whey protein's satiating effect. The study confirms the age-related reduction in energy intake but finds that obesity may counteract or remove the differential effects of whey protein on energy intake seen in non-obese men. This suggests that the use of liquid whey as a weight-loss strategy might be less effective than previously thought. Furthermore, while whey protein might not aid weight loss, it's potential for preserving muscle mass in older adults warrants further investigation.

This study demonstrates that a 30g whey protein drink does not suppress appetite or energy intake in obese younger or older men, suggesting that obesity may negate the age-related difference in protein's satiating effect previously observed in lean individuals. Liquid whey protein supplementation is unlikely to be beneficial for weight loss in men of any age. Future studies should explore the impact of solid, high-protein diets on appetite and weight management in both obese and non-obese older adults, and investigate alternative mechanisms underlying the effects of protein on satiety.

The study's limitations include relatively small sample size and the use of only one dose of whey protein. Subjective perceptions of taste and pleasantness were not assessed, and gastric emptying was measured indirectly using 2D ultrasound which might be less accurate in obese individuals than 3D ultrasonography. The study only included men, limiting generalizability to women.