The foods we eat supply the energy needed by the body to drive its complex chemical, mechanical and electrical systems. Where does this energy come from, how is it locked into food molecules and how is it released?

Energy from the Sun

The energy content of all food molecules can be traced back to the Sun. It is the process of photosynthesis that locks the Sun’s energy into simple carbohydrates like glucose. Although the photosynthetic process, which is carried out in the chloroplasts present in plant cells, involves numerous steps, it can be summarised in the following equation.

Energy from sunlight trapped by chlorophyll present in the chloroplasts

+ 6CO2
carbon dioxide
+ 6H2O
water

C6H12O6
glucose

6O2
oxygen

This reaction only proceeds with an input of solar energy, and the product glucose stores this energy as chemical potential energy.

Plants are able to convert some of the glucose formed into starch and other macronutrients such as proteins and lipids. The origin of the energy locked into these molecules is from the Sun.

Releasing the locked-up energy

The food we eat supplies the body with energy-rich molecules like glucose. On entering the cells of the body, these molecules are broken down in a series of steps to reform carbon dioxide and water, releasing energy to be used by the body.

Some of the chemical potential energy locked into these molecules is transferred within the cell to a substance called adenosine triphosphate (ATP).

ADP + P + energy → ATP

ATP is often referred to as the energy currency of the cell as it is used to drive the body’s complex chemical, mechanical and electrical systems.

This energy transfer pathway, which occurs within all body cells, is called aerobic respiration, and for carbohydrates like glucose, it is effectively the reverse of photosynthesis.

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

The amount of energy released for each glucose molecule consumed produces between 36 and 38 ATP molecules.

When the cell needs energy to drive its complex chemical, mechanical and electrical systems, ATP breaks down, releasing the required energy.

ATP → ADP + P + energy

Efficiency of aerobic respiration

Not all of the energy locked into glucose through photosynthesis is released by aerobic respiration. Several different processes are involved, each of which consists of several steps. This results in some of the energy being converted into heat and not ATP.

The energy conversion efficiency for glucose lies between 38–44%, depending on the cell type. In terms of the body as a machine, this compares very well with the 20–25% efficiency of most machines.

Counting the energy of foods

The standard energy unit is the joule. It is defined in the following way: 4.18 joules is the energy needed to heat 1g of water by 1°C.

The joule is a small unit for everyday purposes, so in food chemistry, the kilojoule is the preferred unit (1000J = 1kJ).

The macronutrients in the foods we eat have been analysed for their available energy content when metabolised in the body. The energy values calculated are:

  • carbohydrate – 17kJ/g
  • protein – 17kJ/g
  • fat – 37kJ/g.

If any given meal is carefully analysed for its macronutrient composition, it is possible to calculate the energy input it gives.

Here is an example:

Main meal items

Portion size (g)

Energy (kJ)

Protein (g)

Fat (g)

CHO (g)

Roast potato

200

866

5.0

1.0

44.0

Chicken (roast)

150

1010

36.9

10.4

0.0

Asparagus

80

88

2.8

0.2

2.0

Carrot

50

55

0.4

0.1

2.8

Butter chicken sauce

40

240

1.0

4.0

3.9

Ice cream (low fat)

120

706

5.8

4.0

29.6

Peaches (canned)

120

270

1.1

0.12

15.0

Orange juice

250

364

1.9

0.3

21.5

Total

 

3,599

     

(Approximate values based on the Concise New Zealand Food Composition Tables 8th edition 2009).

The total daily intake for a moderately active, healthy adult is, on average, about 8,700kJ. Other meals such as breakfast and lunch along with ‘in between’ snacks supply the rest of the daily energy needed.

Useful link

See the 8th Edition Concise NZ Food Composition Tables on the on the Ministry of Health website.
www.health.govt.nz/publication/concise-new-zealand-food-composition-tables-8th-edition

    Published 5 September 2011