BCMB2001 Chap.4 Fasting Fuels: Glycogenolysis, Lipolysis, Ketones
Fasting Fuels: Glycogenolysis, Lipolysis, Ketones
Fasting is a timed redistribution of fuel, not a global shutdown. The immediate objective is to protect circulating glucose for tissues with a strong dependence on it while allowing other tissues to shift toward fatty acids and, later, ketone bodies. In the early post-absorptive state, liver glycogen provides a rapid glucose buffer.
As the fast continues, glycogen's contribution declines and gluconeogenesis carries more of the glucose supply.
White adipose tissue releases fatty acids and glycerol through lipolysis; fatty acids support oxidative metabolism in many tissues, while glycerol can contribute carbon to glucose production.
Protein is also a potential carbon store, but its mobilisation sacrifices functional tissue, so it is better treated as a costly last-resort contribution than as the preferred fasting fuel.
Amino-acid carbon can support gluconeogenesis while nitrogen requires safe disposal. In liver, rising fatty-acid oxidation supplies energy and acetyl-CoA. When oxaloacetate availability is drawn toward glucose production and acetyl-CoA input exceeds Krebs-cycle handling, ketone-body synthesis becomes an alternative route for exporting oxidisable carbon.
Ketone bodies help other tissues use fat-derived carbon without receiving fatty acids in the same form and, during prolonged fasting, reduce pressure on glucose and protein. The core exam move is chronological: identify the current phase, name the organ doing the work, state which store is rising or falling, and explain why the transition protects whole-body function.
What this chapter covers
- 01
The post-absorptive objective: maintain circulating fuel while dietary input has stopped
- 02
Liver glycogen mobilisation as a rapid but finite glucose buffer
- 03
The increasing contribution of gluconeogenesis from lactate, glycerol and glucogenic amino-acid carbon
- 04
White-adipose lipolysis and distribution of fatty acids and glycerol to different metabolic fates
- 05
Proteolysis as a costly contribution that threatens functional tissue if sustained
- 06
Hepatic ketone-body synthesis and the shift in tissue fuel use during prolonged fasting
Compare early and prolonged fasting
- +1 (AskSia)Early fasting uses liver glycogen heavily because it can supply circulating glucose quickly. In prolonged fasting that reserve is depleted and its contribution is much smaller.
- +1 (AskSia)Gluconeogenesis rises as fasting continues and becomes the main continuing route for endogenous glucose production once glycogen can no longer carry the load.
- +1 (AskSia)Adipose lipolysis increases, releasing fatty acids for oxidation and glycerol that can enter gluconeogenic carbon flow.
- +1 (AskSia)Hepatic ketone production is modest early and becomes more important later as fatty-acid oxidation supplies abundant acetyl-CoA and other tissues adapt to ketone use.
- +1 (AskSia)Protein carbon can contribute, especially before ketone adaptation reduces glucose demand, but prolonged survival requires limiting continued protein loss because proteins are functional tissue rather than a dedicated inert store.
Key terms
- Post-absorptive state
- The period after nutrients from the latest meal are no longer entering circulation and endogenous stores must support demand.
- Glycogenolysis
- Mobilisation of glycogen to phosphorylated glucose units, with liver positioned to support circulating glucose.
- Lipolysis
- Hydrolysis of stored triacylglycerol, releasing fatty acids and glycerol for distinct downstream uses.
- Ketogenesis
- Hepatic conversion of acetyl-CoA-derived carbon into ketone bodies for export to other tissues.
- Proteolysis
- Breakdown of proteins to amino acids, providing carbon and nitrogen but sacrificing functional material when sustained.
Fasting Fuels: Glycogenolysis, Lipolysis, Ketones FAQ
Why does the body not use only glycogen throughout a fast?
The store is finite and liver glycogen cannot indefinitely support whole-body glucose demand. Its value is speed during the transition away from dietary input. As it declines, gluconeogenesis becomes essential and non-glucose fuels carry more energy demand. A good answer says both why glycogen rises initially and why its contribution must later fall.
Why does fatty-acid oxidation support gluconeogenesis if fat does not simply become glucose?
Fatty-acid oxidation supplies ATP and reducing power required by the energetically expensive glucose-production pathway. It also generates acetyl-CoA that regulates pathway choice. Glycerol released from triacylglycerol can contribute glucose carbon, but the even-chain fatty-acid carbon entering acetyl-CoA is not the straightforward net source students often assume.
What causes ketone production to rise?
Adipose fatty-acid delivery and hepatic beta-oxidation increase, supplying acetyl-CoA and energy. At the same time, carbon that supports oxaloacetate can be directed toward gluconeogenesis, constraining acetyl-CoA handling through the Krebs cycle. Ketogenesis packages some of that carbon into exportable fuel. State the supply, the bottleneck and the export purpose.
Is proteolysis a normal fasting response or a pathological one?
It can contribute to normal fasting adaptation because glucogenic amino-acid carbon supports glucose production, but it has a high biological cost. Proteins perform structural and catalytic functions, so continued loss cannot be treated like drawing down a neutral storage depot. Ketone adaptation matters partly because it reduces glucose demand and helps spare protein.
Exam move
Build a three-column fasting timeline: early post-absorptive, established fasting and prolonged fasting. In each column list circulating fuels, liver output, adipose output, muscle use and the source of gluconeogenic carbon. Draw arrows only after writing the whole-body objective. Then practise counterfactuals: blocked lipolysis, impaired hepatic beta-oxidation, depleted glycogen or failure of ketone use.
For each, predict glucose support, protein pressure and ketone availability. Avoid vague statements such as fat takes over; name whether you mean fatty-acid oxidation in a peripheral tissue, hepatic energy supply or ketone export. This organ-by-organ precision is the fastest way to prevent contradictory answers.
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