A recent study has shown that the calorie content of food is more important than the viscosity (thickness) in slowing the amount of time it takes for the food to leave the stomach. However, viscosity is more important than calorie content in increasing the perceived fullness, which may be useful in lowering calorie intake.
Satiety is the feeling of being full after eating that suppresses the desire to eat for a period of time. Such feelings of fullness can have a significant role in controlling how much you consume. If you feel full after a meal or “satiated”, then you are likely to go for a longer period of time before you feel hungry again and may consume less at the next meal. An understanding of how to enhance these feelings of fullness or ‘satiety’ may be useful to control how much food is consumed and decrease overconsumption.
Satiety is affected by a number of factors that start when a food or drink is ingested and continue as the food enters the stomach and is digested and absorbed — from the portion size to oral sensory exposure to food. Feedback signals from the stomach and intestinal tract on the volume and nutrient content after ingestion also have an impact on satiety. Evidence from multiple studies has shown that the viscosity (thickness) of food may also have an effect on satiety. However, earlier studies examining whether viscosity directly slows gastric emptying (the amount of time it takes for food to empty from the stomach and enter the small intestine) appears to be inconsistent. Furthermore, the only research to have investigated both viscosity and calorie content of food on gastric emptying in one single study has shown that both viscosity and calorie content slow gastric emptying and that the calorie content slows gastric emptying more effectively than increased viscosity. At the same time, the viscosity of food has a greater influence on perceived satiety than does the calorie content.
In a new study, researchers aimed to further clarify the effects of viscosity and calorie content on gastric emptying and perceived satiety. They conducted a randomized experimental study, which included fifteen male participants between the ages of 18 and 35, with a BMI between 18 and 25, and with self-reported good general health. Participants received dairy-based shake meals (500 mL; 50% carbohydrate, 20% protein, and 30% fat) that varied in viscosity (thin and thick) and calorie content (100 kcal compared with 500 kcal). Upon consuming the milkshakes, participants entered an MRI scanner where gastric (stomach) scans and satiety/appetite ratings (i.e. of hunger, fullness, prospective consumption, desire to eat and thirst, on a 100 point scale) were taken every 10 min until 90 min after consumption. Afterwards, the participants were offered a sandwich meal of which food intake was recorded.
The findings, published in The American Journal of Clinical Nutrition, showed that in men who consumed meals with a higher calorie content, it took a longer time for the drink to empty from the stomach; an increase of viscosity of the meal only significantly slowed the emptying when the consumed meal was low (i.e. 100 kcal) in calorie content. Both 100 kcal meals that had high viscosity (were thicker) and 100 kcal meals that had low viscosity (were thinner) resulted in a very similar gastric empty rate (GER) despite the difference in viscosity. The findings suggest that the difference in the time it took for the meal to leave the stomach between a high viscosity-low caloric (i.e. 100 kcal) fluid and a low viscosity-low caloric (i.e. 100 kcal) meal can be completely explained by the faster drainage within the very first moments after ingestion. In fact, the viscosity showed little to no effect on gastric emptying when the researchers corrected for the draining in the beginning by measuring the GER every 10 minutes.
Moreover, the study showed that the viscosity of the meal had a greater influence than the calorie content on the participants’ subjective ratings of satiety. The viscosity of the meal considerably changed the satiety ratings, where fullness scored higher, and the desire to eat scored lower, which suggest that increased satiety is affected by viscosity through oral exposure and mouth feel of food. As well, participants showed no tendency to make up for the caloric intake during the meal that was offered afterward. Participants who consumed high viscosity (thick) meals containing 100 kcal gave higher fullness ratings and had similar food intake afterwards in comparison to those who had low viscosity (thin) meals containing 500 kcal, even though the former emptied from the stomach much more quickly. This effect is often referred to as the “empty calories” in sodas. Not only was an “empty calories” effect shown after the thin 500-kcal meal, but in contrast, it was shown that there were greater feelings of fullness even when the meal was emptied from the stomach quickly. Therefore, an increase in the perceived thickness created a sense of fullness and satiation by the taste and mouth feel of food, regardless of the food’s caloric content and the actual content in the stomach. This may offer an alternative explanation to findings in previous studies that suggested that greater satiety was caused by slowed gastric emptying, both of which may have been caused by a greater oral and sensory exposure of viscous (thick) food.
The results of this study show that both the calorie content and the viscosity of food slow gastric emptying. However, increasing the viscosity is less effective in slowing gastric emptying than increasing the calorie content. The effect of viscosity in slowing gastric emptying was found to be reduced with increasing calorie content (i.e. 500 kcal).Thus, this effect suggests that the viscosity may not affect satiety through slower gastric emptying, as viscosity had less of an effect in slowing gastric emptying in food with increased calorie content. In addition, the viscosity is more important than calorie content in increasing the perceived fullness or satiety. These findings emphasize the lack of the satiating effect of empty calories in quickly consumed drinks such as sodas. The increase in perceived fullness that is due entirely to the increased viscosity may be beneficial in lowering calorie intake. Low caloric liquid meals that have more viscosity (thickness) may leave people feeling fuller and more satiated than low caloric liquid meals that are thinner, thereby preventing overconsumption afterward.
Written By: Nigar Celep, BASc