The physiology of feeding and satiety Flashcards
Describe the connection between obesity and metabolic disorders Describe the role of the CNS in energy homeostasis Specify the peptides and hormones involved in central control of food intake and energy balance Describe the concept of leptin resistance and its relationship to obesity Discuss potential therapies for the treatment of obesity.
What is energy homeostasis?
A physiological process whereby energy intake is matched to energy expenditure over time.
It promotes body fuel stability - energy primarily stored as fat.
What leads to obesity?
Accessible, tasty, calorie dense food + a sedentary lifestyle.
Obesity arises due to a small constant mismatch between energy intake and energy expenditure
What BMI range is thin-normal?
Up to 25
What BMI range is overweight?
25-29.9
What BMI range is obese?
30 - 39.9
What BMI range is morbidly obese?
At least 40
Is the prevalence of obesity only in adults?
No- there is also an increased prevalence of obesity in children and young people.
Why is the answer to obesity not as simple as changing our lifestyles to eat less fatty foods and do more exercise?
Because biological factors come into play.
Obesity is not a single disorder, but a heterogenous group of conditions with multiple causes.
What are the major factors influencing obesity?
Genetic factors and environmental factors.
Genetics- this is usually through susceptibility genes, which increase risk of developing disease but are often not per se essential for disease expression.
Environment- unmask latent tendencies to develop obesity. Westernisation of diet.
What diseases are obese people more at risk of developing?
Type 2 diabetes High blood pressure Heart attack Certain cancers (colon) Osteoarthritis
Why is fat required by the body?
For energy storage and prevention of starvation.
It is also an energy buffer during prolonged illness.
Why is it difficult to lose weight once it has been gained?
Increased body fat alters brain function.
Long-term obesity induces brain re-programming.
The result is, your brain views the extra weight (i.e. fat) as normal, and dieting as a threat to survival. Therefore it defends the new weight.
How does the CNS influence energy balance and body weight?
What does the integration of these factors determine?
What is the site of integration of these?
- Behaviour - feeding and physical activity
- ANS activity- regulates energy expenditure
- Neuroendocrine system- secretion of hormones
The integration of these determines feeding behaviour.
The site of integration is the brain, and the neural centre responsible is the hypothalamus.
Give examples of conditions affecting the hypothalamus which can cause obesity or leanness.
Lesioning ventromedial hypothalamus causes obesity.
Lesioning lateral hypothalamus causes leanness.
What underlies the control of energy intake and body weight?
- Satiety signalling
- Adiposity (the state of being obese) negative feedback signalling
- Food reward
What is the difference between satiation and satiety?
Satiation is the sensation of fullness generated during a meal.
Satiety is the period of time between the termination of one meal and the initiation of the next.
What happens to satiation signals during a meal?
They increase, to limit the size of the meal.
Name and describe 5 satiation signals.
- Cholecystokinin: secreted from enteroendocrine cells in the duodenum and jejunum. It is released in proportion to lipids and proteins in the meal. It signals to the hindbrain via sensory nerves and stimulates the hindbrain directly in the nucleus of the solitary tract (NTS).
- Peptide YY- secreted from endocrine mucosal L-cells of the GI tract. Levels increase rapidly post-prandially. It inhibits gastric motility, slows emptying and reduces food intake. It signals to the hypothalamus.
- Glucagon-like peptide 1- a product of the pro-glucagon gene. It is also released from L-cells in response to food ingestion. It inhibits gastric emptying and reduces food intake. It signals to the hypothalamus.
- Oxyntomodulin- also from the pro-glucagon gene and released from oxyntic cells of the small intestine after a meal. It acts to suppress appetite, though the mechanism is unclear.
- Obestatin- a peptide produced from a gene that encodes ghrelin and released from cells lining the stomach and the small intestine. It has been suggested to reduce food intake (possibly by antagonising the actions of Ghrelin), though its actions are unclear at present.
What is Ghrelin?
A peptide produced and secreted by oxyntic cells in the stomach. Ghrelin levels increase before meals and decrease after meals. Levels are raised by fasting and hypoglycaemia.
Peripheral Ghrelin stimulates food intake (by signalling to the hypothalamus) and decreases fat utilization.
Is weight generally stable or unstable for long periods of time?
Is this the case for lean or obese people?
Weight is generally stable in humans over long periods of time, and this is true for both lean and obese people.
How is overall energy balance controlled over long periods of time?
By feedback loops which act to maintain constancy of total body energy stores.
The signals are produced in response to body nutritional status. They are sensed in the hypothalamus, which acts to modulate food intake and energy expenditure.
What increases food intake when injected into hypothalamic centres? How long is the effect?
Glutamate, Gaba and opioids
The effects are modest and short lasting
What act to suppress food intake?
Monoamines
What two hormones send “adiposity” signals to the hypothalamus?
Where are they produced?
What is the purpose of this signalling?
What is different about it in obesity?
Leptin and insulin.
They are produced in peripheral tissues.
Leptin ismade and released from fat cells.
Insulin is made and released from pancreatic cells.
The purpose of this signalling is to communicate the status of fat stores to the brain, in order to control the amount of body fat. They inform the hypothalamus to alter energy balance by eating less and increasing energy burn.
In the obese state, the function of leptin and insulin malfunctions.
