GLP-1 Nutrient Deficiencies: Thiamine, Vitamin D, Iron and What the Research Shows

This article is for informational purposes only and is not medical advice. Talk with your healthcare provider before changing your diet or supplements, especially while taking a prescription medication.

Short answer: nutrient gaps are common in people taking GLP-1 medications. In the largest study so far, 22.4% of more than 461,000 adults were diagnosed with a nutritional deficiency within 12 months of starting treatment. Vitamin D was the most common. Iron, B vitamins including thiamine (B1), calcium and protein come up again and again in the research. The cause is simple: when you eat far less food, you take in far fewer nutrients.

Chris Manderino, founder of LyfeFuel. Formerly at Bariatric Advantage, a nutrition company for weight-loss surgery patients.

I've seen this problem before

Before GLP-1 medications existed, I worked at Bariatric Advantage, a nutrition company built for bariatric surgery patients. I traveled the country educating physicians, their staff and their patients. The company existed for one reason: after surgery, patients eat so much less that food alone usually can't cover their nutrient needs.

What I saw again and again was that the patients who treated nutrition as part of the plan did better than the ones who treated surgery as the whole plan. Now the same story is playing out with GLP-1 medications, at a much bigger scale. Different mechanism, same basic problem.

What the research shows

• More than 1 in 5 developed a diagnosed deficiency within a year. A claims analysis of 461,382 adults newly prescribed GLP-1 medications found nutritional deficiencies diagnosed in 12.7% within 6 months and 22.4% within 12 months. Vitamin D deficiency was the most common, at 13.6% by 12 months (Butsch et al., Obesity Pillars, 2025).
• Researchers now call it a common consequence. A 2026 review of six studies covering 480,825 adults concluded that nutritional deficiencies should be considered "a common consequence" of GLP-1 therapy, and that clinicians should monitor nutritional status. It also reported 26% to 30% lower ferritin (iron stores) in GLP-1 users compared with users of another class of diabetes medication (Urbina et al., Clinical Obesity, 2026, summarized by Harvard Health Publishing).
• Food records show the same gaps. In a study of 69 GLP-1 users who kept three-day food records, average intake fell below recommendations for fiber, calcium, iron, magnesium, potassium, choline, and vitamins A, C, D and E (Johnson et al., Frontiers in Nutrition, 2025). Only 1.4% met vitamin D recommendations.

Keep in mind that diagnosed deficiencies only count the people who were tested. Lower intake that hasn't been measured doesn't show up in those numbers.

Surgery and GLP-1s create gaps in different ways

This is the distinction I wish more people understood. Whether you've changed the anatomy matters.

Gastric bypass reroutes the digestive tract. It bypasses part of the small intestine that's a primary site for absorbing calcium, iron, thiamine and vitamin B12. That's why those deficiencies are so well known after some bariatric procedures, and why bariatric patients are told to supplement for life.

GLP-1 medications don't change the anatomy, but they shrink the amount of food. The gaps I see most are the ones the general population already struggles with: magnesium, zinc, choline, iodine, vitamin D, iron stores (ferritin) and some B vitamins. Eat a lot less, and those existing shortfalls get bigger.

Thiamine (vitamin B1) and GLP-1 medications

Thiamine deserves its own section because it moves fast. Your body doesn't store much of it, so a stretch of very low intake, especially with ongoing nausea or vomiting, can run it down. Thiamine deficiency was one of the classic problems in bariatric nutrition, and it's now showing up in GLP-1 case reports.

One 2025 case report in BMC Neurology described a 37-year-old woman who lost weight rapidly in the three months after starting a GLP-1 medication. She then developed leg numbness and weakness, blurred vision and confusion. Testing showed low thiamine, and her doctors' most likely diagnosis was Wernicke's encephalopathy, a serious neurological condition caused by thiamine deficiency. Case reports like this are rare, but they show why thiamine is worth raising with your doctor if appetite drops sharply or vomiting continues.

In the large claims study above, B vitamin deficiencies as a group were diagnosed in 2.6% of GLP-1 users within a year.

Do GLP-1 medications affect nutrient absorption?

Two things are going on. First, these medications slow how quickly food leaves the stomach, which is part of why people feel full on less. Second, people simply eat less. The 2026 review notes that GLP-1 medications may also alter absorption of certain nutrients, and the lower ferritin it found points to iron as one to watch. The research here is still early, so the safest takeaway is the practical one: when intake drops, each meal has to carry more nutrition.

Protein matters more when you're eating less

A 2025 joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society says sufficient protein should be a priority during GLP-1 therapy, to help preserve muscle mass and bone density, ideally alongside a structured strength training program.

On amounts, the advisory notes that the general adult recommendation is 0.8 grams of protein per kilogram of body weight per day, that higher targets of 1.2 to 1.6 g/kg have been proposed during active weight loss, and that prolonged intake at or above 2 g/kg should be avoided. Your target is a conversation for your doctor or dietitian.

The same advisory suggests small, frequent meals when hunger is low, and lists smoothies and protein drinks among the options people often find easier than heavy foods.

