Fed vs fasting metabolism
Fed vs fasting metabolism describes how the body changes fuel use after eating and during periods without food. In the fed state, glucose use, glycogen storage, and nutrient storage are favored. As fasting continues, insulin falls, glucagon becomes more important, glycogen breakdown increases, fat mobilization rises, and ketone production may become more important.
Core idea
This calculator uses a rule-based teaching model to estimate the dominant metabolic state from time since the last meal, carbohydrate intake, insulin-glucagon setting, exercise state, and liver glycogen availability.
\[
\begin{aligned}
S_{\text{state}} &= f(t,\ C_{\text{meal}},\ G_{\text{liver}},\ H,\ A)
\end{aligned}
\]
Here, \(S_{\text{state}}\) is the score for a metabolic state, \(t\) is time since the last meal, \(C_{\text{meal}}\) is meal carbohydrate amount, \(G_{\text{liver}}\) is liver glycogen availability, \(H\) represents the insulin-glucagon setting, and \(A\) represents rest or exercise state.
Fed-to-fasting progression
Immediately after a meal, especially a carbohydrate-rich meal, insulin tends to dominate. This supports glucose uptake, glycogen storage, and lipogenesis. Several hours later, the post-absorptive state begins, and the liver helps maintain blood glucose through glycogen breakdown.
During early fasting, glucagon influence increases, glycogen breakdown becomes important, and lipolysis begins to contribute more. During prolonged fasting, liver glycogen becomes limited, fatty acids become a major fuel source, and ketone production may rise as a glucose-sparing adaptation.
Main pathway directions
The calculator summarizes metabolic switching using pathway directions:
\[
\begin{aligned}
\text{Fed state} &\rightarrow \text{glucose use, glycogen storage, lipogenesis} \\
\text{Post-absorptive state} &\rightarrow \text{hepatic glucose support} \\
\text{Early fasting} &\rightarrow \text{glycogen breakdown and lipolysis} \\
\text{Prolonged fasting} &\rightarrow \text{fat oxidation, ketone production, glucose sparing}
\end{aligned}
\]
How to interpret results
The dominant metabolic state is the state with the highest rule-based score. The hormone pattern shows whether the model predicts insulin dominance, glucagon dominance, or a transitional balance. The main fuel source is the fuel with the highest relative score among glucose, glycogen, fatty acids, ketones, and amino acids.
- Use time since last meal in hours.
- Use carbohydrate amount in grams.
- Use liver glycogen as an estimated teaching value, not as a clinical measurement.
- Exercise shifts the model toward greater glycogen use and fat mobilization.
- Manual feeding-state selection can be used to force a specific teaching scenario.
Example: a high-carbohydrate meal after 1 hour usually produces a fed, insulin-dominant pattern with glucose use and glycogen storage favored. An overnight fast around 12 hours usually shifts toward a post-absorptive or early fasting pattern, where glycogen breakdown and fat mobilization become more important.
This calculator is designed for physiology learning and metabolic-state comparison. It does not diagnose diabetes, fasting safety, hypoglycemia, metabolic disease, or nutritional adequacy. Real metabolism depends on hormones, liver glycogen, muscle activity, body composition, training status, disease states, and recent diet history.
