Question

Triacylglycerols, with their hydrocarbon-like fatty acids, have the highest energy content of the major nutrients. a. If \(15 \%\) of the body mass of a \(70.0 \mathrm{~kg}\) adult consists of triacylglycerols, what is the total available fuel reserve, in both kilojoules and kilocalories, in the form of triacylglycerols? Recall that \(1.00 \mathrm{kcal}=4.18 \mathrm{~kJ}\). b. If the basal energy requirement is approximately \(8,400 \mathrm{~kJ} /\) day \((2,000 \mathrm{kcal} /\) day \()\), how long could this person survive if the oxidation of fatty acids stored as triacylglycerols were the only source of energy? c. What would be the weight loss in pounds per day under such starvation conditions \((1 \mathrm{lb}=0.454 \mathrm{~kg})\) ?

Short answer

a) 409,500 kJ or 97,940 kcal; b) Approximately 48.75 days; c) About 0.474 pounds per day.

Step-by-step solution

Calculate the Mass of Triacylglycerols

First, we calculate the mass of triacylglycerols. Given that they make up 15% of the body mass of a 70.0 kg adult, we find the mass: \[ \text{mass of triacylglycerols} = 0.15 \times 70.0 \text{ kg} = 10.5 \text{ kg} \]

Determine the Energy Content of Triacylglycerols

Triacylglycerols typically provide about 39 kJ per gram of energy. Calculate the total energy: \[ \text{total energy in kJ} = 10.5 \text{ kg} \times 1000 \text{ g/kg} \times 39 \text{ kJ/g} = 409,500 \text{ kJ} \] Convert to kilocalories using the conversion factor 1 kcal = 4.18 kJ:\[ \text{total energy in kcal} = \frac{409,500 \text{ kJ}}{4.18 \text{ kJ/kcal}} \approx 97,940 \text{ kcal} \]

Calculate Survival Duration

Assuming the only source of energy is the oxidation of these triacylglycerols, the basal energy requirement is 8,400 kJ/day. Calculate how many days the energy reserve would last:\[ \text{number of days} = \frac{409,500 \text{ kJ}}{8,400 \text{ kJ/day}} \approx 48.75 \text{ days} \]

Calculate Daily Weight Loss in Pounds

During this period, the daily weight loss can be found by determining how much triacylglycerol needs to be consumed to meet the daily energy requirement. Given 39 kJ/g, calculate the daily consumption in grams and then convert to pounds:1. Calculate daily consumption:\[ \text{grams/day} = \frac{8,400 \text{ kJ}}{39 \text{ kJ/g}} \approx 215.38 \text{ g/day} \]2. Convert grams to kilograms:\[ 215.38 \text{ g/day} = 0.21538 \text{ kg/day} \]3. Convert kilograms to pounds using 1 lb = 0.454 kg:\[ \text{pounds/day} = \frac{0.21538}{0.454} \approx 0.474 \text{ lbs/day} \]

Key concepts

Energy Metabolism

Energy metabolism refers to the biochemical processes that cells and organisms use to produce, transform, and use energy. Our bodies require energy for every action, from moving and growing to maintaining basic bodily functions at rest. This energy primarily comes from the macronutrients in our diet: carbohydrates, proteins, and fats.
Fats, including triacylglycerols, are significant in energy metabolism because they provide the most energy per gram compared to carbohydrates and proteins. Oxidation of fatty acids, which are the building blocks of fats, results in the release of energy that the body can use for various activities. The energy released during this process is often measured in kilojoules (kJ) or kilocalories (kcal).

• Carbohydrates: 4 kcal/gram
• Proteins: 4 kcal/gram
• Fats (triacylglycerols): 9 kcal/gram

This clearly shows why fats are an essential energy reserve in our bodies. For example, one gram of triacylglycerol provides 39 kJ of energy, making it a crucial fuel source especially when other nutrients are not available.

Triacylglycerols

Triacylglycerols are a type of fat stored in our body and are the most concentrated form of stored energy. They consist of three fatty acid molecules bonded to a glycerol backbone. These molecules are stored mostly in adipose tissue, which acts as a long-term energy reservoir.
Due to their dense energy content, triacylglycerols can supply a continuous and extensive amount of energy during periods of fasting or low food intake. When the body needs energy, enzymes called lipases break down triacylglycerols into fatty acids and glycerol, which are then released into the bloodstream and utilized by cells for energy.

• High energy density: 39 kJ/g
• Key storage form of energy in the body
• Broken down into fatty acids for energy production

This efficient storage system allows humans and other animals to survive through times when energy intake is less than the energy expended. In the exercise above, a person with 10.5 kg of triacylglycerols can access an enormous 409,500 kJ of potential energy!

Basal Metabolic Rate

Basal Metabolic Rate (BMR) is the amount of energy expressed in kilojoules or kilocalories that a person needs to keep the body functioning at rest. This includes breathing, blood circulation, cell production, nutrient processing, and thermoregulation.
BMR accounts for the largest portion of a person's total daily energy expenditure—up to 60-75% for most individuals. Knowing one's BMR can help with understanding energy needs, managing weight, and promoting a healthy lifestyle.
In the context of the given exercise, if a person solely uses stored triacylglycerols for energy, their BMR would dictate how long these reserves last. With a BMR of about 8,400 kJ/day, a person could, theoretically, sustain themselves on stored fat for approximately 48.75 days if absolutely no food were consumed.

• Key component of total energy expenditure
• Essential for understanding energy needs and potential weight loss
• Affects how quickly energy reserves are used

Thus, understanding BMR is crucial for metabolism management and determining how long stored energy will last during fasting.

Human Physiology

Human physiology examines how our bodily systems work, how they interact, and how they work in unison to maintain a balanced internal state known as homeostasis. It is immensely important in comprehending how the body utilizes energy and maintains functions.
Physiological systems like the digestive system, nervous system, and endocrine system play vital roles in energy metabolism:

• Digestive System: Breaks down food into nutrients, which can be absorbed and converted into energy.
• Nervous System: Regulates hunger and energy usage through signaling pathways.
• Endocrine System: Releases hormones to manage metabolism, such as insulin and glucagon.

In the scenario described, energy from triacylglycerols is mobilized through these physiological systems during fasting or starvation to maintain homeostatic balance. Human physiology ensures that energy intake and expenditure are balanced, helping the body manage energy resources efficiently. This is what allows the body to conserve energy and prolong survival during periods of insufficient energy intake.