Metabolism: How Your Body Handles Food and How to Live with It

Introduction

Why the Same Food Behaves Differently in Different Bodies

About forty years ago, when I was in my thirties, my wife and I went on vacation with our friends.
Two families, two couples, four children.
One house.
One kitchen.
One fridge.

From the outside, everything looked the same.
We cooked together, sat at the same table, shared the same dishes.
But inside our bodies, completely different stories were unfolding.

I have always had a tendency to gain weight easily.
I eat carefully, not much, not very often, yet my body stores energy quickly.
When I relax my control, the scale reacts very fast.

The husband in the other family was the opposite.
He was slim, even thin, and his appetite seemed endless.
He ate large portions, ate more frequently than I did, and often visited the fridge at night.

At first, my wife looked at me, then at him, and said:
“You see? You should learn from him.”

But after some days of living together, she saw the numbers more clearly.
I ate little and rarely.
He ate a lot and often.
Sometimes she would wake up at night and see him in the kitchen, quietly opening the fridge.

Our teenage son sometimes joined him there.
For our son, this made sense: his body was in a phase of rapid growth and needed a lot of energy to build bones, muscles, brain.
For our friend’s husband, this was simply how his body worked: a fast inner engine, always asking for fuel.

One evening my wife said, half‑seriously, half‑amused:
“He is a very expensive husband. They must spend much more just to feed him.”

Same house. Same food.
One man gaining weight easily on small portions.
Another man staying thin on large portions, plus night visits to the fridge.
A teenager who eats at night and grows taller and stronger.

This experience stayed with me for decades.
Today, I am 76 years old, one of the co‑authors of this book.
Our son Andriy is now 43.

He has inherited part of my metabolic pattern, but he has learned to regulate his eating to match his own body. He lives a healthy lifestyle and looks excellent.
I, in turn, have found my own rhythm of eating, with my personal interval between meals, a form of fasting and feeding that fits my slower metabolism and keeps me well.

Both of us – father and son – are physicians and scientists.
We come to this topic not only as a family who has lived with different metabolic speeds across generations, but also as clinicians and researchers who have watched many patients struggle with the same questions.

This book grows out of these long‑term observations – in our family, in our patients, and in ourselves.

What this book is – and is not – about

In this book we speak about fast, balanced, and slow metabolic speed as physiological variations of how bodies handle food and energy. We focus on genetically undisturbed metabolism: people whose metabolic machinery is structurally intact, but whose bodies differ in baseline speed, enzyme patterns, microbiota, and life history. It is written for those who have noticed that the same food behaves differently in different bodies – and who want to understand why, and how to live with the metabolism they were given.

The three‑speed spectrum is a practical way to group recurring patterns in everyday life — not a claim that all of human metabolism can be reduced to three boxes. It is a clinical map laid over a complex landscape, helpful for orientation and decision‑making, but never a substitute for the full biology or for individual medical assessment when something clearly falls outside the usual pattern.

At every point on this spectrum, however, there can also be true diseases – of endocrine glands, nerves, brain, or other organs – that disturb appetite, weight, and strength. When changes in hunger, weight, or energy become very sudden, very extreme, or clearly deviate far from what is typical for your usual pattern or for your metabolic type, this is a reason to see a doctor, not only to change a diet.

There is also a separate, much rarer group of conditions called inherited metabolic disorders (inborn errors of metabolism). In these diseases, key enzymes or transporters are missing or severely impaired, and the products of metabolism themselves become abnormal or accumulate to toxic levels. Taken together, these genetic anomalies are important for medicine but still affect only a small minority of the population. This book is not a guide to those conditions. It stays beside them and speaks about the wide range of physiological metabolisms in which genes and machinery are intact, yet differences in baseline and life circumstances still matter and require a personalized approach.

Part I – Your Metabolic Story: A Built-In Speed

Chapter 1 – Two Families, One Fridge (edited)

If you have read the Introduction, you have already met our two families under one roof: two men, two metabolic patterns, one fridge. You may not have had exactly the same scene in your life, but perhaps you recognize the feeling.

You sit at a table with friends, colleagues, or relatives. Everyone eats more or less the same food. Some people take second and third portions; others stop after one plate. A few stay slim almost without trying. A few struggle with weight gain even when they are careful. Maybe you are the one who gains easily. Maybe you are the one who never gains at all. Maybe you are somewhere in the middle, watching both extremes and wondering where you belong.

