By The Functional Medicine Report™

Originally published October 16, 2026 in Issue #011 of The Functional Medicine Report™

A genetic vulnerability can exist for decades before illness appears. The more interesting question may be what changed.

Key Takeaways

• A genetic variant found today has usually been present since conception. It cannot explain why a patient who functioned well for decades became ill at one particular point.
• Exposome research indicates that physical, chemical, infectious, nutritional and psychosocial influences overlap rather than arriving one at a time.
• Estimated ANA prevalence in stored U.S. survey samples rose from 11.0% in 1988-1991 to 16.1% in 2011-2012. Recorded incidence across 19 autoimmune diseases in the UK rose for some conditions and fell for others.
• After U.S. folic-acid fortification, the homocysteine difference associated with the MTHFR 677TT genotype became substantially smaller. The gene did not change; the environment did.
• Chronology is clinical evidence. Sequence strengthens or weakens a hypothesis, which is why the patient timeline – including the periods of improvement – is worth reconstructing carefully.

A patient discovers at 47 that she carries a genetic variant associated with the condition she has just been diagnosed with. The result feels like an answer.

Except she had the same genes at 27, when she felt great.

And at 35.

And at 42.

So what happened at 46?

That question—why now?—is one of the most useful questions we can ask in functional medicine.

Genetics can tell us something important about vulnerability. They may help explain why one person responds differently than another to the same infection, exposure, nutrient deficiency or physiological stress. In some conditions, genetics can be central to the diagnosis itself.

But genes alone cannot reconstruct the years between feeling well and becoming sick.

They cannot tell us why symptoms appeared after an infection, during menopause, following a major life change or seemingly out of nowhere. They cannot show us what was protecting the patient before illness appeared, what changed around the time symptoms began or which of those changes still matters today.

For that, we need the patient’s story.

And increasingly, science is giving us better tools for understanding why that story matters.

Researchers are studying not only the genome but the exposome: the collection of physical, chemical, infectious, nutritional and psychosocial influences encountered throughout life. A 2026 Nature Medicine study mapped relationships between multiple exposures and hundreds of health measurements and found what clinicians who care for complex patients might expect: these influences do not occur one at a time.

They overlap.

That does not mean we can draw a straight line from one exposure to one disease. Human biology is rarely that tidy.

But it does challenge an equally simplistic idea: that finding a genetic vulnerability ends the investigation.

The genes were already there. What changed?

ARE WE ACTUALLY GETTING SICKER?

There is another question underneath this story.

Are chronic and autoimmune illnesses really becoming more common, or are we simply getting better at finding them?

The answer appears to be some of both.

One useful clue comes from blood collected long before today’s explosion in genetic testing, specialty labs and patient awareness.

In a 2022 study, researchers measured antinuclear antibodies, or ANA, in stored samples from U.S. national health surveys. Among people age 12 and older, estimated ANA prevalence increased from 11.0% in 1988–1991 to 16.1% in 2011–2012.

A positive ANA is not the same thing as an autoimmune diagnosis. But these were stored blood samples tested using standardized methods, not simply a count of how often clinicians ordered an ANA.

Something measurable had changed.

Diagnosis data are more complicated. A large Lancet study examining 19 autoimmune diseases among 22 million people in the United Kingdom between 2000 and 2019 found substantial increases in the recorded incidence of some conditions, including celiac disease and Sjögren’s syndrome. Other conditions, including Hashimoto’s thyroiditis and pernicious anemia, decreased.

There is no single curve describing every autoimmune disease.

Better recognition matters. Better testing matters. Changing diagnostic criteria matter.

But those explanations do not mean biology has remained static.

The more interesting question is what has changed in the environment around genetically susceptible humans—and why some people cross from vulnerability into disease while others do not.

A GENE IS NOT A DESTINY

MTHFR provides a surprisingly useful example because the environmental story moves in the opposite direction from the one we usually hear.

Common MTHFR variants influence folate metabolism. But carrying one does not mean a person cannot process folic acid. The CDC specifically notes that people with common MTHFR variants can process folic acid and that folate intake is an important determinant of blood folate levels.

Then the environment changed.

