GLP-1s and Nutritional Deficiency: A Vitamin B1 Risk Worth Knowing About

GLP-1 drugs such as semaglutide and tirzepatide have changed the treatment of obesity and type 2 diabetes.

They can produce substantial weight loss and important health benefits.

But there's a less glamorous question worth asking:

What happens nutritionally when a medication makes you eat dramatically less?

A growing body of research suggests that some people taking GLP-1 drugs may develop nutritional deficiencies or inadequate nutrient intake.

Most of the evidence is observational, and it doesn't prove that the medications themselves cause those deficiencies.

But one vitamin deserves particular attention because severe deficiency can cause a potentially devastating neurological condition:

thiamine, or vitamin B1.

The important thing is to keep the evidence in perspective.

Here's what we know, what we suspect, and what remains unknown.

What We Know

GLP-1 treatment can substantially reduce food intake

One of the ways GLP-1 drugs work is by reducing appetite and food intake.

That's part of their therapeutic effect.

But when people eat considerably less, they may also consume less protein, iron, calcium, vitamins and other nutrients.

A 2025 joint advisory from the American College of Lifestyle Medicine, American Society for Nutrition, Obesity Medicine Association and The Obesity Society specifically identified nutritional deficiencies from calorie reduction, muscle loss and bone loss as issues that need attention during GLP-1 treatment.

The advisory recommends nutritional assessment, attention to protein intake, resistance exercise and management of gastrointestinal side effects.

That doesn't mean everyone taking a GLP-1 drug becomes nutritionally deficient.

It means the possibility deserves attention—particularly when appetite falls dramatically or gastrointestinal symptoms make eating difficult.

The biggest nutritional dataset is essentially one giant claims database

A 2026 narrative review by Jorge Urbina and colleagues examined six studies involving 480,825 adults taking GLP-1 receptor agonists.

At first glance, that sounds like an enormous body of evidence.

But there's an important detail hiding inside that number.

About 461,000 people—roughly 96% of the entire sample—came from one retrospective insurance-claims cohort.

In that study, nutritional deficiencies were identified through diagnosis codes, not laboratory measurements.

People with a previously coded nutritional deficiency were excluded.

The study found that 7.5% had a newly coded vitamin D deficiency at six months and 13.6% at 12 months.

Those numbers are worth knowing—but they should not be interpreted as blood-test evidence that GLP-1 drugs caused vitamin D deficiency.

Obesity itself is associated with lower vitamin D levels, and people taking GLP-1 drugs may differ from untreated people in many other ways.

Researchers have also raised the possibility of detection bias: people receiving medical treatment may simply have more opportunities for deficiencies to be diagnosed and coded.

So the headline isn't:

“13.6% of GLP-1 users become vitamin D deficient.”

It's:

“One very large claims database found newly coded vitamin D deficiency in 13.6% of GLP-1 users at 12 months.”

That's a much more honest description of the evidence.

Iron may be a more interesting signal

The same 2026 review reported a finding that deserves more attention than the vitamin D headline.

GLP-1 users had 26%–30% lower ferritin levels than people taking SGLT2 inhibitors.

That's a useful comparison because the groups weren't simply being compared with the general population.

They're people being treated with different medications for overlapping metabolic conditions.

The underlying register-based study found that GLP-1 receptor agonist exposure was associated with lower ferritin compared with SGLT2 inhibitor exposure.

That still doesn't prove that GLP-1 drugs directly cause iron deficiency.

Ferritin is also influenced by inflammation and other factors, and the particularly detailed study involved people with type 2 diabetes and hemochromatosis, so it shouldn't automatically be generalized to every GLP-1 user.

But compared with a simple diagnosis code for vitamin D deficiency, a biological measurement such as ferritin—and a drug comparator—is a more informative signal.

Thiamine is the vitamin we don't want to miss

Thiamine, or vitamin B1, is essential for normal energy metabolism and neurological function.

The body doesn't store large amounts of it.

Severe deficiency can cause Wernicke encephalopathy, a medical emergency involving the brain.

Possible symptoms include:

• confusion or altered mental status
• difficulty walking or loss of coordination
• abnormal eye movements or vision problems

The classic three-part combination isn't present in every case, so the condition can be overlooked.

Importantly, severe thiamine deficiency doesn't require alcohol abuse.

