Glance through a popular magazine’s listof healthy breakfast foods and you’ll likely find oatmeal in the group. Among other benefits, oatmeal is touted as having the ability to keep you full, effectively squelching the desire for that midmorning snack. In fact, a group of researchers recently found that a serving of instant oatmeal decreased the desire to eat more than the same amount of Honey Nut Cheerios, an oat-based cereal. I decided to talk with lead author Candida Rebello of the Pennington Biomedical Research Center to find out what makes oatmeal so special.

The story begins with dietary fiber, the part of plant foods that the body can’t digest, which has been shown to promote a feeling of fullness. Both the oats in oatmeal and ready-to-eat oat-based cereals contain β-glucan, a type of soluble fiber.

However, it’s not just the fiber content that matters, according to Rebello and her fellow researchers. Fullness and the desire to eat may also be influenced by a property of fluids called viscosity, which, in this case, is generated by oat β-glucan. Rebello described viscosity as the slimy feel of oatmeal that you can both see in the bowl and feel in your mouth. Scientists think viscosity affects appetite by influencing the way foods interact with the mouth, as well as the stomach and intestines. For example, increased viscosity in the intestinal tract can stimulate the release of appetite-regulating hormones. The oral and gastric effects work together, according to Rebello, meaning a food with both a high initial viscosity in the mouth and a high subsequent viscosity in the gastrointestinal tract will likely produce a greater feeling of fullness.

The study Rebello and colleagues recently authored looked at instant oatmeal, old-fashioned oatmeal and Honey Nut Cheerios and measured each food’s viscosity and effect on satiety. (I should note that though Rebello works for the Pennington Biomedical Research Center, the study was a collaborative effort with researchers from PepsiCo R&D Nutrition and was funded by PepsiCo R&D Nutrition’s Quaker Oats Center of Excellence.)

The researchers found that instant oatmeal improved several measures of satiety, or the feeling of fullness, over a four-hour period more than Honey Nut Cheerios did. Old-fashioned oatmeal improved one measure of satiety when compared to Honey Nut Cheerios. As might be expected, the study found that instant oatmeal had greater initial and subsequent viscosity compared to Honey Nut Cheerios. Old-fashioned oatmeal had greater subsequent viscosity but not higher initial viscosity, which could explain why it was less effective at promoting fullness.

Viscosity of oatmeal or cereal appears to be a key player in promoting fullness, so my next question was naturally about what determines differences in viscosity.

“Viscosity is generated by the particular fiber in oatmeal, which is called β-glucan,” said Rebello, who is a research dietitian. “The fiber is affected by the manner in which it is processed.”

Ready-to-eat oat-based cereals, instant oatmeal and old-fashioned oatmeal are processed differently, which can lead their β-glucan to have a different structure and result in a different viscosity.

So if you find yourself feeling satisfied until lunch after a bowl of instant oatmeal, now you’ll know why.