You’re losing weight. The scale is moving. Your doctor is happy.
But here’s what nobody mentioned at your last appointment: GLP-1 drugs like Ozempic, Wegovy, and Mounjaro are linked to significant nutritional deficiencies — and most prescribers aren’t testing for them.
A review published in Clinical Obesity looked at data from over 461,000 GLP-1 users and found that 12.7% developed a new nutritional deficiency within just six months. Not a minor dip. A real, measurable deficiency.
That’s roughly 1 in 8 users. And it gets worse the longer you stay on the medication.
If you’re on a GLP-1 drug — semaglutide, liraglutide, tirzepatide — and nobody has talked to you about this, keep reading. This isn’t about scaring you off your medication. It’s about making sure you don’t trade fat loss for something worse.
What the Research Actually Found
The review, led by Jorge Urbina, MD, at the Autonomous University of Guadalajara, pulled together data from multiple studies. The picture was clear: GLP-1 drugs crush your appetite so well that users consistently fail to get enough basic nutrients.
Here’s what stood out:
- Vitamin D deficiency was the most common — hitting 7.5% of users at 6 months and climbing to 13.6% at 12 months
- GLP-1 users had a 49% higher risk of vitamin D deficiency compared to people on other diabetes/weight-loss drugs
- Iron absorption dropped significantly — semaglutide reduced how well your gut absorbs iron after just 10 weeks
- GLP-1 users showed 54% higher risk of low ferritin (your body’s iron storage) compared to other drug users
- B vitamin deficiencies, including thiamine (B1), showed up in case reports — some bad enough to cause serious brain and nerve damage
- Calcium, selenium, and zinc deficiencies all got worse over time
- Muscle loss was a consistent finding, driven by not eating enough protein
- Most GLP-1 users didn’t meet the daily recommended amounts for vitamin D, iron, calcium, and protein
This isn’t guesswork. These are real findings from hundreds of thousands of patients.
Why GLP-1 Drugs Cause Deficiencies
It’s not complicated. GLP-1 drugs work by:
- Crushing your appetite — you eat way less food
- Slowing your digestion — food sits in your stomach longer, which can reduce how well nutrients get absorbed
- Causing stomach issues — nausea, vomiting, and diarrhea speed up nutrient loss
When you’re eating 40-60% less food — which is common on these drugs — you’re getting 40-60% fewer vitamins, minerals, and protein. Your body doesn’t care that you’re losing weight. It needs those nutrients to function.
And here’s the part that should worry you: semaglutide appears to directly block iron absorption in your gut, regardless of how much you eat. That’s not just a calorie problem. That’s the drug itself interfering with nutrient uptake.
What’s at Risk and Why It Matters
Vitamin D
How common: 7.5% deficient at 6 months, 13.6% at 12 months. 49% higher risk than other drug users.
Symptoms you might miss: Fatigue, bone pain, muscle weakness, depression, getting sick often.
Why it matters: Bone loss, weak immune system, higher fracture risk.
Iron / Ferritin
How common: 54% higher risk of low ferritin. Absorption drops at 10 weeks.
Symptoms you might miss: Fatigue, brain fog, shortness of breath, cold hands/feet, brittle nails.
Why it matters: Anemia, poor oxygen delivery, trouble thinking clearly.
Thiamine (Vitamin B1)
How common: Documented in case reports of severe deficiency.
Symptoms you might miss: Confusion, balance problems, vision changes, leg weakness.
Why it matters: Can cause serious brain damage (Wernicke encephalopathy) and heart failure (beriberi) — both potentially life-threatening.
Calcium
How common: Below recommended intake in most users.
Symptoms you might miss: Often silent until advanced; muscle cramps, numbness, tingling.
Why it matters: Brittle bones, fractures, heart rhythm problems.
Protein
How common: Most users don’t eat enough.
Symptoms you might miss: Muscle wasting, slow recovery, hair loss, weak immune system.
Why it matters: Muscle loss, slower metabolism, weight regain after stopping the drug.
Zinc and Selenium
How common: Gets worse over time on the medication.
Symptoms you might miss: Hair loss, poor wound healing, loss of taste/smell, thyroid problems, fatigue.
Why it matters: Weak immune system, hormone problems, skin issues.
Look at that list. Now ask yourself: has your prescribing doctor tested for any of these?
The Muscle Loss Problem Nobody Wants to Talk About
Here’s the dirty secret of GLP-1 weight loss: up to 40% of the weight you lose can be lean muscle, not fat.
When you don’t eat enough protein — and the research confirms most GLP-1 users don’t — your body breaks down muscle for energy. You lose weight on the scale, but your body gets worse, not better. Less muscle means a slower metabolism, which means the moment you stop the drug, the weight comes roaring back. And it comes back as fat, not muscle.
This is why “skinny fat” is becoming the unofficial side effect of the GLP-1 era. People look thinner but are weaker and less healthy on the inside.
