Energy Metabolism
The 3 main sources of energy in your body are fat, sugars and protein.
Serum Energy
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]
Mean total serum fatty acid levels may be 6948 μmol/L (mean molar mass 276.4 g/mol)[4] 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 0.00-0.72 mmol/L.[5] / 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. [6]
Sugars
Due to glycerol's (relative to fatty acids') low molecular weight (92.094 g/mol) and caloric value (4.31 kcal/g [7]], 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).
Protein
Activities
Fasting
During starvation, utilisation of fatty acids from intracellular lipid droplets is increased, and autophagy mediated.[8]
Exercise
Not just daily exercise increases lipolysis in general [9], 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.[10]
Better fitness is not associated with lower dietary fat intakes.[11] Increased physical activity reduces serum triglyceride levels.[12] At similar levels mean VO2, tennis match play relies more on glycolysis and glycogenolysis compared to continuous running exercise.[13]
