The Body We Build: Genes, Environment, and the Biology of Adaptation

For most of my life, I thought about genes in a fairly straightforward way. We inherit DNA from our parents, that DNA contains the instructions for building us, and to a large extent that is the biological hand we are dealt. We can exercise, eat well, sleep well, or do the opposite, but underneath all of it there is this permanent genetic blueprint.

That is true, but it is also incomplete.

The more interesting way to think about the human body is that our genes don't simply issue commands. They provide possibilities. The body is constantly deciding which of those possibilities are relevant based on what is happening inside us and around us. Exercise, nutrition, sleep, psychological stress, hormones, inflammation, light, temperature, social interaction and countless other signals are continually being detected and interpreted.

Our DNA may remain largely the same, but the way the body uses that DNA can change dramatically.

That distinction changes the way I think about the body.

Genes Are a Library, Not a Set of Commands

Nearly every cell in my body contains essentially the same genome. A neuron and a muscle cell have the same DNA, yet they look completely different and perform completely different jobs.

Obviously, something more is happening than simply possessing a particular gene.

A muscle cell does not need every instruction contained in the genome at the same time. Neither does a neuron, a liver cell or a skin cell. Different cells express different collections of genes, and even within the same cell, gene activity changes according to circumstances.

I like thinking of DNA as an enormous library.

All the books may be sitting on the shelves, but that doesn't mean every book is being read.

The important question becomes: Which books are open?

That brings us to gene regulation and epigenetics.

DNA inside our cells is packaged with proteins called histones, forming a structure called chromatin. Some regions of chromatin are relatively accessible to the cellular machinery responsible for reading genes, while others are less accessible. Chemical modifications to DNA and histones are among the mechanisms that influence this accessibility.

DNA methylation, for example, can reduce the expression of certain genes depending on where that methylation occurs. Histone acetylation is often associated with more accessible chromatin and increased gene activity.

But the simple explanation that methylation turns genes off and acetylation turns them on is too crude. Biology rarely works as a simple light switch. These processes are part of an enormous regulatory system involving transcription factors, enhancers, repressors, noncoding RNA and many other mechanisms.

The important point is simpler: Having a gene is not the same thing as expressing that gene.

That raises the next question.

What determines which genes are expressed?

To a surprising extent, the answer is information.

The Body Is Constantly Receiving Information

Every cell exists inside an environment.

A cell can encounter hormones, glucose, oxygen, inflammatory molecules, nutrients and signals from the nervous system. Muscle cells experience mechanical tension and changes in energy availability. Circadian signals tell cells something about the time of day. Hormones such as insulin and cortisol communicate information about energy availability and physiological state.

These signals activate biochemical pathways inside the cell.

Eventually some of those pathways reach the nucleus, where they can influence transcription factors and other regulatory machinery. Gene expression changes. Different proteins are produced. Cellular behavior changes.

This is where the relationship between environment and biology becomes fascinating.

Take resistance training.

When I lift a heavy weight, my muscles don't understand the concept of working out. They experience mechanical tension, metabolic changes, calcium signaling, energy depletion and microscopic disruption.

Those are signals.

The body responds to those signals by activating pathways associated with repair and adaptation. Protein synthesis changes. Over time, repeated training can increase contractile machinery, strengthen connective tissues and alter the nervous system's ability to recruit muscle.

The body has received information about the environment and remodeled itself accordingly.

Endurance exercise sends a somewhat different message. Repeated endurance demands can increase mitochondrial content, capillarization and the machinery responsible for producing energy aerobically.

The same principle applies throughout the body.

Bone responds to mechanical loading.

The cardiovascular system responds to repeated aerobic demand.

The nervous system responds to repeated movements and experiences.

Metabolism responds to patterns of energy intake and expenditure.

The immune system responds to repeated challenges.

The organism is continually adapting to what it repeatedly encounters.

That leads to an idea I find particularly powerful: Yesterday's repeated demands can become tomorrow's physiology.

The Body Prepares for What Keeps Happening

I originally described the body as predictive, not reactive. I would modify that now.

The body is both reactive and anticipatory.

It reacts to what is happening now, but repeated exposure changes the organism in ways that can prepare it for similar demands in the future.

The body isn't consciously predicting anything. There is no little person sitting inside the brain deciding what kind of body we will need next year.

Instead, biology is effectively saying: This keeps happening. We should become better equipped to handle it.

Anyone who has trained seriously has experienced this firsthand.

The first time someone performs a difficult exercise, the movement can feel awkward and exhausting. Continue doing it and the nervous system becomes more efficient. Muscles adapt. Connective tissue adapts. Energy systems adapt.

Eventually the same task that once overwhelmed the system becomes routine.

The environment hasn't necessarily become easier.

The organism has changed.

This principle extends far beyond exercise.

The body can adapt to repeated beneficial challenges, but it can also adapt to repeated adverse conditions.

