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 – aboutIn 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.

You can learn more by reading our e-book or listening to our audiobook

Mykola Iabluchanskyi Yabluchansky