Familiar scenes

You may have seen scenes like these:

• A couple where the wife gains weight during the first years of marriage, while the husband still fits into his old trousers.
• Two colleagues on the same diet plan: one loses ten kilograms, the other loses almost nothing.
• A group of friends: one drinks coffee in the evening and sleeps like a child, another cannot drink coffee after lunch or they will not sleep at all.
• A teenager who seems to live on sandwiches and sweets and still grows taller and leaner, while their parent gains weight from finishing the same leftovers.

From the outside, such scenes look unfair or even absurd. From the inside, they can feel painful.

People often respond with moral explanations:
"He is disciplined, you are not." "She has willpower, you don't." "They are lazy." "They are careless."
These explanations are simple. They are also often wrong.

The quiet question many people carry

If you are reading this book, perhaps you have asked yourself quietly:

• Why do I gain weight more easily than others?
• Why does the same diet work for my friend but not for me?
• Why does one pill help someone else but make me feel terrible?
• Why am I always hungry, or almost never hungry?
• Why did my body change so much after growth, pregnancy, menopause, illness, or a period of stress?

You may not have said these questions out loud. You may have tried to follow the usual advice — "eat less, move more" — and sometimes it helped, sometimes it did not. You may have felt that there is something in your body that behaves differently, but you did not have words for it. This book is an attempt to give you those words.

Seeing patterns, not just episodes

As physicians and scientists, we see patients one by one. As family members, we see relatives and friends over many years. When we step back, we start to see patterns:

• The person who has always been "fast": quick to move, quick to get hungry, hard to keep weight, more sensitive to lack of food.
• The person who has always been "slow": slower to get hungry, easier to gain weight, more affected by large meals.
• The many people in between: sometimes fast, sometimes slow, strongly influenced by sleep, stress, work, hormones, seasons.

These patterns are not about laziness or virtue. They are about how each body is built and how it tends to handle food, energy, and signals.

We needed a simple way to talk about these differences with patients and readers — not a complex laboratory classification, but a language you can recognize in your own life. This is where the idea of metabolic speed comes in.

Why we talk about metabolic speed

In modern science, we can measure many separate things: resting metabolic rate, total energy expenditure, glycemic responses, drug metabolism, microbiota patterns. Each of these is like one tree in a forest. For this book, we needed something simpler — a forest map you can carry in your head.

We chose to speak about metabolic speed:

• Slow — bodies that tend to save energy more easily and gain weight more quickly if food is abundant and movement is low. Many people with slow speed can tolerate longer gaps between meals without intense hunger.
• Balanced — the broad middle range where many people live, with a mix of traits and more flexibility. Hunger, weight, and energy here are strongly shaped by sleep, stress, activity, and life circumstances.
• Fast — bodies that tend to burn energy quickly and recover their hunger sooner after meals. Many people with fast speed get hungry often and may struggle to gain weight or keep reserves.

These are patterns, not diagnoses. It is possible to have extra weight and still feel frequent strong hunger, or to be thin with a surprisingly low appetite. Appetite and weight are also shaped by hormones, stress, habits, emotions, and disease. Our three descriptions are a practical map, not a scientific classification system.

Your starting point is not your fault

Your body's way of handling food and energy is not a moral quality. It is a built-in property that you did not choose. You cannot fully control your starting point — but you can influence how it plays out over time through your habits, your environment, and your cooperation with your body rather than constant war against it.

One important note before we go further: this book speaks about metabolic speed in people who are generally medically stable. Severe or sudden changes in appetite, weight, or energy always require medical evaluation — the framework here is a map for understanding patterns, not a substitute for professional care.

What this chapter is for

This chapter has one purpose: to invite you to look at your own life with new eyes. Think of the people you have eaten with, the diets you have tried, the times your body surprised you — in growth, pregnancy, illness, menopause, aging — and the moments when you felt: my body does not behave like theirs.

Bring those memories into the next chapters. Next, we step into the language of physiology and ask what metabolism actually is — and how we can talk about it in plain words without losing its depth.

You can learn more by reading our e-book 

Mykola Iabluchanskyi together with Andriy Yabluchanskiy