May 5, 2010
By Rachel Perez
In April the American Journal of Clinical Nutrition published a new study addressing the much-contested question of how to control eating. Researchers from the Netherlands fed healthy people different textured low- and high- calorie yogurts, and measured the effects on eating behavior and perceived fullness. The results showed no link between food texture and satiety, however other observations were intriguing.
The link between food properties and eating behavior comes from the concept of “learned satiety,” which suggests that the brain learns to adapt food intake to expected fullness. This learning process may occur after repeated exposure to a food’s oral sensory properties—namely its taste and texture.
When it comes to taste, research shows that sugar in the mouth sends quick signals to receptors in the hypothalamus, the region of the brain that registers hunger. However the role of food texture on brain satiety messages is largely unknown. Theories suggest that foods with higher viscosity spend a longer time in the mouth, perhaps altering signaling to the hypothalamus. This connection between the mouth and brain may be a learning process, requiring multiple exposures to a food’s taste and texture.
“Satiety learning” may occur primarily during childhood and decrease with age. Marcia Pelchat, Ph.D, Associate Member at the Monell Chemical Senses Center (and guest speaker at the 2010 Friedman Gershoff Symposium in April), explains that all determinants of eating behavior are learned through culture, a process that starts in childhood. “As we grow older, maybe we substitute food volume as the cue for satiety” she says. “Adults may be more sensitive to volume than calories.”
This breach between eating and fullness prompted researchers from the Netherlands to investigate the role of food texture on breakfast behavior. Their primary tool? Yogurt.
In the six week study, 105 young healthy adults were randomly assigned to one of three breakfast routines: drinking liquid yogurt through a straw, eating liquid yogurt with a bowl and spoon, or eating semi-solid yogurt with a bowl and spoon. The researchers switched high-calorie and low-calorie yogurts for each person every-other day, however the participants did not know which one they were eating.
For four weeks the researchers told the subjects to eat their yogurt until “pleasantly full.” The researchers knew the semi-solid yogurt had the highest viscosity and the longest sensory exposure in the mouth; thus they hypothesized that people would learn to link texture to satiety, and begin to eat less yogurt over time.
After four weeks the researchers noticed that all people were eating less yogurt, regardless of texture or calorie difference. Perhaps the participants simply got bored with the yogurt (whose flavors – rose apple and soda simmered pumpkin – shed an interesting light on Dutch taste preferences). However people who slurped liquid yogurt with a straw ate 20% more yogurt than those with a spoon, suggesting that the eating mode and rate are more important than the food texture in determining fullness.
Dr. Gene-Jack Wang, MD, Senior Scientist at the Brookhaven National Library (and another speaker at the Friedman Gershoff Symposium), noted similar findings in his research. Wang has researched the brain satiety circuit, and one of his previous studies featured a balloon that was inserted and expanded in the stomach. Without the presence of food, the expanded stomach sent hormonal signals to the brain, showing that volume change alone can contribute to satiety.
For the remaining 2 weeks of the Dutch study, researchers gave each participant 300 grams of yogurt, alternating between high-calorie and low-calorie varieties. After finishing the yogurt, participants were given plates of mini currant buns and told to eat until pleasantly satisfied. Interestingly, researchers found that people ate the same number of currant buns every day. People could not distinguish the different calorie yogurts, and the researchers concluded that “people tend to eat a consistent weight of food. When the energy density of a meal increases, energy intake increases.”
Wang echoes similar concern, noting, “People cannot tell if they are eating a high energy or low energy food, which is extremely dangerous.”
So what’s the take-home message? When it comes to “learned satiation,” the mouth and stomach cannot to be trusted. And until further research illuminates the brain’s complete role in eating, it’s best to return to the familiar recommendations of portion control and food volume. As for slurping, gulping, and chewing, put down the straw and pick up that fork! Chew, but chew slowly.
Hogenkamp, P.S., Mars, M., Stafleu, A., de Graaf, C. April 2010. Intake during repeated exposure to low- and high-energy-dense yogurts by different means of consumption. American Journal of Clinical Nutrition. Pg 841-847.
Pelchat, Marcia. Telephone interview. April 14, 2010.
Wang, Gene-Jack. Telephone interview. April 14, 2010.