Problem 79 - Chemistry: The Central Science

Question

(a) What is meant by the term fuel value? (b) Which is a greater source of energy as food, \(5 \mathrm{~g}\) of fat or \(9 \mathrm{~g}\) of carbohydrate?

Step-by-Step Solution

Answer

(a) Fuel value refers to the amount of energy released when a specific quantity of food is completely burned (or metabolized) in the body, usually expressed in kilocalories (kcal) or kilojoules (kJ) per gram of the substance. (b) 5g of fat provides 45 kcal of energy, while 9g of carbohydrate provides 36 kcal of energy. Therefore, 5g of fat is a greater source of energy as food compared to 9g of carbohydrate.

Step 1: Definition of fuel value

Fuel value refers to the amount of energy released when a specific quantity of food is completely burned (or metabolized) in the body. It is usually expressed in kilocalories (kcal) or kilojoules (kJ) per gram of the substance.

Step 2: Finding fuel values of fat and carbohydrates

We will use the standard fuel values: - Fat has a fuel value of about 9 kcal/g (or 37 kJ/g) - Carbohydrate has a fuel value of about 4 kcal/g (or 17 kJ/g)

Step 3: Calculating the energy provided by 5g of fat

To find the energy provided by 5g of fat, we will multiply the mass of fat by its fuel value. Energy from 5g of fat = (mass of fat) x (fuel value of fat) Energy from 5g of fat = \( 5 \mathrm{~g} \times 9 \frac{kcal}{g} \) = 45 kcal

Step 4: Calculating the energy provided by 9g of carbohydrate

To find the energy provided by 9g of carbohydrate, we will multiply the mass of carbohydrate by its fuel value. Energy from 9g of carbohydrate = (mass of carbohydrate) x (fuel value of carbohydrate) Energy from 9g of carbohydrate = \( 9 \mathrm{~g} \times 4 \frac{kcal}{g} \) = 36 kcal

Step 5: Comparing the energy from fat and carbohydrate

Now, we will compare the energy provided by 5g of fat and 9g of carbohydrate. Energy from 5g of fat = 45 kcal Energy from 9g of carbohydrate = 36 kcal Since 45 kcal (energy from 5g of fat) is greater than 36 kcal (energy from 9g of carbohydrate), we can conclude that 5g of fat is a greater source of energy as food compared to 9g of carbohydrate.

Key Concepts

Energy Content of Nutrients

When we talk about the energy content of nutrients, we're really diving into how our food provides us fuel. Every bite of food you consume can be converted into energy, which your body uses to perform its various functions. This energy is usually measured in kilocalories (kcal) or kilojoules (kJ).

• Fats: These are a dense source of energy, providing about 9 kcal per gram.
• Carbohydrates: These provide about 4 kcal per gram.
• Proteins: Similar to carbohydrates, proteins also provide approximately 4 kcal per gram.

These numbers tell us how much energy is stored in different types of nutrients. The human body can then tap into this stored energy to power everything from digestion to muscle contraction.

Calorific Value

The term 'calorific value' is essentially another way to describe the amount of energy that can be extracted from food sources when they are metabolized by the body. This is a key aspect in understanding nutrition and diet.

Understanding calorific values helps in determining how much food one needs to eat to meet energy requirements. For instance:

• A higher calorific value means more energy is available from a smaller amount of food.
• Fats have the highest calorific value, providing more energy compared to equivalent weights of carbohydrates or proteins.

By using calorific values, nutritionists and dietitians can devise meal plans that provide enough energy without excess.

Metabolism of Macronutrients

Metabolism refers to the chemical processes your body uses to transform food into energy. Macronutrients, which include carbohydrates, fats, and proteins, are metabolized differently by the body to extract energy.

• Carbohydrates: Usually metabolized first because they're the body's preferred energy source, being readily convertible to glucose.
• Fats: Used when carbohydrates are not sufficient, providing a long-lasting energy reserve.
• Proteins: Typically used as a last resort for energy, their primary purpose is to build and repair tissues.

A balanced diet ensures that the body efficiently absorbs and uses energy from these macronutrients, supporting both daily activities and long-term health.