The thicker the shake, the thinner your waistline. That at least seems like a good bet given new data showing that a drink thickened with fiber makes you feel fuller. In fact, participants in the study, which appears in the American Journal of Clinical Nutrition, reported feeling fuller after drinking a thick shake with only 100 calories than after drinking a thin shake with five times as many calories. 

Other research has correlated feeling fuller with eating less; and eating less, as we all know, helps keep our figures trim. But as nutritionist Keri Gans, RDN, author of The Small Change Diet, points out, the current study included only 15 individuals. "We can't really draw major conclusions when we're looking at such a sample size," she says. (The participants were young men, all healthy and lean.)

The researchers, from Wageningen University in the Netherlands, asked participants to drink one of four dairy-based shakes which differed in viscosity (some were thick, some thin, due to varying amounts of fiber) and calorie content (100 calories or 500 calories). All drinks were 50% carb, 20% protein, and 30% fat.

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Participants fasted for three hours prior to the experiment, then drank through a straw, without knowing which drink they were consuming. Immediately after, they had their stomachs scanned every 10 minutes for the next 90 minutes in an MRI scanner. They also rated their appetite levels every 10 minutes 

The thin, 100-calorie shake had the lowest "gastric emptying" time, meaning it left the stomach faster than any of the other shakes (in about 30 minutes). Next was the thick, 100-calorie shake (about 40 minutes), followed by the thin, 500-calorie shake (about 70 minutes). The thick, 500-calorie shake was the slowest. It took about 82 minutes to leave the stomach.

Thickness and thinness had very little effect on gastric emptying time, the researchers determined. But viscosity did account for feelings of fullness, what the researchers call "phantom fullness." So even though the thick, 100-calorie shake left the stomach quickly, it still left participants feeling fuller than the thin, 500-calorie shake. That means there may only be a weak link between gastric emptying time and feelings of satiety. 

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The findings really don't change advice on what we should and should not be consuming, says Gans. "A smoothie is great if you put the right ingredients in [it]," she says. In addition to thickening your drink with fiber (bananas and avocado are good options), try adding Greek yogurt or peanut butter. They have the added advantage of providing lots of protein, which also contributes to feeling full.