The average American eats 2500 kcal/day – 40% carbohydrates, 40% fats, 20% proteins
vitamins and minerals do not contribute to energetics
ideally, there should be the same amounts of carbohydrates and fats, and protein should be 1 gram per kg (good protein)
Fuels in the Blood
(none are phosphorylated in serum):
(1) glucose
– from starch (amylose/amylopectin), lactose, sucrose – requires insulin for transport into muscle
(2) fatty acids
– made into acetyl CoA
(3) ketone bodies
– made from fatty acids
(4) glycerol
from triglyceride breakdown – can be used to make glucose
(5) lactate and pyruvate
from anaerobic glycolysis in red blood cells and muscles
(6) ethanol
– causes glucose to rise then fall
Note: cholesterol cannot be used as a fuel – it is excreted in bile
During fasting, the body must make new glucose to feed body tissues, especially the brain (requires 600 kcal/day). Therefore, the body uses different fuels under different circumstances (fuel sparing):
(1) lipids
– stored as triglycerides (long chain fatty acids esterified to glycerol) – hydrophobic, so easy to store
all metabolism involves oxidation of reduced fuels
– lipids are highly reduced, so very energetic (9 kcal/g)
85% of total body stored energy is in lipids, so they are used first
however,
metabolism of lipids produces acetyl CoA rather than glucose – cannot feed all organs (e.g., brain)
lipids can be used to make ketone bodies during starvation (>3 days)
(2) carbohydrates
– hydrophilic, so more difficult to store (must be hydrated) – also not as reduced (4 kcal/g)
stored as glycogen in muscles and liver, but only liver glycogen can release free glucose
less than 1% of total body stored energy is in glycogen, so it cannot be used for long
(3) proteins
– only mechanism for making new glucose after glycogen is depleted
there is no store of protein – must use body tissues (e.g., muscles such as diaphragm) – hence starvation
Fuel Sparing
- due to brain energy requirements, blood glucose is kept constant during starvation (90-100 mg/100 ml)
maintain during shortage by gluconeogenesis from liver glycogen or muscle protein
after 1-2 days of fasting, gluconeogenesis slows (protein no longer broken down)
ketone bodies
are formed instead (acetoacetate, beta-hydroxybutarate, and acetone)
Metabolic Capacities of Various Tissues
Liver
– very versatile and "altruistic" toward other organs – makes glucose, triglycerides, and ketone bodies (from fatty acids); metabolizes urea