Researchers in the CARDIA study examined folate and homocysteine levels before and after the United States began folic-acid fortification. Before fortification, participants with two copies of the MTHFR 677T variant had higher homocysteine than people with the comparison genotype.

After fortification, the difference became substantially smaller.

The gene didn’t change. The environment did—and the measurable effect associated with that gene changed with it.

That is gene-environment interaction made tangible.

It also gives us a better way to think about vulnerability.

The environment is not simply a collection of things that can harm us. It can also contain things that protect us, compensate for vulnerabilities or help keep those vulnerabilities from becoming clinically important.

That changes the questions we ask.

Instead of stopping at What was added before this patient became sick?, ask:

What disappeared?

Was sleep lost? Did nutrition change? Did movement decrease? Did hormonal physiology shift? Was there an infection? A new medication? A major change in work or home life? Did something that had supported resilience for years suddenly disappear?

Sometimes the clue is the thing that entered the patient’s life.

Sometimes it is the thing that left.

WHY THE ORDER OF EVENTS MATTERS

One of the clearest examples of why timing matters comes from research on Epstein-Barr virus and multiple sclerosis.

In a 2022 Science study, researchers examined more than 10 million U.S. military personnel. The risk of developing multiple sclerosis increased 32-fold following EBV infection. A blood marker of nerve injury rose only after evidence of EBV infection appeared.

That sequence matters.

Finding evidence of a previous infection years after someone becomes ill can raise an interesting question. Showing that infection occurred first, followed by biological change and later disease, gives researchers a much stronger reason to investigate the relationship.

That does not mean every infection causes chronic illness. It certainly does not mean every old positive antibody result explains a patient’s current symptoms.

It means chronology strengthens or weakens a clinical hypothesis.

This is where the patient timeline becomes much more than paperwork.

If symptoms began after an infection, document it.

If the patient’s health changed during pregnancy, perimenopause or menopause, put it on the timeline.

If the decline followed surgery, a new medication, a period of severe sleep disruption, a change in diet, a significant exposure or another major physiological event, record that too.

Then ask whether the proposed explanation actually fits what happened.

Functional medicine should be exceptionally good at this because we are willing to ask what came before the diagnosis.

THE THRESHOLD MATTERS

Complex chronic illness does not always begin with one dramatic event.

Sometimes there is a clear before and after.

Sometimes there isn’t.

A patient may carry genetic susceptibility for decades while remaining remarkably resilient. Then several things change close together: an infection, disrupted sleep, a hormonal transition, nutritional changes, a medication, increased physiological stress or another environmental influence.

No single factor has to explain everything.

Think instead about a threshold.

For years, the patient’s physiology may have been able to compensate. Then the balance changes. Symptoms appear. Recovery becomes harder. Eventually the patient receives a diagnosis.

The diagnosis may explain what is happening.

The timeline can help us investigate why it became clinically important when it did.

That distinction is especially important when genetic testing enters the case.

A variant discovered last Tuesday did not suddenly appear last Tuesday.

If it has been present since conception, the practitioner still has to account for the decades during which the patient functioned well.

That gap is not an inconvenience in the story.

It may be where some of the most useful information lives.

THE HISTORY A PANEL CANNOT GIVE YOU

Modern testing can produce an extraordinary amount of information about a patient.

It can also tempt us to confuse more data with more understanding.

A genetic panel cannot tell you what life looked like before the patient became sick. A laboratory result cannot tell you whether the patient stopped sleeping six months before symptoms appeared or six months afterward. It cannot tell you whether a restrictive diet preceded the decline or was adopted because of it.

You have to ask.

Start with the last period when the patient remembers feeling and functioning reasonably well.

Then move forward.

What happened next?

Was the decline sudden, gradual or episodic? What was the first symptom? What came later? When did the patient stop being able to do something she had previously done easily? Which diagnoses came years after the symptoms they eventually explained?

Separate the date something was discovered from the date something changed.

Those are often very different dates.

And do not build the timeline only around deterioration.

Ask when the patient felt better.

What was different during those periods? What had changed in sleep, food, movement, medications, stress, environment, hormones or daily life? What improvement lasted? What disappeared as soon as an intervention stopped?

Improvement does not prove a mechanism.

But it is data.

