GLP-1 and Nutrient Deficiencies: Screening & Bloodwork
Here’s the conversation that most people are missing when it comes to GLP-1 medications like Ozempic and Mounjaro: nutrient deficiencies. And yet, it makes perfect sense. You’re eating fewer calories, so you’re consuming fewer nutrients overall. You’re also eating less food overall, so you’re probably having less variety in your nutrition than you normally would. Plus, your gut absorption isn’t running at the same capacity as it normally is. That combination sets the perfect stage for GLP-1 nutrient deficiencies. As a Registered Dietitian working with clients on GLP-1s, I see this gap firsthand. Most providers aren’t proactively screening for nutrient deficiencies or running comprehensive labs after starting the medication. So, I’m going to break down the most common deficiencies, what to ask your doctor about, and how to prevent these gaps.
Why GLP-1 Medications Increase the Risk of Nutrient Deficiencies
Reason #1: You’re Eating Less Food
Let’s start with the most obvious reason you’re at higher risk of nutrient deficiencies on GLP-1 medications: you have less appetite, and you’re eating less food. Data shows anywhere from 20–40% fewer calories consumed while on the medication.
It’s important to note here that the single most important way to prevent nutrient deficiencies is to consume enough calories on medications like Ozempic. In my professional opinion as a Registered Dietitian, consuming under 1,000 calories a day is insufficient and unsustainable. Yes, you want to be in a calorie deficit for weight loss, but consuming too few calories creates a different set of problems. With too few calories, you’re now at risk of muscle loss, nutrient deficiencies, fatigue, loss of bone density, and more. While it may be difficult with a suppressed appetite, make sure you’re eating small, protein-rich meals every few hours to keep your body properly fueled.
Reason #2: Food Aversions
The next most likely reason nutrient deficiencies show up is food aversion. If you’re on one of these medications, you’ve probably noticed you’re not only eating fewer foods, but also eating different foods than before. Maybe you’ve developed a new aversion to greasy foods, sugary foods, fast food, or other junk food. Some people avoid entire food groups once they start the medication. Maybe you’ve learned the hard way after eating something that didn’t agree with you. Whatever the case, your diet has changed, and you’re probably not eating the same variety of foods you once did. Less variety typically means less nutrition.
Reason #3: The Medication’s Mechanism Itself
For reason number three, let’s dig into how GLP-1 medications actually work. One of the primary ways they help you lose weight is through delayed gastric emptying. What does that mean? Food moves more slowly from your stomach into your digestive tract. When that happens, your brain gets the signal that you’re still full. Unfortunately, it can also lead to those pesky side effects like bloating, constipation, and acid reflux.
Here’s the part that doesn’t get talked about enough: GLP-1 drugs slow gastric emptying by roughly 40–50%, which means food has less contact time with the small intestine, where most nutrient absorption actually happens. This delayed transit can reduce absorption efficiency by an estimated 15–25%, even when you’re eating adequate nutrients. So now you’re eating less and absorbing less. To make matters worse, nausea, vomiting, and diarrhea, all common side effects, create their own absorption problems on top of everything else. It’s the perfect trifecta for deficiencies.
What the Research Actually Shows
Let’s dig into the research and what has actually been discovered about nutrient deficiencies in patients on GLP-1 medications. We have several studies to look at, and they point to some concerning trends.
The largest dataset comes from a retrospective study of 461,382 adults newly prescribed GLP-1 medications, with over 22% developing at least one nutritional deficiency within 12 months of starting. The timeline matters here: 12.7% were newly diagnosed with a nutritional deficiency by 6 months, and that number nearly doubled to 22.4% by 12 months. What this tells us is that the longer someone stays on the medication, the higher the risk.
In this study, the most common nutrient deficiency was vitamin D, a vitamin we already see chronically low across the general population. Iron was another big-ticket item: GLP-1 users showed 26–30% lower ferritin (iron stores) than people on other diabetes medications.
But there’s a laundry list of other nutrients to keep an eye on, too. A broader nutrient-intake study found that vitamin D, potassium, choline, magnesium, and iron had the largest gaps compared to recommended daily intakes among GLP-1 users, with over 98% of participants falling short on several of these.