It can occur when someone has prolonged vomiting, starvation, very poor food intake or other conditions that substantially reduce thiamine availability.

There are now multiple signals involving GLP-1 drugs and Wernicke encephalopathy

This is where the evidence becomes particularly interesting.

A 2026 systematic review of published case-based evidence identified six reported cases of Wernicke encephalopathy following semaglutide treatment for obesity.

But there's an important correction to our earlier framing:

These were not specifically older-adult cases.

The mean age was about 47, with a range of 37 to 74.

Several patients also had other chronic medical conditions.

The average duration of semaglutide treatment was roughly five months.

So these cases demonstrate that the problem can occur during GLP-1 treatment in adults.

They do not demonstrate that older adults are particularly susceptible.

The risk logic for an older person is reasonable because older adults can be vulnerable to inadequate nutrition and muscle loss, but that's an inference—not something established by these six cases.

A larger pharmacovigilance study found another signal

A separate 2026 study examined the FDA Adverse Event Reporting System and published literature.

It identified 15 cases of Wernicke encephalopathy associated with GLP-1 receptor agonists:

• 13 from FDA adverse-event reports
• one from published literature
• one from the researchers' own medical center

Most involved semaglutide or tirzepatide.

Thirteen of the 15 patients had gastrointestinal problems such as vomiting, loss of appetite, weight loss or malnutrition.

The researchers found that Wernicke encephalopathy was reported disproportionately more often with GLP-1 drugs than with other medications.

But this was a pharmacovigilance signal, not an incidence study.

Voluntary adverse-event reports cannot tell us how many people taking GLP-1 drugs actually develop the condition, and they cannot establish causation.

That's why this finding is best regarded as a safety signal that deserves investigation, not proof that GLP-1 drugs cause Wernicke encephalopathy.

What We Suspect

The problem may be inadequate nutrition rather than a direct vitamin-blocking effect

The emerging picture doesn't look like:

GLP-1 drug → blocks vitamin B1 → deficiency.

A more plausible pathway is:

GLP-1 → appetite falls → food intake falls → weight drops rapidly → nutritional reserves become depleted.

If persistent nausea or vomiting is added to the equation, the risk can increase substantially.

That's consistent with the pharmacovigilance data: most of the reported Wernicke cases involved gastrointestinal symptoms or nutritional compromise.

This distinction matters.

It means the risk may be concentrated in people who are eating very little or becoming nutritionally compromised, rather than being an inevitable consequence of taking a GLP-1 drug.

Rapid weight loss may be part of the problem

Rapid weight loss can involve more than body fat.

It can also involve lean mass, including muscle.

The 2025 multidisciplinary nutritional advisory specifically recommends attention to adequate protein intake and resistance training during GLP-1 treatment because preserving muscle and bone is an important part of healthy weight loss.

That may be especially important as people get older.

Losing 30 pounds isn't necessarily a health victory if a disproportionate amount of that weight comes from muscle.

The goal should be fat loss while preserving physical function.

The iron findings deserve more investigation

The lower ferritin findings are particularly interesting because they were observed against an active medication comparator.

There is also preliminary evidence that semaglutide may affect intestinal iron absorption.

In a small pilot study of 51 adults with type 2 diabetes, researchers measured iron absorption before and after 10 weeks of semaglutide treatment. The study found changes in iron absorption, although it was small, short-term and not designed to establish that semaglutide causes iron deficiency.

So we now have several pieces of a possible puzzle:

lower ferritin in observational data + altered iron absorption in a small mechanistic study.

That's interesting.

It's not yet proof of clinically important iron deficiency caused by GLP-1 treatment.

What Remains Unknown

We don't know how often GLP-1 treatment actually causes nutritional deficiency

This is the biggest unanswered question.

The available evidence includes:

• insurance claims
• observational studies
• dietary surveys
• small laboratory studies
• case reports
• adverse-event databases

Those are useful for detecting signals.

They are not the same as large, long-term randomized trials measuring nutritional status before and after treatment.

We still need prospective studies that measure actual nutrient levels before treatment and follow them over time.

We don't know how much of the vitamin D signal is caused by the medication

The 7.5% and 13.6% figures should not be interpreted as a medication-caused deficiency rate.

They came primarily from one enormous claims cohort, using diagnosis codes rather than measured blood concentrations.

That study tells us that vitamin D deficiency was being diagnosed more often after GLP-1 treatment.