And that brings us to stress.

Stress Is Not the Enemy

One of the biggest mistakes in modern discussions of health is treating stress as inherently harmful.

Stress is essential to adaptation.

Exercise is stress.

Learning is stress.

Fasting is a metabolic stress.

Temperature extremes are stressors.

Competition is stressful.

Even building muscle requires deliberately disturbing the body's existing equilibrium.

The important distinction isn't between stress and no stress.

It is between stress → recovery → adaptation and stress → stress → stress → inadequate recovery.

The first can make an organism more resilient.

The second can gradually wear it down.

There is a concept called hormesis that captures part of this idea. A manageable biological challenge can stimulate protective and adaptive mechanisms that leave the organism better prepared afterward.

Exercise is probably the clearest example.

During a hard workout, I am temporarily making things worse.

Energy stores fall. Oxidative stress rises. Muscle tissue is disrupted. Homeostasis is disturbed. Inflammatory signaling occurs.

If I looked only at what was happening during the workout, I might conclude that exercise was harmful.

But that would miss the entire point.

The benefit occurs because the body responds to the challenge and rebuilds.

Challenge alone isn't the complete stimulus.

Challenge plus recovery produces adaptation.

That distinction becomes important when thinking about chronic stress.

Homeostasis and Allostasis

Most of us learned about homeostasis: the body's tendency to maintain relatively stable internal conditions.

Body temperature has to remain within a certain range. Blood glucose has to be regulated. Blood pressure, pH, oxygen and thousands of other variables are constantly controlled.

But the body doesn't maintain stability simply by keeping everything constant.

Sometimes stability requires change.

That idea is called allostasis.

If I suddenly have to sprint away from danger, my body shouldn't maintain my resting heart rate. Heart rate should increase. Blood pressure should change. Glucose should become available. Attention should narrow. Digestion can temporarily become less important.

Those responses aren't signs that something has gone wrong.

They're signs that the system is working.

The problem occurs when an emergency configuration becomes something closer to a permanent operating condition.

A stress response designed to last minutes or hours is very different from one repeatedly activated for months or years.

Persistent stress, poor sleep, inactivity and metabolic dysfunction can contribute to chronic sympathetic activation, altered hormonal signaling, inflammation, impaired glucose regulation and cardiovascular changes.

Researchers sometimes describe the cumulative physiological burden of repeated adaptation as allostatic load.

This doesn't mean that being worried automatically causes insulin resistance or stiffens the arteries. Human physiology is far too complicated for that. Genetics, age, exercise, nutrition, disease, medication, environment and dozens of other variables matter.

But it does mean that the body's repeated physiological state matters.

The body is constantly responding to the conditions in which it finds itself.

And over time, repeated conditions can influence what kind of organism it becomes.

The Construction Layer

This is where I think the concept becomes especially interesting.

The body isn't merely regulating itself moment by moment. It is physically remodeling itself.

Muscle is remodeled.

Bone is remodeled.

Blood vessels are remodeled.

Connective tissue is remodeled.

Neural connections are remodeled.

Mitochondrial populations change.

Enzyme concentrations change.

Even receptor sensitivity can change.

In other words, adaptation eventually becomes structure.

If I repeatedly ask my body to produce force, it becomes better equipped to produce force.

If I repeatedly ask it to perform aerobic work, it becomes better equipped to generate energy aerobically.

If I repeatedly practice a movement, neural circuits involved in that movement become increasingly efficient.

This means that behavior isn't simply something we do with the body.

Behavior is one of the things that builds the body.

That is a very different way of looking at exercise, sleep, food and even psychological experience.

They aren't merely events occurring during the day.

They are information.

And repeated information can eventually become biology.

Behavior Is Also Influenced by Biology

This is where the whole system turns back on itself.

We usually think of behavior as flowing in one direction: I make a decision, and my body follows my decision.

But the relationship works in both directions.

My physiological state affects the decisions I am likely to make.

Imagine me after eight hours of sleep, a good meal and a workout.

Now imagine the same person after four hours of sleep, hungry, stressed and mentally exhausted.

Technically I am still the same person.

My values haven't disappeared.

My intelligence hasn't suddenly changed.

But I am not making decisions from the same physiological state.

Sleep deprivation affects executive function and impulse control. Hunger changes the value the brain assigns to food. Stress alters attention and arousal. Hormones and neurotransmitters affect motivation. Under sufficient stress, emotional and threat-processing systems can exert greater influence while careful executive control becomes more difficult.

This doesn't eliminate personal responsibility or free will.

It simply recognizes something obvious once we stop pretending that the mind and body are separate: The brain making the decision is part of the body experiencing the state.

That means behavior can become part of a feedback loop.

The Loop

Imagine someone living under chronic stress.

Stress disrupts sleep.

Poor sleep creates fatigue.

Fatigue makes exercise less appealing.

Reduced activity can worsen mood and metabolic health.