Worth noting: most of this data comes from observational and retrospective studies, not randomized controlled trials. Still, it’s pointing us in the right direction to look.
Protein Deficiencies on Ozempic, Wegovy, and Zepbound
By and large, the most common deficiency I see on GLP-1 medications is protein. You hear the importance of high-protein meals all the time, but here’s what’s actually happening behind the scenes.
When you’re not consuming sufficient protein, studies (e.g., STEP trial data) show roughly 25–40% of weight lost can be lean tissue, not fat. Let’s put that into simpler terms: you’re losing muscle instead of fat. When we think about weight loss, we’re really talking about fat loss. Losing muscle works in the opposite direction of where you want to go.
Here’s why that matters so much. Muscle is metabolically active tissue — meaning it burns calories even at rest. So when you lose muscle, you’re actually weakening your metabolism along the way, which makes long-term weight maintenance harder, not easier. This is one of the biggest reasons people regain weight after stopping a GLP-1: they didn’t just lose fat, they lost the metabolic engine that helps keep weight off.
Muscle also matters far beyond the scale. It protects your bones, supports functional strength (think: carrying groceries, getting up from a chair, avoiding falls as you age), and plays a role in blood sugar regulation. If you’re already managing insulin resistance or type 2 diabetes, losing muscle can actually work against the very condition the medication is meant to help improve.
This is the part that surprises people: even though your appetite is lower, your protein needs don’t shrink to match it. If anything, they become more important to prioritize deliberately, because your body is actively losing weight and needs enough amino acids on board to preserve muscle tissue during that process.
Muscle loss doesn’t always show up as an obvious symptom right away. Watch for:
Feeling weaker during everyday tasks (stairs, carrying items, getting up from the floor)
Noticeably reduced strength if you exercise or lift weights
Excessive fatigue that doesn’t improve with rest
Losing weight quickly with little visible change in body composition (fat loss should feel different than muscle loss)
The Bloodwork You Should Ask Your Doctor For
Now that we’ve covered why it’s so important to screen for nutrient deficiencies, here’s the other part of the equation: actually getting the bloodwork done. Unfortunately, that’s not as straightforward as it should be.
Even if you’re on top of your annual bloodwork with your PCP, not all of these markers are automatically included. Standard annual labs often check things like a basic metabolic panel and maybe a CBC, but iron stores, vitamin D, and B12 are frequently left out unless you specifically ask, or unless you already have a diagnosed deficiency on record.
Ideally, if you’re starting a GLP-1, you’d have baseline testing done before the medication even begins, followed by repeat labs every 3–6 months while you’re on it, and again anytime your dose increases, since higher doses tend to come with lower appetite and more pronounced GI side effects.
Here’s what to make sure is included in your labs. It’s worth emphasizing: you may have to specifically ask your doctor to add these, since they’re not always part of a standard panel.
CBC: catches anemia and gives a general picture of your overall health
CMP: (comprehensive metabolic panel): checks liver and kidney function, along with electrolytes
Ferritin/iron panel: iron stores specifically, not just hemoglobin
Vitamin D (25-hydroxy): the most commonly deficient nutrient seen in GLP-1 users
B12, and consider adding thiamine if you’re experiencing any GI or neurological symptoms (numbness, tingling, confusion, or persistent nausea/vomiting)
A few other optional markers to ask about include magnesium RBC, calcium, and prealbumin. The bigger takeaway here: advocate for yourself and speak up about what you want checked. If your current provider isn’t open to more comprehensive testing, it may be worth seeking out one who is.
Signs and Symptoms That May Signal a Deficiency
Bloodwork is the most reliable way to catch a deficiency, but your body is also giving you clues in the meantime. Here’s the tricky part: a lot of these symptoms overlap with what people already expect to feel on a GLP-1, which means they get written off as “just the medication” instead of a sign that something needs attention.
Fatigue that feels different from normal tiredness. Some tiredness is expected when you’re eating less and losing weight. But if you’re feeling wiped out in a way that doesn’t improve with rest, or that feels heavier than typical fatigue, that’s worth paying attention to. This is one of the most common signs of both iron and B12 deficiency.
