Energy Metabolism

The 3 main sources of energy in your body are fat, sugars and protein.

Serum Energy

Serum Fatty Acid Reference Ranges relevant to Australia, UK, USA, and other countries where fat intakes are similar to New Zealand (about 30% of total energy intake) © Bradbury, K.E. et al

Table 3; Range, mean and percentiles of fatty acid concentrations (μmol/L) of plasma total lipids © Abdelmagid, S.A. et al

Fat

Plasma is composed of all major circulating lipid species including triglycerides, phospholipids, cholesterol-esters and free fatty acids.[1]

Triglycerides (triacylglycerol) are the main constituents of adipose tissue (body fat) in humans. 25% of the molecules in triglycerides are glycerol, and 75% fatty acids. Fatty acid levels in adipose tissue reflect intake in years.[2] Serum fatty acids reflect intake in weeks.[3] Fatty acids are also stored in muscles [4], as resting muscles predominantly use fatty acids for energy, though not as triglyceride droplets (adipose fat).

Mean total serum fatty acid levels may be 6948 μmol/L (mean molar mass 276.4 g/mol)[5] corresponding to about 192 mg/dl. That is about 10 gram per adult human, with a standard deviation of 2.6 gram, which accounts for 23 kcal.

About 2 to 5% of serum fatty acids are free fatty acids. Serum reference levels for free fatty acids are 0 to 720 μmol/L.[6] / with a mean value of 7.5 nM (with a standard deviation of 2.5 nM), and a 1.5 nM increase after overnight fasting. [7]

Sugars

Your brain, red blood cells and retinal cells have an obligatory need for glucose, which may be supplied by all sugars. Due to glycerol's (relative to fatty acids') low molecular weight (92.094 g/mol) and caloric value (4.31 kcal/g [8]], glycerol may account for about 5% of total caloric value of triglycerides, depending on the length of the specific fatty acids. This equals energy expenditure during sleep (95% of energy coming from fatty acids).

Activities

Fasting

During starvation, utilisation of fatty acids from intracellular lipid droplets is increased, and autophagy mediated.[9]

Exercise

Not just daily exercise increases lipolysis in general [10], but also acute exercise increases lipolysis, specifically in the liver (not in skeletal muscle), accompanied by increased serum levels of free fatty acid, glycerol and ketone body.[11]

Better fitness is not associated with lower dietary fat intakes.[12] Increased physical activity reduces serum triglyceride levels.[13] At similar levels mean VO2, tennis match play relies more on glycolysis and glycogenolysis compared to continuous running exercise.[14]