Energy Expenditure

• basal metabolism- involuntary processes required for life

  • makes up 60-75% of energy expenditure

  • includes breathing, circulation, regulating body temperature, making tissues, waste removal, nerve signals

• basal metabolic rate (BMR)- rate of energy expenditure under energy conditions

  • influenced by weight, gender, age, exercise, hormones, fasting

• lean body mass (muscle) has a higher BMR than fat body mass

  • as you age, lean body mass decreases → so BMR decreases too

  • men tend to have higher BMR than women because they generally have more lean body mass

• total energy expenditure = BMR + energy used in digestion/absorption + activity level

Energy Balance

• energy balance- when energy consumption equals energy expenditure

  • results in weight maintenance

• negative energy balance- when energy intake is less than energy expenditure

  • results in weight loss

• positive energy balance- when energy intake is more than energy expenditure

  • results in weight gain

Protein Metabolism

• Protein is an essential nutrient

• Proteins are made of 20 different amino acids

  • There are 9 essential amino acids that cannot be made by the body and must be obtained from the diet

• When we eat protein, that gets broken down into amino acids that circulate in the bloodstream.

• The amino acids then go inside cells to synthesize our own proteins

  • The proteins we synthesize can last for varying amounts of time

    • proteins like hemoglobin or collagen in the bones can last for a long time

    • regulatory proteins may only be around for a little while before they are degraded

    • the longevity of a protein can be expressed by its half-life- the amount of time it takes for 50% of the protein sample to be degraded

• We can also use proteins to synthesize other nitrogen-containing compounds such as the nitrogenous bases of DNA

• If our protein intake fulfills the needs of replenishing the amino acid pool and synthesizing nitrogen-containing compounds, the amino acids get broken down into ammonia and the carbon skeleton

  • ammonia is highly toxic and is made into urea in the liver and kidneys

    • urea is eliminated in the urine

  • the carbon skeletons get stored as fat in the fed state

  • in the fasting state, the carbon skeletons can be used in 2 ways

    • the carbon skeletons of ketogenic amino acids can be modified into a compound that directly is used in energy production

    • the carbon skeletons of glucogenic amino acids can be modified into glucose

Carbohydrates

• 2 main types- sugars (mono or disaccharides) and fiber (usually plant polysaccharides)

• starch and glycogen are glucose polysaccharides

• starch is a glucose polymer where the glucose molecules are joined together in α-1,4 glycosidic linkages (a bond between carbon 1 of one glucose molecule and carbon 4 of another glucose molecule where the OH on carbon 1 points below the ring)

• **$glycogen$**is a glucose polymer where the glucose molecules are joined together in

  α-1,4 linkages and α-1,6 glycosidic linkages (a bond between carbon 1 of one glucose molecule and carbon 6 of another glucose molecule where the OH on carbon 1 points below the ring)

• cellulose is a glucose polymer where the glucose molecules are joined together in $β-1,4 glycosidic linkages$(a bond between carbon 1 of one glucose molecule and carbon 6 of another glucose molecule where the OH on carbon 1 points below the ring)

  • cellulose is indigestible in humans because we do not make an enzyme to digest β-1,4 glycosidic linkages

  • cellulose passes through the gut and gives bulk to stool

Fiber and Health

• Fiber is important in the diet because it helps with

  • normalizing bowel movements

  • help maintain bowel health

    • lower risk of hemorrhoids/diverculitis

    • lower risk of colon cancer

  • soluble fiber (fiber that dissolves in water) lowers serum cholesterol

    • fiber can also reduce blood pressure and inflammation

• whole grains control blood sugar levels better than refined carbohydrates

  • help control weight

  • more filling

  • help you eat less

  • longer duration of satiety

• increased fiber intake also reduces risk of death from cardiovascular disease and certain cancers

Why does there need to be glucose in the blood?

Red Blood Cells

• RBCs don’t do the krebs cycle or oxidative phosphorylation because their function is to transport oxygen, so they shouldn’t be using it

  • RBCs have no mitochondria

• RBCs exclusively do anaerobic glycolysis

• basic rundown of glycolysis

  • glucose gets phosphorylated to glucose-6-phosphate (G6P)

  • G6P gets broken down into 2 pyruvates

• 2 pyruvates are then made into 2 lactates so 2 H+ is released to continue glycolysis

• This only nets 2 ATPs per glucose

Brain and Nervous Tissue

• nervous tissue carries out all 3 steps of cellular respiration (glycolysis, Krebs/Citric acid/TCA cycle, and oxidative phosphorylation)

• It is very dependent on glucose and cannot use fatty acids for energy

  • fatty acids do not cross the blood-brain barrier easily

• during a prolonged fast, the brain can adapt to using ketone bodies for energy

Metabolism of Glucose

• insulin is released when blood glucose levels increase → fed state

• glucagon is released when blood glucose levels decrease → fasting state

• fatty acids provide the energy for gluconeogenesis to occur but the carbons themselves come from amino acids

Fats

• fats are stored in the body as triglycerides- a glycerol molecule with 3 fatty acid chains

• the H in the carboxylic acid of the fatty acid and OH of the glycerol are removed to make a bond between glycerol and the fatty acid- ester linkage

• this happens 3 times because glycerol has 3 alcohol groups

Lipid Metabolism (Fasted State)

• hormone-sensitive lipase in adipose tissue gets activated by high glucagon & low insulin levels

• lipase breaks triglycerides into 3 fatty acids and glycerol by hydrolysis

• the fatty acids get bound to albumin to be released into the bloodstream

Ketone Body Synthesis & Utilization

• acetoacetate and beta-hydroxybutyrate are called **$ketone bodies$**because they have ketone groups in them

• acetoacetate can spontaneously break down into acetone

  • acetone gives off sweet smell in breath/sweat/urine → how HCPs know when pt is in ketoacidosis