Which of the following substances has the highest amount of energy in joules per unit weight?

Analyze the question: You're asked which food substance releases the most energy per unit mass (per gram) when it is oxidised in the body.

Explain the concept: The energy value of a food is the amount of energy released when 1 g of it is completely oxidised. The standard values are:

| Food substance | Energy value (approx.) | |---|---| | Fats and oils | 37-39 kJ/g (about 9 kcal/g) | | Carbohydrates | 17 kJ/g (about 4 kcal/g) | | Proteins | 17 kJ/g (about 4 kcal/g) | | Vitamins | Negligible (not used as an energy source) |

Why fats give the most energy: Fat molecules have long hydrocarbon chains with many C-H bonds and very little oxygen. Because they are so highly reduced, more energy is released when they are oxidised. Carbohydrates already contain a lot of oxygen, so they are partly oxidised and release less energy per gram.

Solution:

• Carbohydrates: about 17 kJ/g
• Proteins: about 17 kJ/g
• Vitamins: they regulate body processes and provide almost no energy
• Fats: about 39 kJ/g, which is more than double the others

So fats have the highest energy per unit weight.

Answer: B. Fats

Exam tip: Remember "Fats = 9, Carbs and Proteins = 4" (kcal per gram). Whenever a question asks about the highest energy per gram, think fats. This is also why fat is the body's main long-term energy store. Keep in mind that the quickest source of energy is carbohydrate, which is a different question that JAMB sometimes asks.