In human nutrition, approximately how many kilocalories of energy are released when one gram of dietary fat is completely oxidised in the body?

Introduction / Context:
Macronutrients such as carbohydrates, fats, and proteins provide energy to the human body. Nutritionists express this energy in kilocalories, often called calories in everyday language. Each nutrient has a characteristic energy value per gram. This question asks about the approximate energy released by one gram of fat, which is important for understanding food labels, diet planning, and energy balance in the body.

Given Data / Assumptions:

• The nutrient being considered is fat (lipid).
• The mass considered is one gram of dietary fat.
• The options list several possible energy values in kilocalories per gram.
• We assume standard physiological fuel values used in human nutrition.

Concept / Approach:
The physiological fuel value of macronutrients is usually approximated using Atwater factors. Carbohydrates and proteins each provide about 4 kilocalories per gram, while fats provide more energy because their molecules are more reduced and yield more ATP upon oxidation. Typical nutrition references quote values near 9 kilocalories per gram for fats. The more precise values used in some calculations are approximately 9.45 kilocalories per gram, which is why such an option appears in many examination questions. Therefore, among the values given, 9.45 kilocalories per gram is the most accurate representation of the energy released by one gram of fat.

Step-by-Step Solution:
Step 1: Recall the standard energy values: carbohydrates ≈ 4 kcal/g, proteins ≈ 4 kcal/g, fats ≈ 9 kcal/g. Step 2: Recognise that fats yield more energy per gram than carbohydrates or proteins. Step 3: Look for the option closest to the accepted value for fats, which is around 9 kilocalories per gram. Step 4: Compare 9.45 kcal and 9.54 kcal with the typical 9 kcal figure. Step 5: Select 9.45 kcal as the commonly used precise value in exam oriented nutrition questions.

Verification / Alternative check:
Food composition tables and dietetics references often list the physiological fuel value of fats as approximately 9.3 to 9.5 kilocalories per gram, depending on the type of fat and exact calculation method. When rounded for simplicity, this is commonly quoted as 9 kilocalories per gram. Among the options given, 9.45 kilocalories per gram falls within this typical range and is a recognised value in many textbooks. Lower values like 4.59 kilocalories per gram resemble carbohydrate or protein values and are clearly too small for fat.

Why Other Options Are Wrong:
Option A: 5.49 kcal per gram is too low for fats and does not match standard dietary energy values. Option B: 4.59 kcal per gram is close to the value for carbohydrates or proteins, not fats. Option D: 9.54 kcal per gram is slightly higher than the more commonly accepted 9.45 kcal figure and is less frequently cited in basic exam questions. Option E: 7.25 kcal per gram does not correspond to any standard macronutrient energy value and is not used in human nutrition guidelines.

Common Pitfalls:
Students often memorise that fats provide roughly 9 kilocalories per gram but may become confused when presented with more precise decimal values. They might also mix up the energy values of carbohydrates, proteins, and fats. A reliable memory aid is the pattern 4-4-9: 4 kilocalories per gram for carbohydrates, 4 for proteins, and 9 for fats. When faced with similar decimal options, select the value close to 9 that aligns with standard references, such as 9.45 kilocalories per gram for fats.

Final Answer:
Approximately 9.45 kilocalories of energy are released when one gram of dietary fat is completely oxidised in the body.