I was recently reading Biohacking Lite by Andrej Karpathy, particularly the section called The Four Batteries of Your Body. For the first time in my life, health-related topics are starting to make sense to me because Karpathy explains them from first principles.

TLDR

This is my TLDR after an hour of studying the topic. Any mistakes or misunderstandings are my own, and I may update this as my knowledge improves.

• Most of the human body is just battery storage that runs itself.
• There are different types of batteries to power various activities:
  • A quick, short-term battery that wakes up fast but finishes quickly.
  • An on-demand, medium-term battery that wakes up a bit slower but works longer.
  • A serious-effort, long-term battery that wakes up even slower but works much, much longer.
  • A danger zone where your vital organs become the battery (!).
• At the end of the day, most of your body is just a big battery.
• All the batteries are used to convert ADP to ATP, which in turn powers all bodily activities.
• Short-term and medium-term batteries activate quickly but are mostly anaerobic (running without oxygen).
• The long-term battery is mostly aerobic (operating with oxygen).

The Detailed Version

The human body is like an iPhone with a battery pack. The only difference is that this battery pack can charge beyond 100% with an almost infinite capacity to keep adding to it.

The modern environment is filled with all sorts of foods and drinks around us, creating an oversupply of food. So, the charging outlet is always there, paired with this “always hungry” battery pack.

The technical terms for the important battery packs are: Adipose tissue & Triglycerides (fat).

These are stockpiled and sometimes synthesized, leading to increased volume. Stockpiling made sense in historic times when there were no guarantees about when one could get the next meal. However, with an abundance of food available today, this storage capacity is working against us.

From ADP to ATP

While there are many ways to stock up on adipose tissue and triglycerides, there’s only one way they get consumed: to synthesize ATP from ADP. ATP stands for adenosine triphosphate, and ADP stands for adenosine diphosphate.

ATP is very important for the body because it is the universal currency through which internal work gets done:

• Transporting molecules across cell membranes.
• Untying DNA against hydrogen bonds.
• Moving myosin to operate muscles.
• Assisting with protein synthesis.

At any point in time, there’s only a very small amount of ATP available. ADP is attached to a phosphate group to become ATP. When the phosphate group is detached, energy is released to perform any work.

So, that’s the universal work done by the body to accomplish higher-level goals. To convert ADP to ATP, there are four types of batteries available:

• Super short-term battery
• Short-term battery
• Long-term battery
• Lean body mass

Super Short-Term Battery: The Phosphocreatine System

This system provides an immediate but very limited energy buffer, allowing ATP to be rapidly regenerated. Many athletes take creatine supplements to enhance this buffer.

• Stores phosphate groups attached to creatine.
• Enables quick, localized recycling of ADP into ATP.
• Has an extremely small capacity, so it’s not a major factor in overall energy storage.

Short-Term Battery: Glycogen {24 Hours}

Glycogen is the body’s primary short-term energy store, located in the liver and muscles. It provides about 2,000 kcal, roughly one day’s worth of energy at basal metabolic rate (BMR). However, it is inefficient due to its low energy density and high water retention.

• Storage Locations:
  • Liver: ~120g
  • Skeletal muscle: ~400g
  • Blood glucose: ~4g
• Energy Yield:
  • 4 kcal per gram of glycogen.
  • Total glycogen energy: ~2,000 kcal.
• Efficiency Issues:
  • Glycogen binds ~3g of water per gram, making it a poor long-term storage medium.

Long-Term Battery: Adipose Tissue (Fat) {50-80 Days}

Fat is the body’s main high-capacity energy reserve, storing significantly more energy than glycogen. It is more than twice as energy-dense as carbohydrates and can sustain the body for weeks or even months if needed.

• Energy Density:
  • Fat: 9 kcal per gram (vs. glycogen’s 4 kcal/g).
• Example Calculation (from my 2019 data):
  • Body weight: 200 lbs
  • Fat mass: 40 lbs = 18,000g
  • Total energy in fat: 18,000g × 9 kcal/g = 162,000 kcal
  • Survival estimate: 162,000 kcal ÷ 2,000 kcal/day = ~81 days
• Comparisons for Scale:
  • Equivalent to 678 sticks of dynamite (1 MJ ≈ 239 kcal).
  • Nearly enough energy to fully charge a 100 kWh Tesla battery twice!

Last Resort Battery: Lean Body Mass

When fat reserves are depleted, the body breaks down muscle tissue for energy—a last-ditch survival mechanism.

• Occurs only in extreme fasting or starvation.
• Uses muscle proteins as the primary fuel source, converting them into glucose or ketones.
• This process weakens the body over time, making it highly undesirable.

How Your Body Uses Energy

Your body constantly charges and discharges all four energy stores at different rates, depending on your activity and food intake.

• After Eating (e.g., a Cookie):
  • The cookie is broken down into glucose, which enters the bloodstream.
  • If there is excess glucose (as there usually is with cookies), your body:
    • Stores it as glycogen in the liver and muscles.
    • Rarely, if glucose is in extreme excess, converts it to fat.
• During Exercise (e.g., Jogging):
  • First 3 seconds: The phosphocreatine system (1) provides immediate energy.
  • Next 8-10 seconds: Glycogen (2) is used anaerobically (without oxygen).
  • Longer duration:
    • Glycogen (2) and fat (3) become the main energy sources.
    • These rely on aerobic metabolism, which takes longer to activate but provides sustained energy.
    • Your body increases heart rate, breathing, and oxygen transport to keep up.
  • In Starvation or Carb Deprivation: The body eventually resorts to breaking down muscle (4) for energy, a last-resort survival mechanism.

The Computer Memory Analogy (Brilliant Stuff)

This energy hierarchy can be compared to a computer’s memory hierarchy:

• Phosphocreatine System (1) → L1/L2 Cache
  • The fastest and most immediate energy source, but with very limited capacity.
• Anaerobic Glycolysis (2) → RAM
  • Offers quick access, but has limited storage and is less efficient.
• Aerobic Metabolism (3) → Disk Storage
  • Provides high capacity and efficiency, but is slower to access due to the need for oxygen and the transport of fatty acids from adipose tissue.

Just like in computing, moving energy around is costly, so the body prioritizes faster but smaller energy stores before tapping into slower but larger reserves.