The energy costs of living
Physics Narrative
for 11-14
Keeping a human alive for a day
Let's start with thinking about the energy costs of keeping a human alive and functioning for one whole day. The average daily energy food intake (carbohydrate, protein, fat and alcohol) in the UK is equivalent to 10,500 kilojoule for men and 8400 kilojoule for women. The different components of our diet are not equally good at providing this energy. The following table shows how much energy is provided by one gram of each component:
| component | energy/kilojoule |
|---|---|
| carbohydrate (starch or sugar) | 16 |
| protein | |
| fat | 37 |
| alcohol | 29 |
Over the course of a typical day, if we are not to retain this energy in a chemical store as fat, we need to be active. Different activities help with this energy balance to greater or lesser extents. Just to maintain the basic bodily functions of an adult costs
about 4.6 kilojoule a minute.
This Basal Metabolic Rate (BMR) varies among individuals, and with age and population group. It is always measured at rest and usually accounts for 75% of human energy requirements.
The following figures showing how further activity increases the energy cost:
| energy for each minute of activity | energy cost/kilojoule |
|---|---|
| sitting | 6 |
| standing | 7 |
| washing or dressing | 15 |
| walking slowly | 13 |
| walking moderately quickly | 21 |
| walking up and down stairs | 38 |
| light work (most domestic work, lorry driving, carpentry, bricklaying) | 10-20 |
| moderate work (gardening, tennis, dancing, jogging, cycling up to 20 km/hour, digging) | 21-30 |
| strenuous work (coal mining, cross-country running, football, swimming the crawl) | >30 |
Feeding up the UK population
Where do people in the UK get their energy from?
This is how people in the UK satisfy their energy needs:
| food group | percentage |
|---|---|
| milk and milk products | 11 |
| meat and meat products | 16 |
| eggs and egg dishes | 2 |
| fish and fish dishes | 2 |
| fat spreads | 6 |
| sugar, confectionery and preserves | 6 |
| vegetables | 12 |
| fruit and nuts | 2 |
| cereals (including bread, cakes, pastries) | 30 |
| other foods | 4 |
| beverages | 9 |
These days foods are marked with the energy that they provide:
| food | energy provided per 100 gram / kilojoule |
|---|---|
| white rice | 1450 |
| whole milk | 258 |
| lettuce | 49 |
| margarine | 3036 |
| lamb chop | 1468 |
| white flour | 1456 |
| plain chocolate | 2126 |
| white bread | 965 |
Energy balance in humans
Comparing the amount of energy coming in with the amount going out, over a day or a longer period, allows us to make simple predictions about whether or not particular activities will be viable. Food input limits possibilities.
For example, you might calculate whether or not your energy intake is sufficient to sustain a week long back-packing holiday in the Scottish Highlands.
If the energy balance falls short on the input side you will soon start to feel lethargic and weary, It's just the same as with cash-flow problems, if expenditure outstrips income, the enterprise soon grinds to a halt. There are choices, but these are restricted by the ultimate limiting factor: energy.
