The 7 Secret Pathways Your Body Uses to Stay Alive

Here’s the wild truth: at any given time, only 4 grams of glucose are circulating in the blood of a healthy adult. That’s barely a teaspoon of sugar—and yet, your body manages to keep you alert, energetic, and functioning like a boss. How? Through an incredibly complex and efficient metabolic orchestra that most people don’t even know exists. Whether you’re deep into a fast, sprinting on a track, or just strolling through your day—your body is constantly switching gears between different fuel systems. It’s not magic. It’s biochemistry on beast mode.

In this post, I break down the 7 metabolic pathways your body uses to regulate blood sugar and generate energy—especially during fasting or exercise. From glycogen storage and fat-burning to ketones and ATP generation, get ready for a deep dive into how your body fuels itself like a survivalist genius.

To get a better understanding of how glucose levels remain stable in a healthy individual on an extended fast, look at this mind-map of "7 Metabolic Pathways".

• Glycogenesis: Liver converts glucose to glycogen and stores it for future use (100g max). Skeletal muscle cells also convert blood glucose to glycogen and stores it in the muscles (400g max). Glycogen stores in the muscles can only be used locally whereas liver glycogen can be used anywhere.

• Lipogenesis: When the liver and muscle glycogen stores are full, the liver (on the working of Insulin) then converts any remaining glucose to Triglycerides, which are then stored in fat cells, because the body has limited glycogen reserves/stores but infinite fat stores. Also, it is dangerous to have excess glucose circulating in the blood stream. So the primary work of insulin is to convert excess glucose to stored fat.

• Lipolysis: The breakdown of triglycerides (stored in body fat) into glycerol and free fatty acids. This happens when blood sugar is low, then the hormone Glucagon signals fat cells to release free fatty acids into the blood. Glycerol is half a glucose molecule. Free fatty acids are efficient fuel that the body can utilise.

• Ketogenesis: The liver converts some of these free-fatty acids into ketones.

• Glycogenolysis: Sensing low blood sugar, glucagon signals to the liver to release glycogen as glucose from its stores (100g remember?) thereby converting glycogen back to glucose for energy. This can happen simultaneously with Lipolysis.

• Glycolysis: The metabolic pathway that converts glycogen in muscle (400g remember) into pyruvate, which can then enter the Citric Acid Cycle, also known as Krebs cycle to generate ATP in the presence of oxygen. This happens during aerobic exercise. In the absence of oxygen, like anaerobic exercise, pyruvate is further converted to lactate.

• Gluconeogenesis: When the liver has depleted its glycogen reserves, then to through a series of metabolic pathways, it makes glucose from non-carbohydrate substrates, namely - lactate, glycerol, and amino acids. The glucogenic amino acids are glycine, serine, aspartic acid, glutamic acid, glutamine, valine, methionine, histadine and arginine.

What is Cellular Respiration?

Cellular respiration is the process by which cells convert glucose and oxygen into energy, carbon dioxide, and water. This process is essential for the survival of living organisms, as it provides the energy necessary for cellular functions. Cellular respiration consists of three main stages:

1. Glycolysis

2. The Kreb's Cycle (or) The Citric Acid Cycle

3. Oxidative phosphorylation (or) Electron Transport Chain

Glycolysis breaks down glucose into pyruvate, which then enters the citric acid cycle to produce energy-rich molecules. Finally, oxidative phosphorylation uses these molecules to create ATP, the energy currency of the cell.

What is ATP?

ATP, or adenosine triphosphate, is a molecule that plays a crucial role in energy transfer within cells. It is often referred to as the "energy currency" of the cell because it provides the energy needed for cellular processes such as muscle contraction, protein synthesis, and transportation of molecules across cell membranes. ATP is made up of a nitrogenous base, a sugar molecule, and three phosphate groups. During muscle contraction, when one of the phosphate groups is broken off, energy is released, which can be used by the cell and what remains is (ADP) adenosine diphosphate. ATP is constantly being produced and consumed in cells to maintain energy balance.

Fact to remember: the body spends ATP to get more ATP! (some what like how mutual funds work, I guess)

Another cool fact to remember: One molecule of glucose can generate a net of 38 ATP, whereas one molecule of fatty acid can generate a net of 129 ATP. Which is why some of us who are 'fat-adapted' can work out and train (with the exception of certain exercises) with little to no dietary glucose intake.

When does the Phosphagen System kick in?

In the phosphagen system, creatine phosphate is broken down releasing phosphate and energy, which is then used to rebuild ATP. These reactions take place in the absence of oxygen and therefore termed as Anaerobic. Examples of exercises that utilise the ATP-CP system are:

• Lifting the heaviest weight you possibly can for one or two repetitions.

