Question

Metabolism prompt:

There is a wide variation in the energy requirements between individuals (what we commonly refer to as our metabolism). Imagine that you are trying to explain how our body balances food intake (energy acquisition) with activity (energy expenditure). You should address the hormonal control of appetite and satiety, and also give examples of why the overall energy requirements of individuals might be different.

Answer 1

ENERGY INTAKE AND EXPENDITURE

• Energy is needed by the body to stay alive, grow, keep warm, and move around.
• Energy is provided by food and drink. It comes from the fat, carbohydrate, protein, and alcohol the diet contains.
• Energy requirements vary from one individual to the next, depending on factors such as age, sex, body composition, and physical activity level.
• Energy expenditure is the sum of the basal metabolic rate (the amount of energy expended while at complete rest), the thermic effect of food
• To maintain body weight, it is necessary to balance the energy derived from food with that expended in physical activity. To lose weight, energy expenditure must exceed intake, and to gain weight, energy intake must exceed expenditure

• Energy expenditure is the sum of the basal metabolic rate (BMR) (the amount of energy expended while at rest at a neutral temperature and in the fasting state), the thermic effect of food (TEF) (otherwise known as dietary-induced thermogenesis) and the energy expended in the movement of all types.

  A substantial proportion of total energy expenditure is accounted for by BMR, which is determined principally by body mass and body composition both of which vary with age and sex

• The TEF is the energy cost of digesting food and is rarely assessed separately.

Energy is obtained, from the food and drink we consume, by oxidation of carbohydrate, fat, protein, and alcohol, known as macronutrients. The amount of energy that each of these macronutrients provides varies:

• Fat is the most energy-dense nutrient and provides 9kcal (37kJ)/g.
• Alcohol is the second most energy-dense nutrient, providing 7kcal (29KJ)/g.
• Protein provides 4kcal (17kJ)/g
• Carbohydrate (starch and sugars) is the least energy dense nutrient, providing just 3.75kcal

Energy requirements of individuals

The actual amount of energy needed varies from person to person and depends on their basal metabolic rate (BMR) and how active they are.

Basal metabolic rate

The basal metabolic rate (BMR) is the rate at which a person uses energy to maintain the basic functions of the body – breathing, keeping warm, keeping the heart beating – when at complete rest.

An average adult will use around 1.1kcal each minute just maintaining these functions and BMR differs from one person to the next, both within a population and between population groups.

Infants and young children tend to have a proportionately high BMR for their size due to their rapid growth and development.

Men usually have a higher BMR than women since they tend to have more muscle.

Older adults usually have a lower BMR than younger people since their muscle mass tends to decrease with age.

The BMR accounts on average for about three quarters of an individual's energy needs.

Physical activity level

. The amount of energy an individual will need can be calculated by multiplying their BMR by a factor appropriate to the amount of activity that person does (known as the Physical Activity Level (PAL)). A PAL of 1.4 is associated with a very low level of physical activity at work or during leisure

Energy expenditure = BMR x Physical Activity Level (PAL)

HORMONAL CONTROL OF APPETITE

The hypothalamus acts as the control center for hunger and satiety. Part of the hypothalamus, the arcuate nucleus (or, in humans, the infundibular nucleus), allows entry through the blood-brain barrier of peripheral peptides and proteins that directly interact with its neurons. These include neurons that coexpress peptides that stimulate food intake and weight gain, specifically, neuropeptide Y (NPY) and agouti-related peptide (AgRP), as well as those expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) which inhibit feeding and promote weight loss . Together, these neurons and peptides control the sensations of hunger and satiety and ultimately weight gain and weight loss.