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On a GLP-1 and eating a lot less? Mind your vitamin B1.
Your body can't make vitamin B1, and it keeps only a few weeks of it. Every bit comes in with food. When a GLP-1 medicine turns your appetite way down, less comes in, and nausea or vomiting can push it lower still. Here is what the research shows so far, the test to ask for by name, and the formula I built around a B1 that gets in, with magnesium beside it.
Rick Pescatore, DO. Board-certified emergency physician, Editor-in-Chief of Emergency Medicine News, former Chief Physician of the Delaware Division of Public Health, 100+ publications. More than 400,000 people follow his public-health videos.
Step 1. Tap what sounds like you.
Step 2. Tap what is true for you.
Less food in means less B1 in.
GLP-1 medicines work in part by making you less hungry and keeping food in the stomach longer. That is the point of them. It also means you take in less of everything, vitamins included.
Most vitamins can ride out a few lean months. B1 can't. Your body doesn't make it and stores only a few weeks' worth, and every carbohydrate you burn spends some. Add a stretch of nausea or vomiting and what you did eat may never get absorbed.
Low B1 doesn't announce itself. It looks like fatigue, fog, a poor appetite, nausea, or tingling feet, all of which are easy to blame on the medicine or the weight loss.
Researchers are starting to look.
This is early. Here is what has been published, and what each one does and doesn't say.
A 2026 review of six studies covering 480,825 adults on GLP-1 medicines found that B1 and B12 shortfalls grew the longer people stayed on treatment. Vitamin D and iron were the most common gaps.
Urbina J, Salinas-Ruiz LE, Valenciano C, Clapp B. Clinical Obesity. 2026;16:e70070.A 2025 expert consensus on supportive care for GLP-1 therapy lists thiamine (B1) among the nutrients people living with obesity already run short on, alongside vitamin D, B12, iron and magnesium.
Sievenpiper, Ard, Blüher, et al. Obesity Pillars. 2025;17.Four obesity and nutrition societies made preventing micronutrient deficiencies one of their eight priorities for people on GLP-1 therapy, with supplements as needed. Their advisory does not single out B1.
Joint advisory of the American College of Lifestyle Medicine, American Society for Nutrition, Obesity Medicine Association and The Obesity Society. Obesity. 2025.What it can't tell you. These are records and expert opinion, not trials. Nobody has shown that the medicine itself drains B1; the likely reason is simply eating far less, plus nausea and vomiting. And they can't tell you where you stand. A blood test can.
Ask for it by name.
Vitamin B1 is not on routine bloodwork, so it only gets checked if someone asks. Ask for a thiamine level. Ask before you start any B1 supplement, because the supplement will raise the number.
"Can we check my thiamine level? I'm on a GLP-1, I'm eating far less than I used to, and I've been more tired than usual."
Want the full list of when to ask? The thiamine checklist is free.
Do not wait for a visit if the vomiting will not stop, you can't keep fluids down for a day, or you notice confusion, double vision, or an unsteady walk. That is an emergency department visit today. Tell them you're on a GLP-1 and haven't been eating.
B1 is a small tank, and the gut is the only door.
The tank holds a few weeks. Everything that refills it comes in through the gut. Tap what empties it.
Four of those empty the tank. One refills it.
A B1 that gets in, always with magnesium. That's Clear.
Ordinary B1 relies on a gut transporter that gets less efficient as the dose rises, and metformin, often taken alongside a GLP-1, blocks it in lab studies. So Clear is built around allithiamine, a lipid-soluble form of B1, with magnesium beside it, because the enzymes that run on B1 need magnesium to work. Eight actives, two capsules a day with food, even a small meal.
Allithiamine, the lipid-soluble form of B1 first found in garlic, with magnesium glycinate, the gentle form that is easy on the stomach. Supports normal nerve function and energy metabolism.*
Ginger for queasy days, artichoke leaf for after-meal heaviness, PEA, a calming lipid your body already makes, plus shatavari, saffron, and a trace of lithium about the amount in mineral water.*
See every dose
| The B1 and its partner | |
| AllithiamineA lipid-soluble form of vitamin B1. | 75 mg |
| MagnesiumAs magnesium glycinate. B1's working partner. | 100 mg |
| The rest of the day | |
| PEA (palmitoylethanolamide) | 300 mg |
| Artichoke leaf extract, 5% cynarin | 150 mg |
| Shatavari root extract, 4:1 | 150 mg |
| Ginger extract, 20% gingerols | 100 mg |
| Saffron extract, 4:1 | 30 mg |
| LithiumAs lithium orotate. A trace. | 5 mg |
One bottle is a month. The protocol is three, and the 12-week guarantee covers all three.
My guarantee: 12 weeks, your first order, up to three bottles. Not helping? One email and it's refunded. Keep the bottles.
New customers: $119 today, then $129 every 90 days.
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For adults eating less, on a GLP-1 or otherwise, who want a daily, well-absorbed B1 with magnesium alongside it. Not for pregnancy, nursing, or under 18, or anyone taking prescription lithium. Ask your prescriber first if you take a blood thinner or live with kidney disease, and tell them you take it.
Clear is a supplement. It does not diagnose, prevent, or treat B1 deficiency or any disease, and it does not replace the protein, fluids, and checkups your GLP-1 care calls for. If your level comes back low, your clinician treats that, often with plain thiamine. Vomiting that will not stop, confusion, double vision, or an unsteady walk is an emergency department visit today, not an order.
Should I stop my GLP-1?
No. These medicines work, and nothing here is a reason to stop one. Eating less is the point of them. The point of this page is to keep B1 from quietly running low while you do.
Benfotiamine or allithiamine?
Both are well-absorbed forms of B1, and either is a reasonable choice. Allithiamine, the one in Clear, is the lipid-soluble one. Benfotiamine is often called fat-soluble, but chemically it isn't; it still reaches the blood at several times the level of ordinary thiamine, which is why my Calm formula uses it.
Why magnesium?
B1 does its work as part of enzymes that need magnesium to run. If magnesium is low, B1 can't do its job well, and low magnesium is common when people eat less. So I don't recommend one without the other.
Will a supplement change my test?
Yes. Any B1 supplement raises the number. If you want to know where you stand, get the level drawn first, then start.
When do I take it on a GLP-1?
Two capsules with food, once a day. A small meal is fine. If capsules sit badly on a full-feeling stomach, take them with the meal you tolerate best.
How does the guarantee work?
Twelve weeks from delivery. If Clear is not helping, email support@belly-md.com and your first order is refunded, up to three bottles. You keep the bottles. One refund per household.
Eat less. Don't run low.
Start the 90-day protocolThis is an advertisement for MGB+ Clear, a dietary supplement made by BellyMD. Dr. Pescatore is the founder of BellyMD. The 2026 review summarizes observational studies of people on GLP-1 medicines; the 2025 consensus and advisory are expert recommendations; the 2020 study is a laboratory study of drug effects on a transporter. None tested MGB+ Clear or allithiamine. *These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Individual experiences vary. Consult your clinician before starting any supplement, especially if you are pregnant, nursing, taking medication, or living with a medical condition.