Metabolism—How do we obtain energy from foods?

Susan Algert, Ph.D., R.D.

Metabolism

• Human body releases energy from chemical bonds in nutrients the body uses for fuel.

• As bonds break they release energy

• During metabolism, energy, water and carbon dioxide are released

Energy yielding nutrients

• From carbohydrates—glucose

• From lipids (trigylcerides)—glycerol and fatty acids

• From proteins—amino acids

The Cell

• Cells are work centers of metabolism

• Cells have similar structures

• Two basic parts—nucleus and cytoplasm

• Mitochondria are power generators that contain energy generating pathways

Breaking down glucose for energy--aerobic

• 6-C glucose split in half making two 3-Carbon compounds

• Glycolysis means glucose splitting• -Carbon compounds become 2

pyruvates• Pyruvates will break down further to

form ATP and heat

Glucose retrieval via the Cori cycle--anaerobic

• When less oxygen is available, pyruvate is converted to lactic acid

• Liver can convert lactic acid to glucose in a recycling process

• Pathway is muscle glycogen to glucose to pyruvate to lactic acid ( in liver) to glucose to glycogen

Pyruvate to Acetyl Co-A

• Irreversible step

• Aerobic

• Acetyl Co A to Carbon Dioxide via the TCA cycle

Electron Transport Chain

• Acetyl Co-A to fat

Pyruvate is pivotal

• ATP levels are low—metabolic pathways flow toward the production of ATP

• Depending on O-2; ATP routes pyruvate to acetyl Co-A or lactate

• ATP is abundant; pyruvate converted to oxaloacetate or amino acid alanine; oxaloacetate converted to glucose and then glycogen

Acetyl Co-A at the crossroads

• Breakdown pathways for glucose, fatty acids and some amino acids converge at acetyl-CoA.

• Acetyl Co-A cannot return to pyruvate, but enters energy making pathways

• Acetyl Co-A can also make ketone bodies and fatty acids

Glycerol and fatty acids

• Glycerol to pyruvate

• Fatty acids to Acetyl Co-A

Beta oxidation

• Glucose not retrievable from fatty acids

• Breakdown of acetyl-Co-A

• Fat burns in flame of carbohydrate

When a person draws on stores

• Fat used to fuel brain

• Acetyl Co-A fragments from fatty acids combine to produce ketone bodies

• Ketone bodies can provide some fuel for brain cells

• When ketone bodies contain an acid group they are called keto acids (COOH)

Energy Compounds

• ATP used to power cellular functions

• NADH and FADH-2 carry energy for synthesis of ATP

• NADPH delivers energy for biosynthesis

Amino Acid Catabolism

• Amino acids are deaminated and enter TCA cycle

• Amino acids used to make pyruvate can make glucose

• Amino acids that make Acetyl Co-A provide energy or body fat but not glucose

Energy retrievable from amino acids-

• Glucogenic amino acids- a.a. broken down into pyruvate or intermediate of the TCA cycle; gluconeogenesis

• Ketogenic amino acids—an a.a.a broken down into acetyl CoA which can be converted into ketone bodies

Transamination

• Transfer of amino group from one amino acid to a keto acid, producing a new non essential amino acid and a keto acid

Electron Transport Chain

• Series of proteins that serve as electron carriers

• Mounted in sequence on membrane inside mitochondria

• Carriers receive electrons, it passes electrons and gives up energy until end when any usable energy has captured body’s ATP molecules

TCA and ETC

• Body’s most efficient means of capturing the energy from nutrients and transferring it into the bonds of ATP

• Last step of ETC low energy electrons with H atoms combine with O2 from the lungs to make H2O

Which fuels can make glucose

• Parts of protein and fat that can make pyruvate can provide glucose; parts that make acetyl Co A cannot, but provide fat

• Glucose is needed to fuel CNS and red blood cells

• If there is not enough glucose the body will break down protein

Making glucose on low Carb diets

• Fat delivers mostly acetyl, so that you need to break down protein tissue to make glucose

• High protein diets make your body convert protein to glucose and convert ammonia to urea in the liver

• Urea is excreted via the kidneys• Water is needed to excrete urea

Energy yielding nutrients-fat provides most kcals per gram

• Nearly all bonds in a fatty acid are between carbons and hydrogens

• Oxygen can be added to all of them (making CO2 and H2O)

• Energy in bonds is released as they are oxidized

• Glucose has less potential for oxidation as oxygen is already bonded to each C

Feasting

• Surplus protein—Deaminate and convert to acetyl Co-A and fat

• Surplus carbohydrate--Glycogen

• Surplus fat--Lipogenesis