The person seeks stimulation or comfort - food, social media, television, alcohol, isolation, constant distraction or whatever temporarily changes the state.

Some of those behaviors may further disturb sleep and recovery.

The next morning the person begins from an even worse physiological position.

Now the cycle repeats.

Stress leads to poor sleep.

Poor sleep leads to fatigue.

Fatigue changes behavior.

Behavior worsens recovery.

Poor recovery increases stress.

The system begins reinforcing itself.

This is what I mean by a loop.

The disturbing part is that after enough repetition, a person may stop recognizing it as a state.

They may start calling it an identity.

I'm lazy.

I have no discipline.

I'm just an anxious person.

I've never been someone who exercises.

But at least some of what appears to be a fixed personality trait may actually be a self-reinforcing physiological and behavioral state.

That matters because identities feel permanent.

States can be interrupted.

Healthy Loops Work the Same Way

Fortunately, the exact same feedback mechanism can work in the opposite direction.

Exercise can improve sleep.

Better sleep improves energy and executive function.

More energy makes exercise easier.

Exercise can improve insulin sensitivity.

Improved metabolic health can produce more stable energy.

Successful training creates psychological reinforcement.

The person becomes more likely to train again.

Eventually something interesting happens.

The behavior that initially required tremendous discipline begins to feel normal.

Anyone who has exercised consistently for years understands this. Eventually missing the workout can feel stranger than doing it.

The system has developed a new equilibrium.

This is why I don't think discipline is simply a personality characteristic.

Discipline can create conditions that eventually reduce the amount of discipline required.

At first I force the behavior.

Then the behavior changes the system.

Eventually the changed system begins supporting the behavior.

That is a fundamentally different way of understanding habit.

From Environment to DNA and Back Again

Now the entire model can be connected.

Something happens in the environment.

I respond behaviorally.

My nervous, endocrine, metabolic and immune systems respond physiologically.

Those systems generate chemical signals.

Cells detect those signals.

Intracellular signaling pathways are activated.

Gene regulation changes.

Different proteins are produced.

Cells adapt.

Tissues remodel.

The remodeled tissues change my physiological state.

My physiological state influences what behaviors become easier or harder.

Those behaviors alter my environment.

And the loop begins again.

So the sequence looks something like this:

Environment → Behavior → Physiological State → Cellular Signaling → Gene Regulation → Tissue Remodeling → New Physiological State → Behavior → Environment

There isn't really a beginning or an end.

It is a continuous biological conversation.

Same DNA, Different Body

This is what I meant by the phrase: Same DNA. Different body.

Identical twins provide an obvious example. They begin with almost identical genomes, yet as they move through life their bodies and health can diverge.

They experience different illnesses.

They eat differently.

They exercise differently.

They have different occupations.

They encounter different psychological stresses.

They sleep differently.

They develop different relationships and habits.

They experience different injuries and environments.

Over decades, differences can accumulate in physiology, gene expression and epigenetic patterns even though the underlying DNA sequences remain extraordinarily similar.

The genome establishes constraints and possibilities.

Life influences which possibilities become expressed.

That doesn't mean we can simply think ourselves into perfect health. It doesn't mean genetics don't matter. And it certainly doesn't mean that every illness is the result of lifestyle or attitude.

That would turn an extraordinary biological concept into pseudoscience.

Genetics matter enormously.

Randomness matters.

Age matters.

Disease matters.

Environment matters.

Behavior matters.

The point isn't that we control biology.

The point is that biology is responsive.

The Larger Idea

When I step back from all of this, epigenetics isn't even the most interesting part.

The larger principle is adaptation to repeated information.

The body is constantly receiving evidence about the kind of world it inhabits.

Heavy resistance training tells it that force production matters.

Endurance exercise tells it that sustained energy production matters.

Food availability tells it something about the metabolic environment.

Light tells the circadian system what time it is.

Sleep tells the nervous system something about recovery.

Repeated psychological threat tells the stress system something about the environment.

Social connection sends another set of signals.

None of these signals single-handedly determines what happens to us.

But together, repeated over months and years, they help shape the organism.

That changes the way I think about the human body.

I don't see the genome as a rigid blueprint anymore.

I see it more like an enormous set of biological possibilities operating inside a system that is constantly listening to the world.

And that system remembers - not necessarily as conscious memory, but through altered neural circuits, proteins, cellular machinery, metabolic capacity and physical structure.

The body carries a history of what it has repeatedly been asked to do.

That may be the most important idea of the entire model:

The genome provides the possibilities. The environment provides information. Behavior repeatedly exposes us to that information. Biology adapts to it. And eventually, some of what we repeatedly do becomes part of what we physically are.

We are certainly constrained by the biology we inherit.

But we are not merely the passive expression of a genetic program written before we were born.

We are biological systems continuously interacting with the world, responding to it, adapting to it and, within limits, rebuilding ourselves because of it.