Hair thinning or shedding. A little hair shedding can happen with any rapid weight loss, but noticeable thinning is often your body’s way of telling you it’s not getting enough protein, iron, or key vitamins to support normal hair growth. It could also mean you’re simply not consuming enough calories. When the body has to prioritize calories, hair is one of the first things to go.
Dizziness or lightheadedness. This one gets dismissed a lot, especially if you’re also dealing with lower food intake or GI side effects. But dizziness can point to low iron, dehydration, or an electrolyte imbalance, all of which are more common on a GLP-1 than people realize.
Muscle weakness that goes beyond expected fat loss. If stairs feel harder, if you’re struggling with things you used to do easily, or if your strength at the gym has noticeably dropped, that’s a signal worth taking seriously.
Brain fog or trouble concentrating. This is a big one, especially given everything we discussed earlier about thiamine and B12. If you’re having a hard time focusing, feeling mentally “slow,” or noticing memory issues that are new for you, please don’t brush this off as just part of the GLP-1 experience.
None of these symptoms are something you just have to live with while on a GLP-1. Yes, some fatigue and appetite changes are expected. But when symptoms are persistent, worsening, or interfering with your daily life, that’s your body asking for a closer look, not evidence that you’re doing something wrong. Trust that signal and advocate for the bloodwork to back it up.
How to Prevent Deficiencies While Losing Weight on a GLP-1
With a little intention, you can absolutely lose weight and protect your nutrient status at the same time. Here’s what actually makes a difference.
Since your overall food volume is so much smaller than it used to be, every meal needs to work harder for you. Instead of thinking about protein as one part of the plate, flip the order: decide on your protein source first, then build the rest of the meal around it. When you’re only able to eat a few bites at a time, protein comes first. Try to eat a small meal every few hours, even if you aren’t hungry. It’s not about being “perfect,” it’s about being intentional with the limited appetite you have.
While a multivitamin isn’t a substitute for real food or a fix for a diagnosed deficiency, it can serve as a helpful safety net while your intake is lower than usual. This is a great conversation to have with your Registered Dietitian, since the right choice really does depend on your individual labs and needs.
We talked earlier about how much muscle can be lost alongside fat on a GLP-1. Protein gives your body the raw materials to preserve muscle, but strength training is what tells your body to actually hold onto it. You don’t need to become a bodybuilder here. Even 2–3 sessions a week of resistance-based movement can make a meaningful difference in preserving lean mass while you lose weight.
Why Working With a Dietitian on a GLP-1 Matters
Here’s something I see all the time in my practice: someone starts a GLP-1, their doctor hands them a prescription, and that’s about it. Maybe there’s a quick mention to “eat more protein” or “watch your portions,” but no real plan for what that actually looks like day to day. And then a few months in, the fatigue sets in, the labs come back showing a deficiency, or the scale stalls, and they’re left wondering what went wrong.
This is exactly the gap I help fill. Getting the prescription is one thing. Actually knowing how to eat while on it, especially when your appetite has completely changed, is a whole different challenge. Unfortunately, it’s not something most doctors have the time (or training) to walk you through in a quick appointment.
Labs can tell you what’s low. But knowing your vitamin D or iron is low doesn’t tell you what to actually put on your plate when you can barely finish half a meal. That’s where my team and I come in. I take that lab work and turn it into a real, doable eating plan, one that accounts for your appetite, your food preferences, your schedule, and the reality of what it’s actually like to eat on these medications.
If you’re on a GLP-1 and feel like you’ve been left to figure out the nutrition piece on your own, you don’t have to. Book a consult and let’s build a plan that actually works for where you are right now.
References
Butsch, W. S., Sulo, S., Chang, A. T., Kim, J. A., Kerr, K. W., Williams, D. R., Hegazi, R., Panchalingam, T., Goates, S., & Heymsfield, S. B. (2025). Nutritional deficiencies and muscle loss in adults with type 2 diabetes using GLP-1 receptor agonists: A retrospective observational study. Obesity Pillars, 15, 100186. https://doi.org/10.1016/j.obpill.2025.100186