What I wish more people asked their doctor

Ask for help building an accountability plan for real behavior and lifestyle change. Don't treat surgery or a GLP-1 like a magic bullet that will deliver perfect results on its own.

These medications can be an amazing tool to start the process. But without guidance and without reshaping daily habits, a lot of people regain, and that's exactly what I saw with surgery too. Long term, the habits do the work.

One honest caveat: a lot of doctors get very little nutrition training. If yours doesn't have the time or background, a registered dietitian, an independent nutritionist or a health coach can fill that role. The large claims study above also found that patients who saw a dietitian early were more likely to have deficiencies identified, which is a good argument for getting checked rather than guessing.

Where a shake fits, and where it doesn't

A shake isn't a medical treatment and it won't fix a diagnosed deficiency. What it can do is pack protein and a broad range of vitamins and minerals into a small, easy serving on days when a full meal isn't appealing.

Here's what one scoop of LyfeFuel Essentials (Chocolate) carries for the nutrients discussed above:

Nutrient	Per scoop	% Daily Value
Protein (pea + organic rice)	18g	30%
Vitamin D	30mcg	150%
Thiamin (B1)	0.75mg	62%
Vitamin B12	500mcg	20,833%
Zinc	7.5mg	68%
Iodine	75mcg	50%
Magnesium	200mg	48%
Calcium	100mg	8%
Iron	1mg	6%
Choline	8mg	1%

To be straight with you: Essentials is not a meaningful source of iron, calcium or choline, three of the nutrients that come up in the research. If those are your gaps, you'll need food or a targeted supplement your doctor recommends. Each scoop is 110 calories with 3g of added sugar in Chocolate and 1g in Vanilla Chai, along with organic greens, a six-berry blend, flax and five probiotic strains. Every batch is third-party tested by Eurofins.

If you take a prescription medication, check with your prescriber before adding any supplement.

The bottom line

GLP-1 medications are helping a lot of people. But eating far less without a plan for nutrition creates real gaps, and the research now backs up what bariatric clinics learned years ago. Get your levels checked, prioritize protein, pay attention to thiamine, vitamin D and iron, and build the habits that will last after the medication.

Frequently asked questions

What nutrient deficiencies are common with GLP-1 medications?

Vitamin D is the most common diagnosed deficiency in the research, followed by iron and B vitamins. Food-record studies also show low intake of calcium, magnesium, potassium, choline, fiber, and vitamins A, C and E. Protein intake is often below recommended targets.

Can GLP-1 medications cause thiamine (B1) deficiency?

Case reports have linked GLP-1 use to thiamine deficiency, usually alongside rapid weight loss, very low intake or ongoing vomiting. It appears to be rare, but thiamine stores are small, so it's worth discussing with your doctor if appetite drops sharply.

Do GLP-1 medications affect nutrient absorption?

They slow how fast food leaves the stomach, and a 2026 review notes they may alter absorption of certain nutrients, with lower iron stores reported in GLP-1 users. Most of the gap, though, comes from simply eating much less food.

Can people taking GLP-1 medications drink protein shakes?

Generally, yes. A 2025 joint advisory from four obesity and nutrition societies lists smoothies and protein drinks among the options people often find easier when appetite is low. Check with your prescriber before adding any supplement.

How much protein is recommended during GLP-1 weight loss?

The same advisory notes the general recommendation is 0.8 g per kg of body weight per day, that higher targets of 1.2 to 1.6 g/kg have been proposed during active weight loss, and that prolonged intake at or above 2 g/kg should be avoided. Ask your doctor or dietitian for your number.

Is bariatric surgery nutrition the same as GLP-1 nutrition?

No. Gastric bypass changes the anatomy and bypasses a main absorption site for calcium, iron, thiamine and B12. GLP-1 medications don't change the anatomy. The gaps mostly come from eating less, which magnifies shortfalls in nutrients like vitamin D, magnesium, zinc and choline.

Sources

• Butsch WS, et al. "Nutritional deficiencies and muscle loss in adults with type 2 diabetes using GLP-1 receptor agonists: A retrospective observational study." Obesity Pillars, 2025. PMC12205620
• Urbina J, et al. "Micronutrient and Nutritional Deficiencies Associated With GLP-1 Receptor Agonist Therapy: A Narrative Review." Clinical Obesity, 2026. PMID 41549912
• Harvard Health Publishing. "Study: Taking GLP-1 drugs may increase risk of key nutrient deficiencies." March 18, 2026. health.harvard.edu
• Johnson B, et al. "Investigating nutrient intake during use of glucagon-like peptide-1 receptor agonist: a cross-sectional study." Frontiers in Nutrition, 2025. doi:10.3389/fnut.2025.1566498
• "Nutritional Priorities to Support GLP-1 Therapy for Obesity: A Joint Advisory From the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and the Obesity Society." 2025. PMC12125019
• "Neurological complications associated with rapid weight loss and nutritional deficiencies following GLP-1 agonist use: a case report." BMC Neurology, 2025. doi:10.1186/s12883-025-04540-7

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LyfeFuel Essentials Shake is a food-based nutritional supplement. It is not intended to diagnose, treat, cure, or prevent any disease. These statements have not been evaluated by the Food and Drug Administration.