Nutrient Partitioning: How the Body Allocates Energy

Oxidation Pathway

Oxidation refers to the complete breakdown of macronutrients to produce ATP, the primary energy currency of cells. When carbohydrates are oxidised, glucose undergoes glycolysis and subsequent steps of the citric acid cycle. Fatty acids enter the citric acid cycle through acetyl-CoA. Amino acids may be oxidised directly or converted to other substrates before oxidation.

The rate of oxidation is influenced by energy demand, hormonal signals, and available substrate. In the fed state (shortly after eating), insulin levels are elevated, which promotes glucose oxidation. In the fasted state, glucagon and cortisol promote fatty acid oxidation.

Storage Pathway

Carbohydrates consumed in excess of immediate energy needs are stored as glycogen in liver and skeletal muscle. Glycogen storage capacity is limited (approximately 300–500 grams total across all tissues). Once glycogen stores are repleted, additional carbohydrates are converted to fat through lipogenesis and stored as triglycerides in adipose tissue.

Dietary fat is readily stored as adipose tissue without substantial metabolic conversion. The efficiency of fat storage from dietary fat exceeds that of carbohydrate or protein, as less energy is expended in the storage process.

Dietary protein is less readily stored as fat but can be converted through gluconeogenesis and subsequent lipogenesis if consumed in excess of protein synthesis requirements.

Synthesis Pathway

Amino acids from dietary protein are utilised for synthesis of new proteins, enzymes, hormones, antibodies, and other functional molecules. The rate of protein synthesis is influenced by mechanical tension (exercise), hormonal signals (particularly mTOR and amino acid availability), and nutritional status.

Carbohydrates serve as precursors for synthesis of nucleotides, glycosaminoglycans, and other molecules. Acetyl-CoA derived from various substrates is used in fatty acid synthesis and cholesterol production.

Metabolic State

Fed state: Following food consumption, elevated insulin and other anabolic hormones shift partitioning toward storage and synthesis. Glucose oxidation increases. Fatty acid oxidation decreases.

Fasted state: In the absence of dietary input, glucagon and cortisol increase, promoting fatty acid oxidation and gluconeogenesis. Protein synthesis decreases.

Macronutrient Composition

The ratio of carbohydrate, protein, and fat influences partitioning through multiple mechanisms. Protein consumption stimulates protein synthesis more substantially than carbohydrate or fat. Protein also increases thermic effect of food (energy expenditure during digestion) more substantially than other macronutrients.

Carbohydrate composition (simple vs. complex, glycaemic index) influences insulin response and oxidation rates. Fat composition (saturated vs. unsaturated, chain length) influences absorption efficiency and metabolic effects.

Physical Activity

Exercise increases energy demand and alters partitioning through multiple signals. Muscle contraction activates AMP-activated protein kinase (AMPK) and increases glucose uptake independently of insulin. Resistance exercise stimulates protein synthesis through mechanical tension and hormonal signals.

The timing and composition of nutrient intake relative to exercise influences whether dietary amino acids are partitioned toward muscle protein synthesis or oxidation.