2.4. Describe the storage, transport forms and transitions for metabolic fuels

This guide will help you answer 2.4. Describe the storage, transport forms and transitions for metabolic fuels.

Metabolic fuels are the substances our bodies use to generate energy for all functions, from basic cellular processes to complex physical activities. These include carbohydrates, fats, and proteins. Understanding how these fuels are stored, transported, and utilised is fundamental in health and social care to promote good nutritional care for individuals.

Storage of Metabolic Fuels

The body stores metabolic fuels for later use to ensure a constant energy supply. Each type of fuel has a specific storage form and location:

Carbohydrates

• Storage Form: Carbohydrates are primarily stored as glycogen, which is a complex, branched form of glucose.
• Storage Sites: Glycogen is stored in the liver (around 100g) and skeletal muscles (approx. 400g).
• Purpose:
  • Liver glycogen maintains blood glucose levels, especially between meals or during fasting.
  • Muscle glycogen is reserved for use during physical activity, supplying energy directly to muscles.

Fats

• Storage Form: Fats are stored as triglycerides, which are molecules made up of glycerol and three fatty acids.
• Storage Sites: These are primarily stored in adipose tissue (fat tissue) under the skin and around internal organs.
• Purpose: Fat acts as a high-energy reserve and is mobilised when the body needs long-term energy, particularly during extended fasting or endurance activities.

Proteins

• Storage Form: Proteins are not stored in the same way as carbohydrates or fats. Instead, they are found in body tissues, primarily muscles.
• Purpose: Proteins are primarily used for structural and functional roles in the body. They are only broken down and used as fuel during prolonged fasting or energy deficits when glycogen and fat stores are depleted.

Transport Forms of Metabolic Fuels

Metabolic fuels cannot always be used in their storage forms. During transport in the bloodstream, they need to be broken down into smaller, readily usable molecules.

Glucose (from Carbohydrates)

• Transport Form: Glucose is the primary form used for energy transport. After digestion, carbohydrates are broken down into glucose, which enters the bloodstream.
• How It Travels: It is dissolved in blood plasma and is readily available for cells to take up as an energy source.
• Role of Hormones:
  • Insulin helps regulate glucose uptake by cells after meals.
  • Glucagon promotes the release of stored glycogen as glucose when blood sugar levels are low.

Fatty Acids and Glycerol (from Fats)

• Transport Form: Fats are broken down into free fatty acids and glycerol for transport.
• How They Travel:
  • Free fatty acids bind to a protein called albumin to move through the bloodstream.
  • Triglycerides are packaged into lipoproteins, such as very-low-density lipoproteins (VLDLs), for transport to tissues.
• Special Transport: In a fed state, dietary fats are absorbed and transported in specialised particles called chylomicrons.

Amino Acids (from Proteins)

• Transport Form: Proteins are digested into amino acids.
• How They Travel: Amino acids are absorbed into the bloodstream and transported to the liver and other tissues.

Transitions Between Metabolic Fuels

Under varying energy needs and conditions, the body transitions between different metabolic fuels to maintain energy balance.

During the Fed State (After Eating)

• Carbohydrates are the main fuel source.
• Glucose from food enters the bloodstream. Cells take this up to generate immediate energy through a process called glycolysis.
• Excess glucose is stored as glycogen in the liver and muscles. Any surplus is converted into fat and stored in adipose tissue.

During Fasting (Between Meals and Overnight)

• The body shifts to stored glycogen as the primary energy source to maintain blood sugar levels.
• Glycogen in the liver is broken down into glucose through a process called glycogenolysis.
• If fasting continues beyond 12–18 hours, glycogen stores become depleted, and the body starts to use fat for energy.

During Prolonged Starvation or Fasting

• Fats become the predominant metabolic fuel.
• Stored triglycerides are broken down into fatty acids and glycerol through a process called lipolysis.
• Fatty acids are converted into ketone bodies by the liver, which serves as an alternative fuel for the brain and muscles during prolonged energy shortages.

During Intense Physical Activity

• Stored muscle glycogen is the primary energy source at the start.
• As exercise continues, the body supplements energy with glucose from the bloodstream and free fatty acids.
• During prolonged or extreme exercise, the body may also break down protein from muscle tissue to provide amino acids for energy.

During Situations of High Protein Need

• If energy demands exceed glycogen and fat availability, such as in severe illness or starvation, proteins may act as a fuel source.
• Proteins are broken down into amino acids, which are converted into glucose through a process called gluconeogenesis in the liver.

Fuel Transition Control by Hormones

The body’s hormone system tightly regulates when and how it transitions between fuels. Key hormones include:

• Insulin:
  • Lowers blood glucose levels by promoting glucose uptake in cells.
  • Encourages carbohydrate and fat storage during the fed state.
• Glucagon:
  • Raises blood glucose levels during fasting by stimulating glycogen breakdown.
• Adrenaline:
  • Mobilises glycogen and fat stores, especially during physical activity or stress.
• Cortisol:
  • Promotes protein breakdown and gluconeogenesis during prolonged fasting or stress conditions.

Energy Production from Metabolic Fuels

The ultimate purpose of breaking down and transitioning between metabolic fuels is to produce energy in the form of ATP (adenosine triphosphate). ATP powers cellular functions. Each type of fuel undergoes different processes to generate ATP efficiently.

Carbohydrates (Glucose):

Glucose breaks down through glycolysis, followed by the citric acid cycle (Krebs cycle) and oxidative phosphorylation. This process generates a high yield of ATP.

Fats (Fatty Acids):

Fatty acids undergo beta-oxidation in cell mitochondria, producing energy slowly but efficiently. They provide more ATP per molecule than glucose but take longer to metabolise.

Proteins (Amino Acids):

Amino acids are deaminated (removing nitrogen) and fed into glucose or fat metabolic pathways, depending on the body’s needs. This process is less efficient and is used only as a last resort.

Final Thoughts and SUmmary

To summarise the key ideas about metabolic fuels:

• Carbohydrates are stored as glycogen, fats as triglycerides, and proteins in body tissues.
• Glucose, fatty acids, and amino acids are transported through the bloodstream in specific forms.
• The body transitions between fuels based on energy needs, which are influenced by factors like fasting, physical activity, and stress.
• Hormones such as insulin, glucagon, adrenaline, and cortisol play essential roles in fuel storage, release, and utilisation.
• Energy from these fuels is used to produce ATP, which powers all cellular processes.

Understanding these principles is important in care settings where individuals may have specific dietary or metabolic needs. Tailored nutritional approaches help meet these needs and promote overall health and well-being.