Part 1 — Understanding the basics
Chapter 1
How the Body Uses Energy
Think about the last meal you ate. Maybe it was a bowl of rice with vegetables, a sandwich, a plate of pasta, or a warm tortilla wrapped around beans. Whatever it was, within an hour or two it had vanished from your plate and, in a very real sense, from the world of food altogether. Part of it became the electricity that lets you read this sentence. Part of it was quietly packed away for later. And a small, carefully guarded portion of it entered your bloodstream as a single molecule: glucose.

Right now, at this moment, there is only about a teaspoon of glucose circulating in all the blood in your body, roughly 4 to 5 grams.[@wasserman2009] Your body works hard to keep it there. Too little, and your brain starts to falter within minutes. Too much, for too long, and blood vessels, nerves, kidneys, and eyes slowly pay the price. Diabetes, in all of its forms, is what happens when the systems that keep that teaspoon in balance stop doing their job.

This chapter is the foundation for everything else in the book. We will follow a single meal on its journey: from your mouth, through your gut, into your blood, into your cells, and finally into storage. Along the way we will meet the hormone that runs the whole operation, insulin, and then see how the system begins to fail. Some of the terminology may look intimidating at first. It is not as difficult as it sounds, and I have marked the most technical parts as "Science Corner" boxes, so that you can skip them on a first read and come back later. Understanding this machinery is not just academic. Once you can see why a blood sugar number behaves the way it does, every later decision, from what to put on your plate to when to take a walk, stops feeling like an arbitrary rule and starts making sense.

1.1 The Metabolic Engine

From food to fuel: how carbohydrates become glucose

Carbohydrates are the body's most flexible fuel. They come in many forms: the starch in rice, wheat, corn, potatoes, and cassava; the sugars in fruit, milk, and honey; the fiber in vegetables and beans. But your cells cannot use any of these directly. Almost all digestible carbohydrate has to be reduced to its simplest building blocks, single sugar molecules called monosaccharides, before it can pass from your gut into your body. The most important of these is glucose.

Digestion of starch begins earlier than most people realize: in your mouth. Your saliva contains an enzyme called salivary amylase, which starts snipping the long chains of starch into shorter fragments. You can actually taste this at work. Chew a piece of plain bread or a spoonful of plain rice for a minute or so, and it begins to taste faintly sweet as the starch is broken into sugars. Salivary amylase is soon switched off by stomach acid, so its head start is modest. The main event takes place further down.

When the partly digested meal leaves your stomach and enters the first part of the small intestine, your pancreas releases pancreatic amylase, a far more powerful version of the same enzyme. It continues cutting starch into smaller and smaller pieces. The final steps are handled by enzymes on the surface of the intestinal lining itself (maltase, sucrase-isomaltase, and lactase, among others), which split the remaining two-sugar molecules, or disaccharides, into single units. Table sugar (sucrose) becomes glucose and fructose. Milk sugar (lactose) becomes glucose and galactose. Starch, which is really just a very long chain of glucose units, becomes glucose and only glucose.[@goodman2010]

Notice something important here. Whether you eat white rice, a sweet potato, or a spoonful of sugar, the end product in the gut is largely the same. What differs is the speed. Fine, refined starch is broken down quickly; starch locked inside intact grains, seeds, or fiber-rich structures is broken down slowly. That single idea, speed rather than identity, will come back again and again, especially in Part 2 of this book.

Science Corner: Two kinds of starch Starch comes in two forms. Amylose is a long, straight chain of glucose molecules, tightly packed and relatively slow to digest. Amylopectin is heavily branched, giving enzymes many more places to attack at once, so it digests fast. Foods differ in their ratio of the two, which is one reason different rices and potatoes behave so differently in your blood.

Getting glucose into the bloodstream

Once glucose has been freed, it has to cross the lining of the small intestine. This lining is one cell thick, folded into millions of tiny finger-like projections that create a surface area of roughly 30 square metres, about the size of a studio apartment.[@helander2014] The cells that form it are called enterocytes, and they have to move glucose from the inside of your gut (which, remarkably, is technically still "outside" your body) into the blood vessels underneath.

