Transcript Document

Nutrient Regulation Helps Prepare for Future Needs
Aside from obtaining energy from ingested food, nutrients are
needed for growth, maintenance, and repair of the body.
The process of digestion—breaking down food—is controlled by the
nervous system, which also anticipates future requirements.
Basal metabolism is energy used for heat production, maintenance
of membrane potentials and life-sustaining processes.
Kleiber’s equation for the rate of basal metabolism—a rule that
relates energy expenditure to body weight:
kcal/day = 70 × weight0.75
The Relation between Body Size and Metabolism
Nutrient Regulation Helps Prepare for Future Needs
Energy expenditure is adjusted in response to nutrition.
At the start of a diet (less nutrition), the basal metabolic rate
will fall to prevent losing weight.
Restricted food intake promotes longevity, perhaps due to
trophic factors that promote cell growth.
Why Losing Weight Is So Difficult
The Benefits of Caloric Restriction in Monkeys
Nutrient Regulation Helps Prepare for Future Needs
Glucose is the principal sugar used for energy.
Glycogen is a complex carbohydrate, made by the combining of
glucose molecules, stored for a short term in the liver and
muscles.
Glycogenesis is the process of converting glucose to glycogen,
regulated by the pancreatic hormone insulin, released by beta
cells in the islets of Langerhans.
Glucagon, another pancreatic hormone released by alpha cells in the
islets of Langerhans, mediates glycogenolysis –conversion of
glycogen back into glucose when blood glucose levels drop.
Lipids (or fats) for longer-term storage, are deposited in adipose
tissue.
Gluconeogenesis is the process of converting fat and proteins to
glucose and ketones, a form of fuel.
The Role of Insulin in Energy Utilization
Three Phases of Energy Metabolism
Digestive Phase
The Fasting and Absorptive Phases of Metabolism
Insulin
(parasymp.)
Glucagon
(sympath.)
Regulation of Eating
• Satiety is the feeling of fulfillment or satisfaction.
• Hunger is the internal state of an animal seeking food.
• The brain integrates insulin and glucose levels with other
information to decide whether to initiate eating.
• No single brain region has control of appetite, but the
hypothalamus is important to regulation of:
– Metabolic rate
– Food intake
– Body weight
• A dual-center hypothesis proposed two appetite centers in the
hypothalamus: (now considered outdated)
– One for signaling hunger
– One for signaling satiety
The Hypothalamus Coordinates Multiple
Systems That Control Hunger
• Ventromedial hypothalamus (VMH) lesions cause animals to eat
to excess (hyperphagia) and become obese, suggesting the VMH
is a satiety center.
• Lateral hypothalamus (LH) lesions cause aphagia—refusal to
eat—suggesting LH is a hunger center.
• The dual-center hypothesis proved to be too simple.
• VMH-lesioned animals exhibit a dynamic phase of obesity with
hyperphagia (overeating) until they become obese, on a rich diet.
• Their increased weight stabilizes in a static phase of obesity; this
weight is maintained even after food manipulations.
Changes in Body Weight after Hypothalamic Lesions
The Hypothalamus Coordinates Multiple Systems
That Control Hunger
• The arcuate nucleus of the hypothalamus
contains an appetite controller governed by
hormones, like insulin.
• Other peripheral peptide hormones are
• Leptin,
• Ghrelin
• PYY3–36.
Role of Leptin
• Fat cells produce leptin and secrete it into the
bloodstream.
• Leptin works to suppress hunger
• Leptin’s effects on the arcuate are long-lasting.
• Leptin activates POMC/CART neurons but inhibits
NPY/AgRP neurons
Role of Ghrelin
•
•
•
•
Increase during fasting
Decrease after a meal
Increased levels increase appetite
Obese individuals have low baseline levels and
levels do not drop after a meal so no signal for
“just ate a meal”
Role of PYY3-36
•
•
•
•
•
Small peptide from the small intestine
Low baseline levels
Levels increase quickly when eating a meal
Increased levels decrease appetite
Receptors in the arcuate nucleus
Integration of Appetite Signals in the Hypothalamus
CNS leptin and insulin action in the control of energy
Homeostasis (2010) Annals of the New York Academy of Sciences
Volume 1212, Issue 1,
CNS Leptin and Insulin
Regulation of Energy Intake Is A Complex Process
Integration of multiple signals from
Internal homeostatic mechanisms
External sensory cues
social context
availability of food
learned behaviors
cognitive factors
habits
Cognitive and Emotional Influences on Eating
• Cognitive
– Sensory
• Taste & Odor
• Visual
– Memory
• Early childhood eating habits
• Food preferences generally
• Cultural influences
• Emotional
– Food sensory can activate
• Reward system
• Disgust system
– Negative emotions
• Fear, sadness, anger
• Disrupt eating
• Sometimes increases and sometimes decreases eating
Role of Learning in Eating
• Learning can influence eating in a variety of ways
• Most mammals are born with a preference for sweet and salty
tastes and with an aversion to bitter tastes
•
Learn to avoid any taste followed by illness
– conditioned taste aversion
• Learn to prefer tastes that improve their health
– conditioned taste preference
• These forms of learning are robust and adaptive
Taste Aversion
• An example of classical conditioning.
• Will cause people (or animals) to avoid a food or drink that has
been associated with sickness, vomiting or nausea.
• Blue jay that eats a monarch will get sick because the butterfly's
wings is toxic.
• Explains why blue jays avoid eating monarch butterflies.
• Many brain circuits are activated in response to seeing food
cues,
–
–
–
–
–
–
–
–
Prefrontal cortex
Orbitofrontal cortex
Inferior temporal cortex
Insula
Striatum
Amygdala
Hippocampus
Hypothalamus
• High hedonic value food produces greater activation of the
brain circuits
Positive-Incentive Models of Feeding
• Major influences of taste, learning & social factors on
feeding
• Alternative theory of feeding & hunger
– based on idea that we eat because eating is
pleasurable rather than to satisfy some setpoint for
glucose or fat.
– When good food is present, we will eat regardless
– Hunger determined by many factors
•
•
•
•
•
Taste
Previous experience with food
Time of day
Time since last meal
Social environment
Sensory Signals and Positive Incentives
• The homeostasis “set point” explanation of eating regulation can
not explain eating a piece of pecan pie and whipped cream at the
end of a large meal
• In rats, a small amount of artificial sweetener saccharin added to
their diet leads to an increase in consumption and marked weight
gain
• Positive-incentive properties of food (i.e., anticipated pleasurable
effects) rather than internal deficits
• Deprivation increases food's positive incentive properties
• Positive energy balance “Over eating” reduce food’s incentive
signals especially in the Insula and Hypothalamus
– For individuals with good regulation i.e. “thin”
– But not for individual’s whom tend to be overweigh
Sensory-Specific Satiety
• Eating one particular food (chocolate) reduces
incentive value of its taste
• Cafeteria diet has variety so incentive value
does not drop as quickly
– rats increase consumption & body weight
– many choices allows switching as incentive value for
a particular food falls