Thick & Thin Foods

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From sixty-four ounce colas to six hundred calorie health shakes, Americans are drinking far more calories than ever before. We also have record setting rates of obesity. One study suggests the two trends might be connected.


Why eating thin can make you fat. I’m Bob Hirshon and this is Science Update.

Between juices, smoothies, sports drinks, and diet shakes, it’s now possible to drink three meals a day through a straw. And according to Purdue University psychologist Susie Swithers, that might be one reason why Americans are so overweight.

She and her colleague Terry Davidson recently put two groups of rats on identical diets, except for an occasional chocolate treat.


And for one group of animals, the treat was thin, the consistency of chocolate milk. And for the other group, they got the same exact supplement, only it had been thickened. So it was something like chocolate pudding. And over the course of about five weeks, the animals that got the thin supplement, the chocolate milk, gained more weight than the animals that had gotten the thick chocolate pudding supplement.

This was true even though both treats had the same nutrition and calorie content. Dr. Swithers suggests several possible explanations.


One of the possibilities is something about the mouth feel, so animals may be able to detect whether something is thin or thick in the mouth, or it may be something that happens further on. Thick items may stay in the stomach longer, they maybe take longer to digest, and all of those processes may be contributing to the outcome.

She says understanding the relationship between food thickness and food intake could help us fight the battle of the bulge. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Our bodies have many built-in rules for judging how much food we should eat. These rules have been shaped over thousands of generations, based on the diet that's been available to humans throughout most of history.

The problem now is that we're eating a lot of foods that didn’t exist ten years ago, much less in prehistoric times. So our bodies are unprepared to deal with them. Examples include highly processed grains, refined sugar, artificial ingredients, and liquid calories. Prehistoric humans ate almost all of their calories in solid form. They certainly didn't have hundreds of brands of soft drinks, juices, "health" shakes, and other sugary liquids to round out their diets.

That's the thinking behind Swithers and Davidson’s experiment in rats. They wondered if the body perceives calories differently when they come in the form of a liquid rather than a solid. Their experiment shows that rats gained more weight when they drank rather than ate their extra calories. Since the calorie content of the thick and thin chocolate treats was the same, the rats either drank more of the treat when it came in liquid form, or ate more of their regular food on top of the liquid treat, or both.

The findings are significant in light of modern Americans’ dietary habits. Not only do we drink far more of our calories than our ancient ancestors did, but the percentage of total calories we get from beverages has risen dramatically in just the last twenty years. Look at the drink sizes in any movie theater, fast-food restaurant, or convenience store, and it's easy to see why. Swithers and Davidson's work suggest that it's not just the extra calories themselves that may make us fat–it's the fact that our bodies don't realize how much we're really drinking.

Now try and answer these questions:

  1. Describe the design and result of Swithers and Davidson's experiment.
  2. Why was it important that the thick and thin chocolate treat be nutritionally identical?
  3. Propose an experiment that would determine exactly where the extra weight gain in the "thin treat" group came from.
  4. Suppose you repeated the experiment. Your results show that again, Rat Group A (thin treats) gained more weight than Rat Group B (thick treats). However, you find that both groups of rats consumed exactly the same total calories (from all foods). How would you interpret that result?

For Educators

Purdue University's Ingestive Behavior Research Center, where Swithers and Davidson work, is devoted to the scientific study of hunger, satiety, and how we eat.

Liquid Candy, a report published by the Center for Science in the Public Interest, provides an overview of soft drink consumption over the past several decades, and its consequences.

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