They do this using two different kinds of transport proteins working in sequence:

Think of SGLT1 as a doorman pulling guests in from the street, and GLUT2 as the exit door on the other side of the lobby. Everything that comes in through the front eventually goes out the back into the blood.

{{fig:F1-A}}

From here, glucose travels through the portal vein directly to the liver, which acts as the body's first checkpoint. The liver takes up a portion of the glucose and lets the rest pass into general circulation, where it is delivered to every tissue in your body. Your blood glucose begins to rise.

Turning glucose into energy

What do cells actually do with glucose? They burn it, though not in the sense of flames. The process is a slow, controlled, multi-step chemical release of energy, and its end product is a molecule called ATP (adenosine triphosphate). ATP is the universal energy currency of life: the muscle contraction in your leg, the nerve impulse in your brain, the beat of your heart, and the repair of every cell in your body are all paid for in ATP.

The conversion happens in three stages:

  1. Glycolysis. In the fluid of the cell, one molecule of glucose is split into two smaller molecules called pyruvate. This yields a small, quick payout of ATP and does not require oxygen.
  2. The citric acid cycle (also called the Krebs cycle). Pyruvate enters the cell's power plants, the mitochondria, where it is broken down further, releasing carbon dioxide (the CO₂ you breathe out) and loading up carrier molecules with high-energy electrons.
  3. Oxidative phosphorylation. Along the inner membrane of the mitochondria, those electrons are passed down a chain of proteins, using oxygen at the end, and the energy released is used to manufacture the bulk of the ATP.

A single glucose molecule can generate roughly 30 to 32 ATP in total, most of it from that final stage.[@rich2003] Your brain alone, which makes up only about 2% of your body weight, uses roughly 20% of your resting energy, and glucose is its preferred fuel.[@raichle2002] That is why very low blood sugar makes people confused and shaky, and why the body defends blood glucose so fiercely.

Science Corner: Why oxygen matters The reason you breathe faster during exercise is largely to keep this last stage running. Without oxygen, cells can only rely on glycolysis, which is fast but yields a tiny fraction of the energy and produces lactate. This is also why regular aerobic exercise, which builds more mitochondria, makes you a more efficient user of glucose. We will build on this in Chapter 9.

Storing the surplus: glycogen

Your body rarely needs all of the glucose from a meal at once. Rather than let it accumulate in the blood, where it would be damaging, the body stores the excess in a compact form called glycogen, essentially a densely branched chain of glucose molecules, like a tightly wound spool.

Storage happens mainly in two places, and they have different jobs:

The process of building glycogen, called glycogenesis, is switched on by insulin. In other words, after you eat, insulin is the signal that says: "Fuel is plentiful. Use what you need and store the rest."

There is an obvious limit here. Together, your liver and muscle glycogen stores hold only a few hundred grams of glucose, equivalent to a day's worth of carbohydrate at most, and for a person who is inactive and eating more than they burn, those stores are often already full. When glycogen tanks are full, the liver begins converting surplus glucose into fat, and fat can be stored almost without limit. This is the seed of the trouble we will explore in section 1.3. It also reveals a hopeful point that we will use later: because muscles are large glycogen reservoirs, emptying them through physical activity creates real room for glucose to go.

1.2 The Role of Insulin and the Pancreas

If glucose is the fuel, insulin is the traffic controller. Insulin is a hormone, a chemical messenger, produced by the pancreas, a leaf-shaped organ tucked behind your stomach. Scattered throughout the pancreas are an estimated one to three million tiny clusters of cells called the islets of Langerhans,[@ionescu2015] and within those islets sit the beta cells, the cells that make insulin.

