(3) Metabolism, body mass and what are our energy requirements?

Metabolism is the scientific term used for all the chemical reactions that go on inside an organism's body. Metabolic reactions synthesise molecules of all shapes and size for specific uses in an organism. Metabolic reactions break down larger molecules down into smaller ones (e.g. in digestion). Metabolic reactions supply the energy needs for the chemistry of every cell from respiration in the mitochondria.

• What is your metabolism?   What is metabolic rate? What energy do you need and why?

• The rate at which all the chemical reactions in the cells of the body are carried out (the metabolic rate) varies with the amount of activity you do and the proportion of muscle to fat in your body.

• A person loses mass when the energy content of the food taken in is less than the amount of energy expended by the body.

• You need energy to fuel the chemical reactions in your body to keep you alive and these reactions are called your metabolism.

• Different people have small differences in their resting metabolic rates because e.g. muscle needs more energy than fatty tissue, so more muscular people tend to have a higher metabolic rate because of the higher ratio of muscle to fat.

• Bigger people tend to have a higher metabolic rate because more energy is needed to maintain a greater mass of cells.

• On average men tend to have a greater metabolic rate because they tend to have a greater proportion of muscle.

• If your job involves heavy manual labour, not surprisingly, you energy intake needs to be higher.

• Different people have different energy requirements depending on you body mass ('weight').

• For every kg of body mass, it is reckoned that you need 5.4 kJ of energy per hour, as the basic energy requirement (BER) that you need to maintain all your bodily functions.

• This can be calculated: BER (kJ/day) = 5.4 x 24 hours x body mass (kg).

• All the cells of your body need chemical energy, therefore the bigger you are and the more physically active you are, the more energy you need e.g. very physical work or movement e.g. running or physically demanding hard labour.

  • Average daily energy requirements: men 12180 kJ (2900 kcal)  and women 9240 kJ (2200 kcal)

  • Remember this varies widely because of your weight or age, how you spend the day - an office worker sitting at a desk will need far less energy than somebody doing a hard physical job working on a building site.

• For example, a 70 kg person may use 450 kJ of energy walking for 30 minutes, but use 1800 kJ running for 30 minutes.

• Note on energy units

  • Calories are 'old' units of energy, but still in common use.

  • 1 calorie = 4.2 joules of energy (1 kcal = 4.2 kJ, k indicating a factor of 1000)

  • If a person needs 2000 kcal/day, that is equal to 8400 kJ/day.

Know that inherited factors also affect our health

• Metabolic rate may be affected by inherited factors.

• Your DNA profile ==> gene expression ==> inherited characteristics, can have some bearing on your metabolic rate eg an underactive thyroid gland which can lower metabolic rate, increase fatigue and ultimately cause obesity. Any energy not used will be stored as fat.

• People can inherit factors that affects the blood cholesterol level.

• Cholesterol is an essential fatty substance for good health and its in every cell in the body but if it is too high it causes fatty deposits on the inner surfaces of blood vessels causing higher blood pressure and heart disease.

• People who exercise regularly are usually healthier than people who take little exercise.

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