Metabolic fuel use

This graph illustrates the percentage of maximal energy supplied by various sources over time, ranging from a few seconds to two hours. The X-axis represents duration of exercise, and the Y-axis represents % Maximal energy by source. The lines represent Stored ATP, Creatine phosphate, Anaerobic metabolism, Aerobic metabolism, and Overall performance.

#boundingbox

Caloric Values

• 1g carb/protein = 4 kcal
• 1g alcohol = 7 kcal
• 1g fatty acid = 9 kcal

As exercise continues beyond a few seconds, the body shifts to other energy systems. Anaerobic metabolism becomes dominant for activities lasting up to a minute, followed by aerobic metabolism taking over for longer durations. The overall performance curve reflects the combined contribution of these systems, as shown in the previous graph.

Fasting and starvation

Priorities are to supply sufficient glucose to the brain and RBCs and to preserve protein.

Fed state (after a meal)

Glycolysis and aerobic respiration. Insulin stimulates storage of lipids, proteins, and glycogen.

Fasting (between meals)

Hepatic glycogenolysis (major); hepatic gluconeogenesis, adipose release of FFA (minor). Glucagon and epinephrine stimulate use of fuel reserves.

Starvation days 1-3

Blood glucose levels maintained by:

• Hepatic glycogenolysis
• Adipose release of FFA
• Muscle and liver, which shift fuel use from glucose to FFA

• Hepatic gluconeogenesis from peripheral tissue lactate and alanine, and from adipose tissue glycerol and propionyl-CoA (from odd-chain FFA—the only triacylglycerol component that contributes to gluconeogenesis)

Glycogen reserves depleted after day 1. RBCs lack mitochondria and therefore cannot use ketone bodies.

Starvation after day 3

Adipose stores (ketone bodies become the main source of energy for the brain). After these are depleted, vital protein degradation accelerates, leading to organ failure and death. Amount of excess stores determines survival time.

This graph shows the depletion of stored energy in kilograms over weeks of starvation. The X-axis represents Weeks of starvation, and the Y-axis represents Stored energy (kg). Lines indicate Carbohydrate, Fat, and Protein stores.

#boundingbox

Topics Covered:

• Metabolic Fuel Use: Initial Exercise
• Metabolic Fuel Use: Longer Exercise and Caloric Values
• Fasting and Starvation: Introduction and Fed State
• Fasting and Starvation: Between Meals
• Starvation Days 1-3: Fuel Shifts
• Starvation Days 1-3: Gluconeogenesis and RBCs
• Starvation After Day 3 and Stored Energy Depletion