How the pancreas senses a meal

Beta cells are exquisite glucose sensors. When blood glucose rises after a meal, glucose flows into beta cells, where an enzyme called glucokinase acts as a kind of meter, measuring how much is present. As glucose is metabolized inside the cell, its energy level rises and a chain of electrical events unfolds: ion channels close, the cell membrane depolarizes, calcium floods in, and the insulin that was stored in small packets (vesicles) is released into the blood.[@henquin2009]

Insulin release is biphasic, meaning it comes in two waves[@henquin2009]:

The first-phase burst is one of the earliest things to be lost as type 2 diabetes develops, long before blood sugar reaches diagnostic levels. That is one reason a person can look "normal" on a fasting blood test while their post-meal glucose has already become abnormal.[@gerich2002]

How insulin delivers its message

Insulin, once in the bloodstream, travels to every tissue in the body. But insulin does not itself enter cells. It delivers its message from the outside. The popular analogy is a key in a lock: insulin (the key) fits into the insulin receptor (the lock) on the surface of a cell. I would add a refinement, because it makes the picture more accurate: the receptor is less like a lock on a door and more like a doorbell. Insulin rings the bell; it never walks through the door. What happens next is that the cell responds from within.

Here is that sequence, in plain terms:

  1. Insulin binds to the receptor on the cell surface. The receptor is a type of enzyme called a tyrosine kinase, and binding causes it to activate itself, a process called autophosphorylation.
  2. The activated receptor switches on a relay of messenger proteins inside the cell. Two key names you will see in the literature are IRS-1 (insulin receptor substrate 1) and PI3K (phosphoinositide 3-kinase).
  3. This signal cascade ultimately travels to storage compartments inside the cell that hold a special glucose transporter called GLUT4.[@petersen2018]

Science Corner: Insulin does more than move sugar Insulin is a growth and storage hormone with wide effects. In the liver, it tells cells to stop making new glucose and to build glycogen. In fat tissue, it stops the release of stored fat and promotes fat storage. In muscle, it promotes glucose uptake and protein building. This is why insulin resistance affects far more than blood sugar, and why it often accompanies raised triglycerides, fatty liver, and abdominal weight gain.[@petersen2018]

The GLUT4 doorway

This is the elegant heart of the process. In muscle and fat cells, GLUT4 transporters are mostly kept inside the cell, packed into small storage vesicles, like chairs stacked in a back room. When the insulin signal arrives, these vesicles travel to the cell surface and fuse with the membrane, inserting GLUT4 into the wall. Now the "doors" are open. Glucose, which is plentiful outside the cell, flows in by facilitated diffusion, and the level of glucose in the blood falls.

When insulin levels drop again, GLUT4 is drawn back inside, and the doors close. The whole system therefore behaves like a thermostat: glucose rises, insulin rises, glucose is pulled into tissues, glucose falls, insulin falls.

{{fig:F1-B}}

Two things are worth holding on to as we move on:

1.3 Insulin Resistance Unpacked

In a healthy person, the system we have just described runs smoothly for decades. Insulin resistance is what happens when cells stop responding properly to the doorbell. The pancreas rings louder, and for a while, this works.

To be clear about what "resistance" means: it is not that the cells refuse insulin entirely. It is a reduced sensitivity. It takes more insulin than before to produce the same effect. Someone with insulin resistance is often making two, three, or more times the normal amount of insulin, keeping their blood sugar in the normal range through brute force. This is why fasting insulin is often high years before fasting glucose ever rises.[@tabak2009]

Insulin resistance is not a single defect. Several forces converge, and they tend to feed each other.

Force one: inflammation and constant high insulin

Fat tissue, particularly the fat stored deep in the abdomen around the organs (visceral fat), is not inert. It behaves like an active endocrine organ, releasing inflammatory messengers such as TNF-alpha and IL-6. In small amounts these are part of a healthy immune response; in chronic excess they interfere with insulin signaling inside cells, specifically by disrupting the IRS-1 relay step we met earlier.[@hotamisligil2006] The doorbell is ringing, but the wiring has been tampered with.

A second, subtler problem is that constant high insulin can itself dull the response. When cells are exposed to elevated insulin around the clock, they respond, as most biological systems do, by reducing the number of receptors on their surface and dampening their signaling. It is the same principle as walking into a room with a strong smell: after a few minutes, you stop noticing it. (Researchers still debate how much high insulin is a cause of insulin resistance versus a consequence of it. In practice, it is likely a vicious circle in which each feeds the other.[@shanik2008])

Force two: fat in the wrong places

Your body is designed to store fat in fat tissue. When fat tissue reaches its limit, or when the capacity of a person's fat cells to expand is exhausted, fat begins to accumulate in places it does not belong, particularly the liver, the muscles, and the pancreas. This is called ectopic fat.

