W11 & W12 | Starting and Stopping Eating

Starting and Stopping Eating

• The last lecture covered peripheral factors affecting starting and stopping eating
• This lecture focuses on the consequences of digestion
• Digestion breaks down food, bodies respond to products, and related blood chemicals
• These factors influence hunger and satiety, potentially impacting short and long-term energy regulation

Glucostatic Theory

• Glucose (blood sugar) is the primary energy source for the brain and other cells

• Blood sugar levels drop before a meal and rise quickly afterward

• Mayer's theory proposes a correlation between blood sugar levels and hunger/satiety

• High blood sugar in arteries, but low in veins, doesn't indicate hunger

• Low blood sugar in both arteries and veins indicates hunger

• Injecting insulin (lowers blood sugar) triggers hunger

• Reducing blood sugar by 50% increases caloric intake by 200%

Lipostatic Theory

• Focuses on long-term body weight regulation

• The body stores energy as fat during periods of food scarcity

• 1 kg of fat equals approximately 7800 kcals

• Humans typically have around 10% body fat, which equates to about 20 days of energy

• The body maintains a 'set point' for body fat, vigorously defending this point

• Factors such as environment, genes, and other factors potentially influence set point

Set Point

• Body works to restore the set point if there are fluctuations
• Factors such as shortening daylight, pregnancy/lactation, and puberty can affect set point
• Body uses indicators such as leptin levels to measure fat stores

Leptin

• Leptin is a hormone released by fat cells, released into the blood
• Higher leptin levels are associated with reduced hunger and increased satiety
• Leptin levels may be elevated in morbidly obese individuals, but this doesn't always reduce appetite
• Leptin levels fluctuate according to a circadian rhythm
• Leptin levels, typically highest at night, and lowest during the day
• People with defective leptin signaling experience obesity and this often reverses with injections

Insulin

• Pancreatic beta cells secrete insulin which helps lower blood sugar

• Insulin is important in regulating carbohydrate (glucose) metabolism by storing glucose as glycogen (inactive form)

• Clusters of approximately 20,000 glucose units form plants to starch

• Blood Sugar Level (BSL) regulation is crucial, influenced by insulin

• Abnormalities in BSL regulation (due to lack of or resistance to insulin) can be fatal and/or have long-term adverse consequences

Appetite and Insulin

• Artificially high insulin levels can trigger hunger before meals
• This is paradoxical because low BSL should trigger hunger
• Insulin likely indicates the body's current energy needs
• Cephalic Phase Responses (CPRs) are anticipatory physiological responses to food cues.

Satiating Agents - CCK

• Cholecystokinin (CCK) is a hormone released during feeding and affects digestive activity
• Higher protein and fat cause a greater CCK release into the blood, reducing food intake
• CCK is part of a negative feedback loop that slows stomach emptying when there is increased CCK concentration

Satiating Agents - Glucagon

• Glucagon, released by the pancreas, operates in opposition to insulin by increasing blood glucose levels
• Glucagon is also linked to glycogen breakdown and subsequent protein conversion to glucose
• A rise in glucagon is associated with lowered food intake, particularly when glycogen stores are depleted

Food as a Satiating Agent

• Food type (e.g., caloric density, macronutrient composition, and texture) influences satiety
• Fatty foods, although initially reducing intake, can lead to increased intake over time due to adaptative mechanisms
• Low-calorie foods, in contrast, are often associated with increased intake.
• Solid food tends to be more satiating than liquid calories

Low-Calorie Food

• People often eat more of a low calorie food after repeated exposure.
• Associative learning links certain flavour or sensory stimuli to low calories
• Protein based foods are more filling than carbohydrate food

Texture

• Crunchier food tends to be more satiating than softer food because the chewing process is more time-consuming
• Faster gastric emptying is linked to lower satiety with liquid vs. solid foods

Central Mechanisms

• The brain plays a role in appetite regulation (e.g., 'hunger' and 'satiety' centers)
• Lesions to the hypothalamus can cause severe eating problems, including overeating (lateral hypothalamus) and under-eating (ventromedial hypothalamus).
• The hippocampus is involved in food-related memories. Alterations could affect how individuals decide whether and how much to eat.
• These centers often work in conjunction with other brain areas and peripheral factors regulating ingestion

This quiz explores the mechanisms behind starting and stopping eating, including peripheral factors and the consequences of digestion. It delves into the glucostatic and lipostatic theories, examining how blood sugar levels and body fat regulation influence hunger and satiety. Test your understanding of these important concepts in nutrition.