Minimum protein targets on GLP-1 therapy should be 1.0–1.2g per kilogram of body weight per day — and many doctors now recommend even more. If you weigh 200 lbs, that’s 90–110g of protein daily. When your appetite is crushed and you’re eating one meal a day, that’s nearly impossible without a plan.
What You Should Be Testing
If you’re on a GLP-1 drug and your doctor hasn’t ordered full bloodwork beyond a basic panel, you’re flying blind. Here’s what should be on the lab order:
Before Starting — and Every 3–6 Months After:
- 25-hydroxyvitamin D — the gold standard for vitamin D levels
- Ferritin + serum iron + TIBC — ferritin alone isn’t enough; get the full iron panel
- Complete metabolic panel (CMP) — includes calcium and electrolytes
- Magnesium (RBC) — regular magnesium tests are nearly useless; RBC magnesium shows what’s actually in your tissues
- Zinc and selenium — often skipped, rarely tested, frequently low
- Thiamine (vitamin B1) — especially if you have stomach issues or any nerve symptoms
- B12 and folate — standard B vitamin markers
- Albumin — tracks your protein status
- DEXA scan or body composition test — track muscle vs. fat, not just total weight
Warning Signs That Need Immediate Testing:
- Unusual tiredness that doesn’t get better with rest
- Hair loss or thinning
- Brain fog or trouble concentrating
- Muscle cramps, weakness, or balance problems
- Numbness or tingling in hands or feet
- Mood changes, especially new depression or anxiety
Don’t wait for symptoms. Deficiencies are easier to prevent than to fix, and some — like thiamine deficiency turning into brain damage — can cause permanent harm before you realize something is wrong.
The Bigger Problem: Prescribe and Forget
Here’s what frustrates us at Viking Alternative Medicine: the “prescribe and forget” approach that’s taken over GLP-1 prescribing.
Telehealth companies are handing out semaglutide prescriptions like candy. Monthly check-ins are just “are you losing weight?” and “any side effects?” That’s it. No bloodwork. No body composition tracking. No nutrition guidance. No supplement plan.
That’s not medicine. That’s a vending machine with a prescription pad.
Real weight management means monitoring the whole patient. That means regular labs, targeted supplements, protein optimization, and adjusting the plan based on what your blood actually shows. It means caring about your muscle mass, your bone density, your hormones, and your nutrient levels — not just the number on the scale.
At Viking Alternative, we take a whole-body approach because we’ve seen what happens when clinics don’t. Whether a patient is on GLP-1 therapy, peptide protocols, or a combination approach, we’re tracking labs, adjusting supplements, and making sure the weight you lose is the weight you want to lose.
The Bottom Line
GLP-1 drugs aren’t the enemy. For many people, they’re genuinely life-changing. But they are not risk-free, and the nutritional problems are real, well-documented, and almost always ignored by prescribers.
If you’re on Ozempic, Wegovy, Mounjaro, or any GLP-1 drug:
- Get full bloodwork — not just A1C and cholesterol
- Take supplements — at minimum: vitamin D, iron (if labs show you need it), a quality B-complex, magnesium, zinc, and calcium
- Make protein a priority — track it, supplement it, make it non-negotiable
- Monitor your body composition — weight loss means nothing if you’re losing muscle
- Work with a provider who actually watches this stuff — not one who writes a prescription and disappears
Your weight loss journey shouldn’t create a new set of health problems. Demand better.
Viking Alternative Medicine provides complete weight management programs with full lab monitoring, targeted supplements, and body composition tracking. Contact us to learn how we approach weight loss differently.
Frequently Asked Questions
Does Ozempic cause vitamin deficiency?
Yes. Research published in Clinical Obesity found that 12.7% of GLP-1 users (including Ozempic/semaglutide) developed a new nutritional deficiency within 6 months. Vitamin D was the most common, followed by iron, B vitamins, calcium, zinc, and selenium.
What supplements should I take while on Ozempic or semaglutide?
At minimum, most GLP-1 users should consider vitamin D3, a B-complex (including thiamine), magnesium, zinc, calcium, and an iron supplement if labs show low ferritin. Protein supplements are also important to prevent muscle loss. Always work with a provider who checks your bloodwork.
Can semaglutide cause iron deficiency?
Yes. Research shows semaglutide significantly reduces iron absorption in the gut after just 10 weeks of use. GLP-1 users also had a 54% higher risk of low ferritin levels compared to users of other medications.
Why am I losing muscle on Ozempic?
GLP-1 drugs crush your appetite so much that most users don’t eat enough protein. Without enough protein, your body breaks down muscle tissue. Up to 40% of weight lost on GLP-1 drugs can be lean muscle rather than fat.
How often should I get bloodwork while on a GLP-1 drug?
Full bloodwork should be done before starting and every 3–6 months during treatment. This should include vitamin D, a full iron panel, B vitamins, minerals (zinc, selenium, magnesium), and body composition tracking.