metabolism

anabolic reactions: building glycogen (glucose), protein (amino acids) and triglycerides (glycerol + fatty acids). they require energy

catabolic reactions: breaking them down. they release energy. much energy is stored in bonds of ATP which fuels anabolic reactions

amino acids, glucose and glycerol (but not fatty acids) can then be converted to pyruvate

glucose

glucose first goes through glycolysis to produce pyruvate

glycolysis : the breakdown of glucose to pyruvate

pyruvate can then be converted either to:

• acetyl-CoA (aerobic)
• lactate (anaerobic): during intense exercise, muscles rely on anaerobic glycolysis to produce ATP quickly. pyruvate is converted to lactate, which travels to liver and gets converted back to glucose, enabling glycolisis to continue (Cori cycle)

conversion of pyruvate to acetyl-CoA produces more ATP than glycolysis

tryglicerides

tryglicerides are broken down to glycerol and fatty acids

glycerol can be converted to pyruvate (and then acetyl-CoA) or glucose and makes up ~5% of the energy in a triglyceride

amino acids

most amino acids can be converted to pyruvate (glycogenic)

some amino acids are converted to acetyl-CoA directly (ketogenic)

some amino acids can enter TCA cycle directly as compounds other than acetyl-CoA (glucogenic)

acetyl-CoA

most energy-yielding nutrients are converted to acetyl-CoA. acetyle-CoA can be used to make ATP through TCA cycle. when there is abundant ATP, acetyl-CoA makes fat

TCA cycle

chemistry...

fat is the preferred energy source due to higher energy yield per gram

feasting (excess energy)

in this case, metabolism favors fat formation

pathway from dietary fat is simplest (costs less energy)

excess of protein and carbs increases oxidation, whereas excess of fat gets stored efficiently

carbs get stored as glycogen, but the capacity is limited and excess is converted to fat.

maintaining glucose levels is critical, the body uses glucose frugally when the diet provides small amounts

fasting (low energy)

gluconeogenesis : the synthesis of glucose from non-carbohydrate sources

as glucose is the primary energy source, lack of carbohydrates becomes a problem

glycerol and amino acids can be converted to glucose. to obtain amino acids, the body breaks down muscle protein

in the first few days of a fast, about 90% of the energy comes from breakdown of body protein (10% from glycerol)

if breakdown of body protein continued at this rate, death would result in less than 3 weeks. fortunately, fat breakdown increases (almost doubles).

as the fast continues, the brain start to use fat as a fuel source (acetyl-CoA fragments from fatty acids produce an alternate fuel for the brain - ketone bodies)

ketone body production rises for about 10days. still many areas of the brain rely exclusively on glucose and to produce it, the body continues to sacrifice protein