Previous research documents that solid foods lead to greater and more prolonged reductions of hunger were compared to low viscosity foods such as liquids [11
]. Specifically, Mattes et al. and Rothacker et al, [11
] compared meal-replacement bar and shake effects on hunger, fullness, and desire to eat and found that hunger and desire to eat remained below baseline for 3 hours following consumption of the liquid MRP. The bar led to a reduced hunger and desire to eat for 5 hours [11
]. Almiron-Roig et al. examined the satiety response following solid and liquid study foods (i.e., cola and cookies) and found no differences in satiety [19
]. However, the study foods were consumed during a low-hunger state, thus limiting the power to observe differential appetitive responses. Our research findings are consistent with the findings of weaker responses to liquids in that the S-MRP elicited lower hunger and desire to eat compared to the L-MRP. While these differential appetite data provide further insight surrounding the physiological responses to solids vs.
liquids, future studies are needed to identify whether these responses would lead to differential food intake.
Consistent with the appetite data, we observed a greater and more prolonged reduction in plasma total ghrelin concentration following the S-MRP compared to the L-MRP. The S-MRP also led to lower plasma insulin concentration throughout the 4-hour period compared to the L-MRP. Lastly, our study showed that despite the higher hunger and desire to eat following the L-MRP, plasma leptin and CCK concentrations remained fairly stable for both S and L-MRPs.
While the S and L-MRPs had similar energy content, the macro-nutrient compositions were not matched. Specifically, the L-MRP contained more carbohydrate, protein, and fiber, and less fat than the S-MRP. These differences may have affected the appetite and hormonal responses, making it difficult to draw quantitative conclusions about an independent effect of solids vs.
liquids. Due to the higher protein, higher fiber, and lower fat content in the L-MRP compared to the S-MRP, a greater reduction in hunger and desire to eat would be expected. However, the S-MRP led to reduced hunger and desire to eat indicating that the properties of solids and liquids were stronger appetite regulators than macronutrient composition. Concerning the hormonal responses to macro-nutrients, insulin responds to glucose and protein and remains virtually unaffected by dietary fat and fiber intake [20
]. Thus, it is possible that the lower insulin response observed during the solid meal may have been due to the lower glucose and protein concentrations. The effect of macronutrient composition on the ghrelin response is limited and controversial. However, recent studies show significant declines in ghrelin following carbohydrate [20
], protein [21
], and fiber consumption [23
]. Thus, we likely biased the trial to yield a greater reduction in post-prandial ghrelin concentration with the liquid meal. The fact that reduced post-prandial ghrelin was observed lends support for a strong role for the properties of solid foods. CCK responds to dietary protein [13
] and fiber [24
]. Specifically, Bourdon et al. found that CCK was twice as high following the consumption of a high fiber meal compared to a low-fiber meal [24
]. In the present study, the liquid meal-replacement had higher fiber compared to the solid meal replacement yet no differences in CCK were found. Several studies suggest leptin is responsive to dietary fat and/or protein [25
]. However, energy density, which is a characteristic difference between solids and liquids of the same energy content, appears to have a greater influence on appetite and the related hormones compared to the macronutrient content [26
]. Our results would also support this statement as the S-MRP was more energy dense than the L-MRP with similar energy content. Further studies comparing liquid vs.
solid meals with identical macronutrient composition are needed to quantitatively and independently identify the effects of solids vs.
liquids on these appetite-regulating hormones.
Numerous studies have examined the effects of meal-replacements on body weight regulation. Heymsfield et al. performed a meta-analysis of 30 weight loss studies utilizing meal-replacement products and found that meal-replacement products are as effective as conventional meal plans for prolonged weight loss and are more convenient [5
]. Recently, Noakes et al. incorporated both solid and liquid Slim · fast®
meal-replacements, similar to the products in our study, into a six-month weight loss program and found that these meal-replacements led to a 9.4 % reduction in body weight [27
]. While meal-replacements lead to successful weight loss, our study shows that the consumption of comparable solid and liquid MRPs leads to differential appetitive responses. While speculative, this would suggest greater compliance and therefore greater and more prolonged weight loss if solid MRPs are incorporated into weight loss programs in overweight and obese older adults. Alternately, Wouters-Wesseling et al. evaluated the effects of consuming a liquid supplement on body weight over a 6-month period in healthy elderly individuals [9
]. The liquid supplement led to greater weight gain (+ 1.6 kg) compared to those individuals not consuming the liquid [9
]. Similar results were also observed in frail, undernourished elderly people over a 16-week period [10
]. Taken together, these studies support the application of L-MRPs in promoting weight gain and the prevention of body weight reduction in lean older adults through reduced suppression of hunger and desire to eat. While our study was not designed to examine whether solid meal-replacements lead to better energy balance or weight control, the differential responses in appetite between solids and liquids may aid in management of energy balance and body weight.