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Ch. 24 Nutrition, Metabolism, and Energy Balance
Marieb - Human Anatomy & Physiology 7th Edition
Marieb, Hoehn7th EditionHuman Anatomy & PhysiologyISBN: 9780805359091Not the one you use?Change textbook
All textbooksMarieb, Hoehn 7th EditionCh. 24 Nutrition, Metabolism, and Energy BalanceProblem 23
Chapter 23, Problem 23

Explain the effect of the following on metabolic rate: thyroxine levels, eating, body surface area, muscular exercise, emotional stress, starvation.

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Thyroxine levels: Thyroxine is a hormone produced by the thyroid gland that plays a key role in regulating metabolism. Higher levels of thyroxine increase the metabolic rate by stimulating cellular respiration and energy production. Conversely, lower levels of thyroxine slow down the metabolic rate. This effect can be explained by thyroxine's ability to increase the activity of mitochondria and the synthesis of enzymes involved in metabolism.
Eating: The act of eating temporarily increases the metabolic rate due to the thermic effect of food (TEF). This is the energy required for digestion, absorption, and assimilation of nutrients. Proteins have the highest thermic effect, followed by carbohydrates and fats. The metabolic rate rises after a meal and gradually returns to baseline levels.
Body surface area: Individuals with a larger body surface area relative to their body mass tend to have a higher metabolic rate. This is because they lose more heat to the environment and need to generate more energy to maintain their body temperature. Smaller individuals or those with less surface area relative to mass lose less heat and therefore have a lower metabolic rate.
Muscular exercise: Physical activity significantly increases the metabolic rate as muscles require more energy during exercise. This energy demand is met by increased cellular respiration, which consumes oxygen and produces ATP. The intensity and duration of exercise directly influence the degree of metabolic rate elevation.
Emotional stress and starvation: Emotional stress can increase the metabolic rate due to the release of stress hormones like adrenaline and cortisol, which stimulate energy production. On the other hand, starvation decreases the metabolic rate as the body conserves energy by slowing down non-essential processes. This adaptive response helps preserve energy stores during periods of insufficient food intake.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Thyroxine and Metabolic Rate

Thyroxine, a hormone produced by the thyroid gland, plays a crucial role in regulating metabolic rate. It increases the basal metabolic rate (BMR) by stimulating the metabolism of carbohydrates, fats, and proteins. Higher levels of thyroxine lead to increased energy expenditure, while low levels can result in a decreased metabolic rate, affecting overall energy balance.
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Impact of Eating on Metabolism

Eating influences metabolic rate through the thermic effect of food (TEF), which is the energy required to digest, absorb, and process nutrients. This effect varies with the type of food consumed; for instance, protein has a higher thermic effect compared to fats and carbohydrates. Additionally, meal frequency and portion sizes can also impact overall energy expenditure.
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Body Surface Area and Metabolic Rate

Body surface area (BSA) is directly related to metabolic rate, as larger surface areas typically correlate with higher energy expenditure. This is due to greater heat loss in larger individuals, necessitating increased metabolic activity to maintain body temperature. BSA calculations often use formulas that consider height and weight, providing a more accurate assessment of metabolic needs.
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Related Practice
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