We obtain our energy from food via digestion. In the process of respiration the glucose from food is burnt for energy and water and carbon dioxide gas are released.
Fuel + Oxygen → Carbon Dioxide + Water
C6H12O6 (s) + 6O2 (g) → 6CO2 (g) + 6H20 (g) (exothermic reaction)
Glucose is produced by plants in the process of photosynthesis.
Carbon Dioxide + Water → Fuel + Oxygen
6CO2 (g) + 6H20 (g) → C6H12O6 (s) + 6O2 (g)
Carbohydrates such as glucose provide greatest proportion of energy in human diets.
Food packaging provides consumers with information about the amounts of fats, carbohydrates, proteins as well as other things such as minerals.
Fats have the most energy per gram followed by proteins and carbohydrates.
Food labels are listed in descending order based on weight. Each has a mass per serving size (may or many not match your own serving size) as well as weight per 100g.
The energy from foods is measured in kJ/g.
Food label image By BruceBlaus - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=57761221
Nutrition guide By Hglu23 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=29349816