The problem is not the fat itself, but the by-products it generates inside cells. Molecules such as diacylglycerols (DAGs) and ceramides accumulate and directly interfere with insulin signaling.[@samuel2016,petersen2018] In the liver, this leads to what has traditionally been called non-alcoholic fatty liver disease (NAFLD), a condition now renamed metabolic dysfunction-associated steatotic liver disease (MASLD) in recent international guidance.[@rinella2023] A fatty liver becomes resistant to insulin's message to stop producing glucose, so it keeps releasing glucose into the blood even when it should not, one reason fasting blood glucose rises.

This is also why people of normal weight can have insulin resistance.[@thomas2012] What matters is not just how much fat you have but where it is, and whether your body has room to store it safely. It is also why waist measurement often says more about metabolic risk than the number on the scale,[@ross2020] a topic we will return to in Chapter 2.

Force three: the pancreas overworks, then tires

For years, or even decades, the beta cells compensate. They grow in number and size, and they produce more insulin to keep glucose in the normal range. From the outside, everything looks fine. A routine blood glucose test may come back normal, while the pancreas is quietly working at several times its normal capacity.

But beta cells are not built for endless overtime. Sustained high demand, combined with the toxic effects of chronically elevated glucose (glucotoxicity) and elevated fat molecules (lipotoxicity), starts to damage them.[@weir2004] Beta cells begin to lose their identity and function, and some die through programmed cell death (apoptosis).[@weir2004,talchai2012] Insulin output starts to fall behind demand, and it is at this point that blood glucose finally begins to rise beyond normal, first after meals, then, as more function is lost, in the fasting state as well. This is the transition from prediabetes to type 2 diabetes.

Science Corner: The "twin cycle" idea One influential model, proposed by Professor Roy Taylor and colleagues,[@taylor2008] suggests that excess calories lead to fat accumulating in the liver, which drives the liver to produce more glucose and more fat-carrying particles. The extra fat then reaches the pancreas and impairs beta-cell function. In a small study of people with recent-onset type 2 diabetes who followed a very-low-calorie diet, liver and pancreas fat fell and beta-cell function recovered.[@lim2011] This does not apply to everyone, and it does not mean the process is always reversible, but it is the scientific basis for the hope we discuss in later chapters.

Why this story matters

There are two lessons to draw from this chapter before we move on.

First, type 2 diabetes is not a sudden event. It is the late stage of a process that usually takes many years. Insulin resistance can be present for many years, often a decade or more, before a diagnosis.[@tabak2009] That is bad news in one sense, since damage may be quietly accumulating, but it is good news in another: there is a long window in which the process can be slowed, halted, and in many cases reversed.

Second, the problem is not a moral failing. Insulin resistance is a physiological state, shaped by genetics, sleep, stress, food environment, activity patterns, and more. Understanding the biology takes away the blame and puts the focus where it belongs: on what can be changed.

Key Takeaways

Action Points

  1. Notice your own patterns. Over the next three days, pay attention to how you feel one to two hours after meals: energy, focus, hunger. You do not need to change anything yet. You are gathering data about your own metabolic engine.
  2. Measure your waist. Using a soft tape measure around your middle, note the number (Chapter 2 shows exactly where to place it). It is a simple and inexpensive marker of visceral fat,[@ross2020] and we will interpret it in the next chapter.
  3. Ask about fasting insulin and HbA1c. If you have never had a metabolic check-up, ask your doctor about fasting glucose and HbA1c (explained in Chapter 4), particularly if you have a family history of diabetes.
  4. Take one 10-minute walk after your largest meal this week. This is the simplest possible way to put the "muscle back door" from this chapter to work.

This book is intended for education and does not replace personal medical advice. If you have diabetes or take glucose-lowering medication, please consult your healthcare team before changing your diet, exercise, or treatment.

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