GLP-1 medications like semaglutide and tirzepatide have been genuinely life-changing for a lot of people. The weight loss is real. The metabolic benefits are real. But there's something that doesn't get talked about nearly enough — and we see it regularly in our patients at The Lee Clinic.

These medications can quietly deplete essential nutrients. Not in everyone, and not all at once. But the research is clear that it happens at rates high enough that every person on a GLP-1 should be monitored.

Here's what you need to know.

Why This Happens

It comes down to three things working simultaneously.

First, GLP-1 medications significantly reduce appetite. Most people eating on semaglutide or tirzepatide are consuming dramatically fewer calories — sometimes 20 to 40% less than before. When total food intake drops that much, total nutrient intake drops with it.

Second, these medications slow gastric emptying — the rate at which food moves through your digestive system. This affects how certain nutrients, particularly iron, are absorbed in the gut.

Third, many people starting GLP-1 therapy already have baseline nutrient deficiencies they don't know about. Obesity itself is associated with lower levels of several key vitamins and minerals, even before any medication is started. The GLP-1 can unmask or worsen deficiencies that were already quietly present.

The result is that in large studies of GLP-1 users, nearly 13% had a nutritional deficiency diagnosis within 6 months of starting — and over 22% within 12 months. And because the symptoms of nutrient deficiency — fatigue, hair loss, brain fog, weakness — can look exactly like the symptoms these women were already dealing with in perimenopause and menopause, they often go unrecognized.

The Nutrients Most Commonly Affected

Vitamin D

This is the most consistently documented deficiency in GLP-1 users. It's also the one that matters most for our patients, because vitamin D is critical for bone density — and women in perimenopause are already in a window of accelerated bone loss. Vitamin D deficiency compounds that risk directly.

Part of why it drops is that vitamin D is fat-soluble and gets sequestered in adipose tissue. As fat mass decreases rapidly with GLP-1 therapy, vitamin D can be released but also poorly replenished if dietary intake is inadequate. At The Lee Clinic, we check vitamin D levels in all of our GLP-1 patients and aim for an optimal blood level — not just the bare minimum that a standard lab flags as "normal."

Iron

Iron is where some of the most striking data is. Research has shown that GLP-1 users have 26 to 30% lower ferritin levels — a key marker of iron stores — compared to patients on other diabetes medications. A prospective study found that intestinal iron absorption dropped measurably after just 10 weeks of semaglutide. More than 60% of GLP-1 users in one dietary analysis were consuming below recommended levels for iron.

Iron deficiency doesn't always show up as obvious anemia right away. The early signs — fatigue, hair shedding, poor exercise tolerance, restless legs — are easy to attribute to something else entirely. This is why we check ferritin specifically, not just hemoglobin, as part of our GLP-1 monitoring panel.

B Vitamins — Especially B12 and Thiamine (B1)

B12 levels have been shown to decline over time with GLP-1 use, and thiamine deficiency is an emerging concern the medical community is only beginning to take seriously. There are published case reports of patients on semaglutide developing Wernicke encephalopathy — a serious neurological condition caused by severe thiamine deficiency — as well as cases of beriberi. These are rare, but they are a signal that thiamine needs to be on the radar.

B12 deficiency develops slowly and is often missed until it's significant. Symptoms include fatigue, tingling in the hands and feet, cognitive changes, and mood disturbance — again, symptoms that overlap heavily with perimenopause and hypothyroidism and are easy to miss without targeted testing. We check B12 as a routine part of our GLP-1 follow-up labs.

Calcium, Magnesium, Zinc, and Protein

A joint advisory from the American College of Lifestyle Medicine and the Obesity Medicine Association identified calcium, magnesium, zinc, and protein as additional nutrients of concern in GLP-1 users — along with vitamins A, E, K, and C. When someone is eating significantly less food overall, these shortfalls add up in ways that aren't always obvious.

Protein deserves special mention because it sits at the intersection of nutrition and body composition. GLP-1-driven weight loss can include a meaningful loss of lean muscle mass if protein intake isn't actively protected. For women in menopause who are already losing muscle due to declining estrogen and testosterone, this is a real concern. We set specific protein targets — typically 1.2 to 1.6 grams per kilogram of body weight per day — alongside GLP-1 therapy in every patient we manage.

What We Do Differently at The Lee Clinic

This is exactly why we don't treat GLP-1 prescribing as a simple transaction. When we start a patient on a GLP-1 medication, nutritional monitoring is part of the protocol — not an afterthought.

We check vitamin D, iron and ferritin, B12, and a comprehensive metabolic panel as part of our baseline evaluation and at follow-up visits. We look at these numbers in the context of your symptoms, your hormone levels, and your overall clinical picture — because a number that looks "in range" on a standard lab report can still be suboptimal for how you actually feel and function.

We also counsel every GLP-1 patient on protein intake and resistance training from day one, because protecting muscle while losing fat isn't optional — it's the whole point of doing this well.

If you're on a GLP-1 medication and nobody has checked your nutrient levels, that's a gap worth closing. Feeling tired, losing hair, or noticing brain fog while on these medications is not something to just push through. It's information — and it deserves a proper evaluation.

The Lee Clinic sees patients in person in Winchester and Reston, VA. Telehealth appointments are available for patients in FL, DC, WV, and MD. Call us at 540-542-1700 to schedule.