Metabolism

Appetite

Hunger and

satiety are regulated by

a complex interaction of

multiple brain centers,

hormones, and

sensory and motor

pathways.

Hunger center

a region in the lateral hypothalamus that triggers the desire for food

stimulated destroyed

stimulated destroyed

Satiety center

a region in the ventromedial hypothalamus that suppresses the desire for food

The satiety center has neurons called glucostats that rapidly absorb blood glucose after a meal.

hypothesis: glucose uptake causes the satiety center to send inhibitory signals to the hunger center and thus suppresses the appetite.

Satiety Center

HungerCenter

Blood glucose

appetite

+

_

+

Gastric peristalsis

stimulates hunger.

Mild hunger

contractions begin soon

after the stomach is

emptied and increase in

intensity over a period of

hours.

- Adipocytes (fat cells) secrete

hormones (leptin) that regulate

appetite and body weight.

Role of Hormones in Appetite Regulation

- Hormones from GI:

cholecystokinin: suppressantghrelin: stimulantPYY: suppressant

(Science 299:846-849 2003)

Leptin's effects. Because of a gene defect, the boy doesn't make leptin, but treatment with the hormone, begun when he was 3.5 years old (top), brought his weight down to normal levels, as shown at age 8.

(Science 299:846-849 2003)

Nutrients

Major Classes of Nutrients :

carbohydrates, lipids, proteins, minerals, vitamins, and water.

Nutrients are used for:

1) providing energy

2) growth, repair, or maintenance of body cells

Metabolic Rate

is the amount of energy released in the

body per unit of time, expressed as kcal/hr

or kcal/day

37 C 38 C

1 Kg

A kilocalorie (kcal) is the amount of heat that will raise the temperature of 1 kg of water by 1oC.

An average adult needs 2,000-5,000 kcal/day, depending on physical activity, mental state, and other factors such as room temperature.

1,000 Kg water x 2 - 5 C

Caloric restriction (by 30%) prolongs life span by 30%-50% and reduces morbidity of aging-related diseases.

These effects have been observed in many animal species, including worms, insects, rodents, and maybe primates.

ProteinsLow fat/cholesterolVitaminsMinerals

Carbohydrates

unchanged

reduced by 30%

- ATP is the universal cellular energy, and can be produced from glucose, fat, and proteins.

- A total of 38 ATP is generated per molecule of glucose in the presence of oxygen.

Role of blood glucose in providing energy

Blood glucose is more important than fat and proteins in providing energy

- Glucose can be used by all tissue cells.

- Neurons and erythrocytes normally obtain

energy only from glucose.

hypoglycemia weakness, coma

Blood glucose level has to be maintained.

discussed in two states.

1) Absorptive State

lasts about 4 hours after a

meal.

2) Postabsorptive State

Maintenance of blood glucose

Absorptive State

- Blood glucose is

readily available for ATP

synthesis.

- Glucose serves as a

primary fuel and spares

the body from having to

draw on stored fuels.

Excessive glucose is stored as glycogen in the liver and muscles or as body fat.

Absorptive State

glycogen

fat

Fats are taken by the tissues,

especially adipose and muscular tissue.

Absorptive State

Amino acidsbecome available for

protein synthesis. Aminoacids

Regulation of the Absorptive State

- regulated largely by insulin, which stimulates nearly

all cells to absorb glucose.

glucose

cell

cell

blood

insulin receptors

insulin

glycogen

muscle

Postabsorptive State (fasting)

- prevails hours after meals and overnight.

- The essence of this state is to regulate

blood glucose levels, which is especially

critical to the brain.

Glucose is drawn from the body's glycogen reserves in liver and muscles, or synthesized from fats (gluconeogenesis).

glycogen

fats

After 4 to 5 days of fasting, the brain

begins to use ketone bodies as

supplemental fuel.

Postabsorptive State

- from fat

- acidosis

After glycogen and fat reserves are depleted

- The body begins to burn proteins.

- The first to go is skeletal muscle proteins.

glycogen

fats

proteins

Regulation of the Postabsorptive State

- by the sympathetic nervous system and several

hormones.

- The sympathoadrenal system can mobilize stored

energy reserves in adipose tissue as needed.

glycogen

fats

Glucagon promotes:

1) glycogenolysis glycogen glucose

2) gluconeogenesis AA/FFA glucose

3) lipolysis triglyceride FFA

glycogen

fats

Growth hormone also raises blood glucose

concentrations.

Body Heat and Thermoregulation

Body Temperature

- Body temperature fluctuates about 1oC in a 24-hour cycle.

lowest in the early morning

highest in the late afternoon

Core temperature

• in the cranial, thoracic, and abdominal cavities

• close to rectal temperature

37.2 -37.6 C

(99.0-99.7 F)

Shell temperature

- skin and oral

36.6 -37.0 C

(97.9-98.6 F)

heat

heat

Heat Production

Body heat is generated from:

1) nutrient oxidation (ATP production)

energy in glucose ATP

2) ATP use

energy in ATP mechanical energy

- At rest, mainly generated in brain, liver, heart, endocrine glands, and skeletal muscles (20-30%).

- During vigorous exercise, skeletal muscles produce 30-40 times as much heat as the rest of the body.

Heat Production

The body loses heat through:

Heat Loss

Conduction Evaporation Radiation

Thermoregulation

- Hypothalamic thermostat

1) monitors the blood

temperature

2) receives signals from

peripheral

thermoreceptors in the

skin.

Thermoregulation

heat-losing center

heat-promoting center

in the hypothalamus

thermostat37.5 C

When blood temperature is

too high

heat-losing center stimulates

1) dilation of dermal

arterioles

2) sweating

When blood temperature drops too low

heat-promoting center stimulates

1) dermal vasoconstriction

2) shivering thermogenesis.

3) later increase in

metabolic rate by 20-30%

as adaptation.

Disturbances of Thermoregulation

Exposure to excessive heat causes:

1) heat exhaustion hypotension, dizziness, vomiting, and sometimes fainting

2) heat stroke brain cell malfunction, convulsions, coma, and finally death

heat

glucose ATP

- refers to low body temperature

- when below 33oC (91oF), the metabolic rate drops so low that heat production cannot keep pace with heat loss, and the temperature falls further.-

Hypothermia

enzyme

Hypothermia

below 24oC (75oF) = fatal.

Which way helps a person survive longer in ice-cold sea water?

a. Swim vigorously

b. Rest and keep awake