How we use food to produce energy: introducing the concept of Cellular Nutrition

The human body can be seen as a biological machine and food is the fuel necessary for the functioning of this machine. Many substrates of dietary origin can be used as metabolic fuel:

·        glucose is the main form of dietary energy and derives from the digestion of carbohydrates (fructose and galactose are minor monomers obtained from carbs);

·        fatty acids, from dietary fats, are the second major source of energy;

·        amino acids, obtained from the breakdown of dietary proteins, are also an important source of energy.   

The conversion of the food into usable energy occurs inside every single cell. Indeed our body doesn’t contain a specific organ able to perform this conversion and then the distribution of the energy to various tissues. Generally any type of cells, a part from few types (e.g. red blood cells),  has specific organelles, termed as mitochondria, able to generate energy (mainly in form of ATP) from dietary compounds.

The demand for energy depends on the metabolic rate of distinct tissues and organs. For instance adipose tissue has the lowest metabolic rate, while liver, brain, heart and kidney are metabolically the most active tissues.

Tissues can use different metabolic fuels depending on the fuel availability and physiological conditions, as showed in the following table

Tissue	Metabolic fuels for distinct tissues
Brain	Glucose (almost exclusively), ketone bodies (prolonged starvation)
Muscle	Glucose, fatty acids, ketone bodies (starvation), acetate (after alcohol ingestion), triacylglycerol, branched-chain amino acids
Liver	Amino acids, fatty acids, glucose, alcohol
Kidney: ·        Cortex ·        Medulla	Glucose, fatty acids, ketone bodies Glucose
Gastrointestinal tract: ·        Small intestine ·        Large intestine	Glutamine, ketone bodies (starvation) Short chain fatty acids, glutamine, glucose
Red blood cells	Glucose
Leukocytes	Glutamine, glucose, fatty acids

 

Ultimately any single cell of our body is an independent energy factory, able to fulfil its own energy requirement using various dietary molecules.

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