Empty calories and phantom fullness: a randomized trial studying the relative effects of energy density and viscosity on gastric emptying determined by MRI and satiety

Background: Stomach fullness is a determinant of satiety. Although both the viscosity and energy content have been shown to delay gastric emptying, their relative importance is not well understood. Objective: We compared the relative effects of and interactions between the viscosity and energy density on gastric emptying and perceived satiety. Design: A total of 15 healthy men [mean +/- SD age: 22.6 +/- 2.4 y; body mass index (in kg/m(2)): 22.6 +/- 1.8] participated in an experiment with a randomized 2 X 2 crossover design. Participants received dairy-based shakes (500 mL; 50% carbohydrate, 20% protein, and 30% fat) that differed in viscosity (thin and thick) and energy density [100 kcal (corresponding to 0.2 kcal/mL) compared with 500 kcal (corresponding to 1 kcal/mL)]. After ingestion, participants entered an MRI scanner where abdominal scans and oral appetite ratings on a 100-point scale were obtained every 10 min until 90 min after ingestion. From the scans, gastric content volumes were determined. Results: Overall, the gastric emptying half-time (GE t(50)) was 54.7 +/- 3.8 min. The thin 100-kcal shake had the lowest GE t(50) of 26.5 +/- 3.0 min, followed by the thick 100-kcal shake with a GE t(50) of 41 +/- 3.9 min and the thin 500-kcal Shake with a GE t(50) of 69.5 +/- 5.9 min, and the thick 500-kcal shake had the highest GE t(50) of 81.9 +/- 8.3 min. With respect to appetite, the thick 100-kcal shake led to higher fullness (58 points at 40 min) than the thin 500-kcal shake (48 points at 40 min). Conclusions: Our results show that increasing the viscosity is less effective than increasing the energy density in slowing gastric emptying. However, the viscosity is more important to increase the perceived fullness. These results underscore the lack of the satiating efficiency of empty calories in quickly ingested drinks such as sodas. The increase in perceived fullness that is due solely to the increased viscosity, which is a phenomenon that we refer to as phantom fullness, may be useful in lowering energy intake.