Metabolism—How do we obtain energy from foods?

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

Metabolism—How do we obtain energy from foods?