Energy Processing & Metabolism

intro

-Storing and releasing energy from food

-Sources of energy expenditure

major sites where energy is processed

Liver

-Receives most nutrients (except fat and cholesterol) directly from the hepatic portal system

-Can store/release glucose and synthesize fats (*and chol)

Muscle

-Takes up glucose and stores as glycogen

-Uses its own glycogen for energy – does not release back into blood

Adipose

-Stores/releases fats

-Synthesizes fats from glucose

*most extra kcal → fat (can store a lot of fat)

Absorptive and Postabsorptive States

Absorptive State

-Nutrients are being absorbed from GI tract

-Carbohydrates from food are used for energy by most cells (*insulin)

-Excess energy mostly stored as fat

Postabsorptive State

-Begins approximately four hours after a meal

-Energy is released from body stores (*release fats from adipose, and break down glycogen)

Summary: what is happening to each nutrient during the absorptive state?

*high insulin levels; goal=store

Glucose:

-used for energy by most cells

-max out glycogen stores

-extra converted to fat and stored

Amino acids:

-used for protein synthesis

-extra converted to fat

Fats:

-used for energy (except brain)

-extra store in adipose

*muscles burn fat if lower intensity activity (ex walking=50%), if higher intensity use more glucose bc use less O2 and easier to break down

Summary: what is happening to each nutrient during the postabsorptive state?

*low insulin levels

Glucose:

-produce from glycogen breakdown OR gluconeogensis

Amino acids:

-use to make glucose

-protein catabolism

Fats:

-released into blood from adipose stores

*tap into stored energy; gluconeogensis AA → glucose; nervous tissue needs steady glucose

response of target cells to insulin

-insulin determines if in absorptive or postabsortive state

*green=promoted by insulin; red=insulin is low

Which organ does not make significant use of fatty acids for energy?

brain

-uses glucose or ketones (from fat breakdown, make when fasting/low glucose as back-up source, so doesn’t directly use fatt acids)

conservation of energy

-Energy neither created nor destroyed but converted between forms

-Energy taken in from food: chemical energy (*energy in bonds)

-Can be converted to:

• mechanical energy: move muscles, do work

• heat energy: byproduct of metabolism and mechanical processes

energy balance in the body: equation + internal vs external heat

-Internal heat: produced by metabolic processes and other activities that generate heat as a byproduct

-External work: energy used to move objects, using skeletal muscle

4 sources of energy expenditure

-Basal metabolism: metabolic cost of living to keep gong; largest fraction!

-Physical activity: “intential exercise”

-Thermic effect of food: energy to digest food; protein > carbs > fats

-Non-exercise activity thermogenesis (NEAT): ex=body posture, standing up, on feet all day; non-intentional movement

Basal metabolic rate (BMR)

-Measured under standard conditions

• rest

• comfortable room temp

• no food in last 12 hours/fasted

-“Metabolic cost of living”

-Thyroid hormone is the main determinant of BMR

• Increased T3 and T4 increase BMR

• Calorigenic effect (*generation of heat from metabolism; upregulate Na/K pump)

Regulation of body weight

-Weight (more accurately, total energy storage) is maintained at a set point by feedback mechanisms (*set point genetic and other factors)

-Decrease food intake to cause 10% weight loss: BMR goes down (*why plateau in 3 months WL, body recalibrates to evolutionary prevent weight loss)

-Increased weight leads to increased BMR (*weaker than last point; bc evolutionary want to maintain storaage and not resist gaining extra)

*higher muscle mass=higher BMR

Leptin

-Produced by adipose tissue (*increase adipose=increase leptin)

-Inhibits neurons in hypothalamus that stimulate appetite

-Increases BMR

-Wonder drug for weight loss?: Nope (*bc can become resistent to it; GLP1s more effective and also effect satiety signals)