• Sprinting as fast as you can for 50 – 100 metres with 2-3 minute recovery intervals before repeating.

• Punching a boxing bag as hard as you possibly can for 2 – 3 punches.

• Darting across the room from your chair to catch your baby before he falls off the sofa (alright it’s not really ‘training’ but as it requires immediate energy for movement the energy comes exclusively from the ATP-PC system).

What is the Aerobic System?

The Aerobic System has three stages:

Stage 1: Glycolysis

Here glycogen from liver and muscles is first converted into glucose and into two pyruvate molecules through a series of many pathways but with the presence of oxygen (hence the term 'aerobic'). Pyruvate is further converted into ‘acetyl coenzyme A’. In addition to the breakdown of glucose, fats are also broken down into glycerol molecules.

Stage 2: The Kreb's Cycle

Acetyl coenzyme A enters Stage 2 of the Krebs Cycle. The by-products of this stage is 2 ATP molecules, hydrogen ions and carbon-dioxide. The Kreb's cycle also known as the citric acid cycle occurs in the mitochondria of eukaryotic cells and involves the oxidation of acetyl-CoA, which is generated from the breakdown of carbohydrates, fats, and proteins. This process ultimately produces energy in the form of ATP, as well as carbon dioxide and water as byproducts.

Stage 3: Electron Transport Chain

The hydrogen ions then enter Stage 3. The Electron Transport Chain (ETC) is a series of protein complexes in the inner mitochondrial membrane that play a critical role in cellular respiration. During this process, electrons are transferred from electron donors to electron acceptors, such as oxygen, resulting in the production of 34 ATP molecules along with water.

Though the complete process produces 36 ATP (net 34) molecules, the generation or production of ATP takes time and is a slow process. Therefore, one can sustain aerobic exercises for about 2 hours but only at lower intensities. The fuel for the aerobic system is glycogen and stored body fats.

What exercises employ the Aerobic System?

Any exercise that doesn't exceed 60% of maximum heart rate employs the Aerobic system. Such as:

• Swimming, Walking, Cycling

• Jogging, Marathon running

• Pilates, Yoga

• Low intensity strength training

• Low intensity circuit training

Note: To calculate your maximum heart rate, subtract your age from 220.

What is the Lactic Acid System?

The Lactic Acid System is one of the three energy systems in the human body. This system kicks in during high-intensity exercise when the body's demand for energy exceeds the oxygen supply. The first part of the cycle is the Aerobic Glycolysis (Glycogen converted to Pyruvate). Now, pyruvate is converted to lactate in the absence of oxygen with hydrogen. This process is called Anaerobic glycolysis. This conversion is catalyzed by an enzyme called lactate dehydrogenase. The process of pyruvate to lactate helps the body to generate energy quickly, but it also results in the accumulation of lactate (remember the feeling of your quads on fire?), which can lead to muscle fatigue and soreness.

What is the Cori System?

The lactate that builds up in the muscles cannot be used by the cell and is transported out of the cell into the bloodstream, and a portion of it reaches the liver where it can undergo gluconeogenesis to produce glucose to transport back to the cells. This is called the Cori Cycle and is an example of one of the critical roles of the liver in assuring an adequate supply of glucose in the body.

It is important to note that the process of producing glucose in the liver requires 6 ATPs, whereas the return through glycolysis to lactate produces only 2 ATPs. So it is an inefficient process for producing useful energy in the cell as it is costly in energy.

This is why high intensity exercises are discouraged during a prolonged fast. In the absence of dietary glucose and when the liver has depleted its glycogen reserves as Gluconeogenesis is a costly process. The aerobic system, as explained earlier, involving Glycolysis -> Krebs cycle ->Electron chain transport system, is more suited for Prolonged fasts or carbohydrate-restricted diets, as more ATPs are generated (34 net).

What exercises uses the Lactic Acid System?

Remember the ATP-CP or Phosphagen system? After about 12 seconds, when the ATP-CP stores are extinguished the Lactic Acid System kicks in. This system is also triggered for high intensity exercises that last more than 2 minutes (anaerobic). Examples of exercises that employ the Lactic acid energy system are:

• A 200m, 400m race/run

• Weight training of 10-12 reps

• Tabata style HIIT workout

• Power Pilates or HIIT Pilates

• High intensity circuit training

• Biking, Swimming, Speed skating

Conclusion:

Your body is an incredible machine with multiple backup systems to keep you fueled—whether you're feasting, fasting, lifting weights, or just chasing toddlers. Understanding these metabolic pathways not only helps you train smarter and eat better, but also gives you deeper respect for how resilient and adaptive the human body truly is. Fuel it well, and it will serve you powerfully.