Which one of the following is not a source of energy?

Concept

Foods supply two categories of nutrients: macronutrients (carbohydrates, proteins, and fats) and micronutrients (vitamins and minerals). Macronutrients are broken down through metabolism to release chemical energy, measured in kilocalories (carbohydrates and proteins give about 4 kcal per gram, fats about 9 kcal per gram). Micronutrients like vitamins are needed only in small amounts — they work as coenzymes and regulators inside metabolic reactions but are not themselves burned for energy.

Application

1. Protein is a macronutrient: its breakdown yields roughly 4 kcal of usable energy per gram, so it does supply energy.

2. Fat is a macronutrient: breakdown of fatty acids yields roughly 9 kcal per gram, the most energy-dense of the three, so it does supply energy.

3. Carbohydrate is a macronutrient: breakdown of glucose yields roughly 4 kcal per gram, so it does supply energy.

4. Vitamin is a micronutrient: many vitamins act as coenzymes or cofactors supporting metabolic reactions (for example, B-vitamins assist enzymes that release energy from other nutrients), while others play different regulatory roles — but no vitamin itself carries metabolisable calories, so it does not supply energy.

Cross-check

Nutrition-facts labels calculate a food's declared energy value using standard conversion factors applied to its macronutrient content, but never assign any calorie value to its vitamin content, confirming vitamins are excluded from the energy-yielding components that determine that count.

Vitamin is therefore the nutrient among the four that is not a source of energy.