Fed or fasted: how your body switches fuels

Science · The Human Body · ages 21-22

Name ______________________ Date ____________

During an overnight fast, which liver response helps maintain blood glucose?
- Breaking down glycogen and making glucose for release
- Building glycogen from blood glucose and reducing glucose release
- Using glycogen-derived glucose locally and reducing glucose release
Blood glucose rises after a carbohydrate-containing meal. Which pancreatic signal increases in response?
- Glucagon from alpha cells
- Insulin from beta cells
- Cortisol from adrenal cells
In the fed state, insulin favors glycogen storage in liver and muscle and triglyceride storage in fat tissue.
Circle one: True False
Which comparison describes the roles of muscle glycogen and fat tissue during fasting?
- Muscle glycogen supports local muscle work; fat favors fatty acid release
- Muscle glycogen supplies blood glucose; fat favors fatty acid release
- Muscle glycogen supports local work; fat favors net triglyceride storage
During a transition toward fasting, insulin falls relative to glucagon. How does this coordinate liver glycogen pathways?
- Synthesis is favored alongside breakdown, increasing glycogen cycling
- Synthesis is favored over breakdown, increasing glycogen storage
- Breakdown is favored over synthesis, increasing glucose release
Liver glycogen is becoming depleted during a fast. Which description fits the overlapping fuel pathways?
- Glucose synthesis begins after glycogen depletion
- Glucose synthesis continues as fatty acid use increases
- Brain fatty acid use replaces glucose synthesis
During several days of fasting, which brain fuel change can reduce protein breakdown for glucose synthesis?
- The brain uses more ketones, lowering its glucose demand
- The brain uses more fatty acids, lowering its glucose demand
- The brain uses more ketones, raising its glucose demand
A student says muscle exports free glucose from its glycogen. Which missing step explains why this route does not work?
- Glucose-6-phosphate formation during muscle glycogen breakdown
- Local use of glucose-6-phosphate to support muscle metabolism
- Glucose-6-phosphatase conversion to free glucose for export

Answer key

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Breaking down glycogen and making glucose for release · Glycogen breakdown and gluconeogenesis supply glucose for release into blood. Storage and local use do not themselves add glucose to blood.
Insulin from beta cells · Rising glucose stimulates pancreatic beta cells to release insulin, promoting glucose use and storage.
True · Insulin promotes storage, but the stored forms differ: liver and muscle hold glycogen, while fat tissue holds triglycerides.
Muscle glycogen supports local muscle work; fat favors fatty acid release · Muscle glycogen supports local metabolism. During fasting, lower insulin permits fat mobilization and fatty acid release rather than favoring net triglyceride storage.
Breakdown is favored over synthesis, increasing glucose release · Less insulin influence relative to glucagon favors glycogen breakdown over synthesis. This supports glucose release and limits wasteful opposing cycling.
Glucose synthesis continues as fatty acid use increases · Glucose synthesis already contributes before glycogen is depleted. It continues while many tissues increase fatty acid use; direct brain fatty acid use is not its replacement.
The brain uses more ketones, lowering its glucose demand · The liver makes ketones from fatty acids. Greater brain ketone use lowers its glucose demand, which can reduce the need for protein-derived glucose carbon.
Glucose-6-phosphatase conversion to free glucose for export · Muscle forms and uses glucose-6-phosphate locally. The missing conversion is glucose-6-phosphatase action, needed to make free glucose for export through this route.