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• A dual-center hypothesis proposed two appetite centers in the hypothalamus: (now considered outdated) – One for signaling hunger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 InsulinCNS leptin and insulin action in the control of energyHomeostasis (2010) Annals of the New York Academy of Sciences Volume 1212, Issue 1,

Regulation of Energy Intake Is A Complex Process

Integration of multiple signals from

Internal homeostatic mechanisms

External sensory cues social contextavailability of foodlearned behaviorscognitive factorshabits

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