And in a complicated case, it may be some of the most useful data we have.

PUT THE PATIENT BACK IN THE CASE

The explosion of genetic testing has given practitioners remarkable new information. It has also made one old-fashioned clinical skill even more important:

Listening closely enough to reconstruct what actually happened.

The goal is not to blame every illness on an exposure, infection or lifestyle factor. Nor is it to force every patient’s history into a neat root-cause story.

The goal is to understand enough of the case to make a better next decision.

Sometimes genetics will provide the answer.

Sometimes an infection will become an important part of the explanation.

Sometimes the most important finding will be autoimmune, hormonal, metabolic, neurological or environmental.

And sometimes several of those things will turn out to matter at once.

That complexity is not a failure of functional medicine.

It is exactly why functional medicine exists.

We look across systems because patients do not become ill inside one laboratory value. We ask about history because a diagnosis tells us what a condition is called, not necessarily how this particular patient arrived there. And we look for modifiable contributors because genetic vulnerability and inevitability are not the same thing.

A genetic result can be extraordinarily valuable.

But it should open the investigation, not end it.

When a patient who functioned well for decades becomes sick, ask what she has.

Then ask the question her genome cannot answer:

The genes were already there. Why now?

DR. Z’S TAKE

One of the first questions I ask when someone tells me they have a genetic variant is very simple:

You’ve had those genes your entire life. So why are you sick now?

That doesn’t mean the genetics don’t matter. Of course they matter. Genetics can create vulnerability. They can affect how we metabolize nutrients, respond to infections, regulate inflammation or tolerate different environmental stressors.

But genetic predisposition is not the whole story. Something usually has to interact with that vulnerability. An infection. Hormonal changes. Sleep disruption. Chronic stress. Nutrition. Environmental exposures. Lifestyle. Sometimes several of those things happen at the same time.

That is where epigenetics becomes so important. We cannot change the genes someone was born with, but gene expression does not happen in a vacuum. The environment around those genes matters.

And I do believe that most patients have a root cause—or, more commonly, root causes.

That distinction matters. We have made “root cause” sound as though there must be one thing buried underneath every complicated patient. Find the infection. Fix the gut. Correct the deficiency. Remove the exposure. Done.

Real patients are rarely that neat.

A woman may have a genetic predisposition, then develop an infection during perimenopause while sleeping five hours a night and going through enormous stress. Which one caused the problem?

Maybe that is the wrong question.

Maybe several things changed the physiology at once until her body could no longer compensate the way it had for the previous 40 years.

That is why the timeline matters so much.

I want to know when the patient last felt really well. What happened before the first symptom? What happened next? What changed around the time everything seemed to fall apart? What made her better? What made her worse?

But I don’t only want to know why.

I want to know: What can we do about it?

Which pieces can we change? Which ones can we support? Which ones require treatment? Which things are outside our control? And where can we make the biggest impact first?

That is ultimately the purpose of all this investigation. We aren’t building a beautiful timeline so we can admire it. We are trying to build a better care plan.

And this is exactly why we built the Care Plan Builder and why I use it with every patient interaction.

Complex patients have too much information for any practitioner to reliably hold in their head at once. There may be years of history, multiple functional and conventional labs, genetics, medications, symptoms, diagnoses, previous interventions and dozens of relationships among them.

I don’t want my patient’s outcome depending on whether I happen to remember one important connection that day.

The Care Plan Builder lets me be what I call a bionic practitioner. We programmed it around the way we think clinically, so it can help me organize the timeline, pull together multiple labs—including genetics—and look across all of that information at once. It doesn’t replace my clinical judgment. It gives my clinical judgment a much bigger field of view.

That matters because the answer may not live in any single lab.

The genetic report may show the vulnerability. The history may reveal the trigger. The labs may show what is happening now. The timeline may show how those pieces fit together.

Then we still have to decide what matters most and what we can actually change.

That, to me, is functional medicine.

Why this person? Why this condition? Why now? And most importantly: what can we do about it?

Those are the questions that turn information into a care plan.

Read the full issue

This cover story appeared in The Functional Medicine Report™, Issue #011, October 9, 2026 – the weekly intelligence brief for functional medicine practitioners. Subscribe at thefmreport.com.

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