It doesn't tell us precisely why.

We don't know whether thiamine deficiency is common

This is perhaps the most important distinction in the entire story.

Wernicke encephalopathy appears to be rare.

But it can be devastating if missed.

The newer pharmacovigilance study strengthens the safety signal: 15 cases were identified, and most involved vomiting, poor appetite, weight loss or malnutrition.

A separate analysis of the WHO global safety database and published cases has also reported additional cases, bringing the total number of reported cases in that analysis to 19.

But these are still reports—not a denominator.

We don't know how many people took GLP-1 drugs during the same period without developing Wernicke encephalopathy.

So we cannot calculate the actual risk from these reports.

We don't know whether routine thiamine testing is necessary

There isn't evidence that every person taking a GLP-1 drug needs routine thiamine testing.

The more reasonable approach is to pay attention to risk circumstances.

Someone eating normally and losing weight gradually is very different from someone who has barely been able to eat for weeks and is repeatedly vomiting.

The latter situation deserves medical attention.

The warning signs worth knowing

This is probably the most useful part of the story.

If someone taking a GLP-1 drug develops persistent vomiting, extremely poor food intake or unusually rapid weight loss, they should tell their healthcare professional.

And if neurological symptoms develop—particularly:

• new confusion
• difficulty walking or severe imbalance
• abnormal eye movements
• double vision
• unusual weakness or neurological changes

don't assume it's simply a side effect, aging or diabetic neuropathy.

Thiamine deficiency is one possibility among several, but Wernicke encephalopathy is time-sensitive and treatment shouldn't be unnecessarily delayed when clinicians suspect it.

The published cases also highlight another important point:

Inadequate or delayed thiamine treatment can matter.

Reports of GLP-1-associated Wernicke encephalopathy include cases in which nutritional compromise was not adequately addressed early, reinforcing the importance of recognizing the condition promptly. The pharmacovigilance literature emphasizes early treatment because neurological damage can become permanent.

Don't respond by taking a handful of vitamins

It's tempting to hear this story and conclude:

“I'll just take a multivitamin.”

That isn't necessarily the answer.

The better first question is:

Are you actually eating enough nutritious food?

Someone taking a GLP-1 drug should ideally still be getting adequate protein and a reasonably varied diet.

If appetite suppression, nausea or vomiting makes that difficult, the solution may involve adjusting treatment, managing gastrointestinal symptoms, working with a dietitian or evaluating specific nutritional deficiencies.

And someone with neurological symptoms shouldn't attempt to diagnose or treat suspected Wernicke encephalopathy with an over-the-counter supplement.

That's a medical problem requiring prompt evaluation.

The bigger lesson

GLP-1 medications can be enormously useful.

But successful weight loss isn't simply about making the number on the scale smaller.

It's also about preserving muscle, maintaining adequate nutrition and avoiding complications from eating too little.

The emerging evidence doesn't justify telling people to fear GLP-1 drugs.

It does justify paying attention to what happens after appetite disappears.

If you're eating substantially less, losing weight rapidly or experiencing persistent gastrointestinal symptoms, nutrition becomes part of the treatment—not an afterthought.

The Watchdog Takeaway

GLP-1 drugs don't automatically cause vitamin deficiencies. But substantial appetite suppression, very low food intake, rapid weight loss and persistent vomiting can create conditions in which nutritional deficiencies become a problem.

Vitamin D deficiency is the most prominent signal in current observational data, but the headline numbers—7.5% at six months and 13.6% at 12 months—come largely from one enormous insurance-claims database and are based on diagnosis codes, not blood tests.

A more informative signal is that GLP-1 users had 26%–30% lower ferritin levels than SGLT2 inhibitor users in observational data, although this still doesn't prove that GLP-1 drugs caused iron deficiency.

And then there's thiamine (vitamin B1).

Wernicke encephalopathy has now been reported in a small number of people taking GLP-1 drugs, particularly semaglutide and tirzepatide, usually in the setting of vomiting, poor intake, rapid weight loss or malnutrition. The cases are concerning but do not establish how common the problem is or prove that the medication itself is the cause.

The practical lesson is simple:

Don't just watch the scale. Watch what you're able to eat, how quickly you're losing weight, whether you're maintaining muscle—and whether persistent gastrointestinal symptoms are putting your nutrition at risk.