Following the Sugar: A Runner's Food in the Exercise Lab

Use a tracer study and fuel data to model how cells break down and rebuild food molecules.

Answer Key

The marked-sugar study

Read the passage and study Table 1. Then answer the questions.

Where Does a Sports Drink Go?

1Saoirse Kavanagh, a seventh grader, runs on her school's cross-country team. Before big races, Coach Darnell Whitfield tells the team to eat a starchy dinner, such as pasta or rice, and to sip a sugary drink on long training runs. Saoirse wondered what actually happens to that food inside a runner's body. Her aunt, Dr. Mei-Ling Tsao, works in a university exercise lab, and she walked Saoirse through one of the lab's studies with adult volunteers.

2Dr. Tsao started with a model. Digestion breaks large food molecules into small ones: starch into glucose, a sugar; proteins into amino acids; and fats into fatty acids. Blood carries these small molecules to the body's cells. Inside cells, two kinds of change happen. In cellular respiration, glucose or fatty acids react with oxygen to form carbon dioxide and water, and energy is released for the cell to use. Cells can also link small molecules into larger ones, such as glucose into glycogen, a starch-like molecule stored in the liver and muscles, or amino acids into new proteins.

3In the study, each volunteer drank 50 g of glucose dissolved in water. Some of the carbon atoms in this glucose were a natural, harmless form of carbon that is slightly heavier than usual, so a lab machine can tell these marked atoms apart from the other carbon atoms in the body. Every 30 minutes, the volunteer breathed into a bag, and the machine measured how much of the marked carbon had been breathed out in carbon dioxide.

4Each volunteer did the test twice, on different days. Once they rested quietly in a chair for the 2 hours after the drink. The other time, they pedaled an exercise bike at a steady, moderate pace. Table 1 shows the averages for eight volunteers.

5The lab also measured how much of the energy the volunteers' bodies used came from carbohydrates, such as glucose and glycogen, and how much came from fat. The bar graph shows the averages at three levels of activity.

Table 1. Marked Carbon Breathed Out So Far (% of the marked carbon in the drink)
Time after drink (min)	Resting (%)	Cycling (%)
30	2	6
60	6	18
90	10	31
120	14	44

Assume the marked carbon shows what happened to all of the 50 g of glucose in the drink.

By 120 minutes, about 22 g of the glucose had been broken down and breathed out as carbon dioxide in the cycling test, and about 7 g in the resting test.

Where did the marked carbon atoms in the volunteers' breath come from?

A. From the air in the lab, which the volunteers breathed in and then breathed back out
B. From glucose in the drink, which cells broke down by combining it with oxygen
C. From the water in the drink, which the body turned into carbon dioxide gas
D. From the volunteers' lungs, which made new carbon atoms as the volunteers worked

Why did the volunteers breathe out the marked carbon faster while cycling than while resting?

A. Working muscle cells needed more energy, so they broke down more glucose in respiration.
B. Cycling made the stomach turn the glucose straight into carbon dioxide without digestion.
C. Cycling made the volunteers breathe in more of the marked carbon from the lab's air.
D. Resting cells pause respiration until the body starts to move again, so they skip glucose.

In the resting test, about 86% of the marked carbon had not been breathed out after 2 hours. Based on the model in paragraph 2, where was much of it most likely?

A. It had been changed into energy, so the carbon atoms no longer exist.
B. It had been turned into oxygen, which the blood carried to the lungs.
C. It had been linked into glycogen and stored in the liver and muscles.
D. It had been destroyed in the stomach, so it could not reach the cells.

Why did the lab use glucose with marked carbon atoms instead of ordinary glucose?

A. Marked glucose is broken down faster, so the test could end in 2 hours.
B. Marked glucose gives cells more energy than ordinary glucose does.
C. Marked carbon atoms are lighter, so they rise into the breath bag more easily.
D. The drink's carbon could be told apart from carbon already in the body.

Suppose a volunteer kept cycling at the same pace and gave a breath sample at 150 minutes. Based on Table 1, what would most likely be measured?

A. More than 44%, because cells would still be breaking down the drink's glucose
B. About 14%, because the volunteer's tired muscles would have switched to burning fat
C. About 44%, because the drink's glucose had been used up by 120 minutes
D. Less than 44%, because some of the marked carbon would return from the air

Fuel for an easy run and a race

Use paragraphs 2 and 5 and the bar graph to answer the questions.

In the lab's measurements, the energy that did not come from carbohydrates came from fat.

At race pace, about 20% of the energy came from fat.

Which pattern does the bar graph show?

A. Carbohydrates gave about half of the energy at each level of activity.
B. As the activity got harder, carbohydrates gave a larger share of the energy.
C. Resting used a larger share of carbohydrate energy than an easy run did.
D. As the activity got harder, fat gave a larger share of the energy.

Sort each change in a runner's body by what it does.

Leg muscle cells combine glucose with oxygen during a race.Liver cells link glucose into glycogen after a pasta dinner.Muscle cells link amino acids into new proteins after practice.Muscle cells combine fatty acids with oxygen during an easy run.Fat cells link fatty acids into stored fat after a large meal.

Releases energy for cells to use	Builds larger molecules to keep
Leg muscle cells combine glucose with oxygen during a race. Muscle cells combine fatty acids with oxygen during an easy run.	Liver cells link glucose into glycogen after a pasta dinner. Muscle cells link amino acids into new proteins after practice. Fat cells link fatty acids into stored fat after a large meal.

10.

Coach Whitfield wants the team to eat a starchy dinner the night before a race. Which reason fits the model and the bar graph best?

A. Starch turns into fat overnight, and fat gives most of the energy for running at race pace.
B. Starch goes into the muscles without being digested, so it gives runners energy faster.
C. Glucose from the starch can be stored as glycogen, which muscles break down at race pace.
D. Starch slows down respiration in the muscles, so runners use less oxygen during the race.

Does food just burn up?

Write your answer in full sentences.

11.

A teammate says, “When you run, the food you ate just burns up into energy and disappears.” Do you agree? Write a claim, use evidence from Table 1 and the bar graph, and use the model in paragraph 2 to explain what happens to the atoms in food and where the energy for running comes from.

Claim: I disagree. Food does not disappear; cells rearrange its atoms, and energy is released as they do. Evidence: In Table 1, the marked carbon atoms from the drink showed up in the volunteers' breath, 44% of them by 120 minutes during cycling, so the drink's atoms left the body in carbon dioxide instead of vanishing. At rest, only 14% had been breathed out, so much of the glucose was kept, most likely as glycogen. The bar graph shows that at race pace about 80% of the energy came from carbohydrates. Reasoning: In cellular respiration, glucose and oxygen react to form carbon dioxide and water, so the same atoms leave in breath and water. The energy for running is released from the chemical energy stored in the food molecules, not made from nothing. Food that is not used right away can be linked into larger molecules such as glycogen or proteins and kept in the body.