Following fructose through the body

This exercise is a part of Educator Guide: Small Intestine is First Stop for Fructose / View Guide

These questions are based on the article “Small intestine is first stop for fructose. 

1. What is fructose? 

Possible student response: Fructose is a sugar, more complex than glucose, which is found in honey, fruit, soda and other foods and drink.

2. How does the body use fructose for energy?

Possible student response: The body converts fructose into glucose or other, smaller molecules that cells can use for fuel.

3. What was previously believed about the metabolism of fructose, and what did this new research find? 

Possible student response: Scientists previously thought that most fructose was broken down, or metabolized, in the liver. This new research shows that when mice are given moderate doses of fructose, most of the sugar is metabolized in the small intestine. The liver metabolized fructose only when mice were given high sugar doses, which caused the small intestine to pass along excess fructose it couldn’t handle.

4. Why is it important to limit how much fructose reaches the liver? 

Possible student response: Too much fructose puts the liver at risk for conditions such as fatty liver disease and raises people’s overall risk of obesity and type 2 diabetes. 

5. How did the researchers keep track of which sugars get metabolized in which organs?

Possible student response: The researchers fed mice glucose or fructose in which certain carbon atoms had been replaced by a slightly heavier form of carbon. Those heavier carbon atoms, called isotopes, let the scientists trace where each sugar ended up. Researchers collected samples from different mouse organs, separated the metabolized molecules, or metabolites, from those organs and identified which organs contained the isotopes associated with fructose or glucose. 

6. What did the researchers find for low sugar doses?

Possible student response: At lower sugar doses, there were more metabolites derived from labeled fructose molecules in the small intestine, and only small amounts in the liver and in the hepatic portal vein that connects the two organs. Researchers found a high concentration of glucose molecules in the vein, with labels showing that some of the glucose molecules were once fructose molecules that had been broken down in the small intestine. 

7. What did the researchers find for high sugar doses?

Possible student response: At high sugar doses, the hepatic portal vein had a much higher ratio of fructose to glucose than at lower sugar doses. The small intestine was forwarding excess fructose along to the liver.

8. How is this research related to human foods? How can these findings be translated into dietary recommendations?

Possible student response: Fructose is naturally found in honey and fruits, and is also found in a wide variety of sweetened products such as soda. It is difficult to compare sugar doses between humans and mice. Mice expend more energy relative to body mass than humans, so the sugar dose cannot be scaled up by body mass.

9. If these results could somehow be translated for humans, what recommendation might be made? 

Possible student response: People should eat sugars in moderation, and should not overindulge in sugars during one sitting. 

10. Draw a diagram to depict the main idea of the article?

Possible student response: If students are familiar with the digestive system or are able to research a mouse’s digestive system, they may draw it and indicate the pathway of fructose metabolism for moderate versus large doses of ingested fructose. Other students may use molecular diagrams, rudimentary depictions of digestive organs and arrows to show high concentrations of labeled fructose metabolites in the small intestine (and low concentrations in the liver) when a moderate dose of fructose is ingested. 

11. What questions do you still have after reading the article?

Possible student response: How relevant are these mouse results for humans, in view of the very large differences in diet, size and other factors between mice and humans? How could related studies be safely performed in humans? What animals intermediate in size between mice and humans would be most relevant for similar studies?