MCAT Biochemistry
Body mass is composed of water, carbohydrates, proteins, and lipids, with lipids playing the most significant role in long-term weight change. Body mass is affected by the balance between energy input (food) and energy output (basal metabolic rate and physical activity). Various feedback loops help maintain this balance by manipulating appetite and metabolism.
Three primary appetite and satiety hormones regulate energy intake: ghrelin, orexin, and leptin. Ghrelin stimulates appetite and prepares the gut for food, while orexin further increases appetite, influenced by ghrelin and low blood sugar levels. In contrast, leptin decreases hunger and increases fat burning at the end of a meal, inhibiting orexin. Leptin secretion increases with weight gain and decreases with weight loss, initiating feedback loops to regulate body mass and bring fat stores back to steady-state levels. Obesity often results from a surplus of energy input and may involve leptin resistance, leading to continuous appetite and weight gain.
Lesson Outline
<ul> <li>Body mass components</li> <ul> <li>Water, carbohydrates, proteins, and lipids contribute to body mass</li> <li>Water, carbohydrates, and proteins are fairly stable over time</li> <li>Lipids are the main source of fluctuations in body mass</li> </ul> <li>Energy balance</li> <ul> <li>Energy in (food intake) vs energy out (energy expenditure)</li> <li>Basal metabolic rate (BMR) and physical activity: the two main components of energy expenditure</li> </ul> <li>Role of feedback loops in body mass regulation</li> <ul> <li>Match energy needs with appetite and food intake</li> <li>Metabolic changes in response to imbalances</li> <li>Various hormones involved in feedback loops</li> </ul> <li>Changing body mass on purpose</li> <ul> <li>Importance of incremental changes over time</li> <li>Building muscle mass to increase day-to-day energy needs</li> <li>Gradually resetting basal metabolic rate for lasting changes</li> </ul> <li>Role of appetite and satiety hormones</li> <ul> <li>Ghrelin: Appetite inducer released at the sight, smell, or thought of food; also induced by empty stomach</li> <li>Orexin: Stimulated by ghrelin and low blood sugar, further increasing appetite</li> <li>Leptin: Released at the end of a meal to decrease hunger drive and increase fat burning</li> </ul> <li>Effect of body mass changes on appetite hormones</li> <ul> <li>Weight loss: Decrease in leptin secretion, increase in orexin and ghrelin production</li> <li>Weight gain: Increase in leptin secretion, decrease in orexin and ghrelin production</li> </ul> <li>Obesity and hormonal malfunctions</li> <ul> <li>Surplus of energy input leading to excessive fat storage</li> <li>Leptin resistance causing continuous appetite and weight gain</li> </ul> </ul>
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FAQs
Appetite and satiety play significant roles in body mass regulation by stimulating the feeling of hunger or fullness, respectively. Appetite, mainly driven by hormones like ghrelin and orexin, promotes food intake when the body requires energy. Satiety, on the other hand, occurs when the body reaches a state of adequate nutrient supply, signaling the brain to inhibit further food consumption. Hormones such as leptin, released by adipose tissue, help regulate long-term satiety, while short-term satiety is regulated by various gastrointestinal hormones and neural signals.
Leptin and ghrelin are hormones that regulate energy balance and body mass by influencing appetite and satiety. Leptin, primarily produced by adipose tissue, signals the brain to suppress food intake and increase energy expenditure, thus promoting a negative energy balance and reducing body mass. Ghrelin, on the other hand, is produced mainly in the stomach and stimulates appetite by acting on the hypothalamus, promoting a positive energy balance and increasing body mass. Therefore, these two hormones work in opposition to maintain homeostasis in body mass regulation.
Basal metabolic rate (BMR) is the rate at which the body uses energy to maintain essential physiological functions while at rest. It constitutes the largest component of total energy expenditure and thus plays a crucial role in body mass regulation. Individuals with a higher BMR burn more calories at rest, making it easier for them to maintain or lose weight. Conversely, a lower BMR makes it more challenging to maintain or reduce body mass. Factors such as age, gender, muscle-to-fat ratio, and genetics determine an individual's BMR, making it a significant variable to consider in body mass regulation.
Adipose tissue, commonly referred to as body fat, plays an active role in body mass regulation by storing and releasing energy in response to the body's needs. Adipose tissue not only acts as an energy reservoir but also as an endocrine organ by releasing hormones such as leptin and adiponectin, which influence energy balance, appetite, satiety, and insulin sensitivity. These hormones facilitate body mass regulation by communicating with the brain and other organs, ultimately helping maintain energy homeostasis and a healthy body mass based on an individual's lifestyle and physiological requirements.
Obesity can have significant consequences on body mass regulation mechanisms and overall health. Chronically increased body mass can lead to a state of leptin resistance, where the brain becomes less responsive to leptin's satiety signals. This resistance may result in increased food intake and reduced energy expenditure, promoting further weight gain.
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