The Nutrient Deficiencies Nobody Warned You About When Starting a GLP-1

Here’s something a lot of GLP-1 doctors don’t explain properly: when you start a GLP-1 like Mounjaro or Ozempic, you eat a lot less food. 

You’re not just losing weight, you’re also getting fewer nutrients.

Nutritional deficiency risks rise fast, causing side effects like constant tiredness, thinning hair, easy bruising, or feeling cold. These GLP-1 side effects aren’t always from the medications and often, your body just misses key nutrients.

Studies show over 1 in 5 people on GLP-1 medications develop a nutritional deficiency within their first year.

Let’s break down the main ones to watch for when starting a GLP-1.

Vitamin D

This is the most common nutritional deficiency in GLP-1 users.

Vitamin D is not just a “bone vitamin.” It also helps regulate calcium absorption, supports muscle function, contributes to immune function, and has been linked to better mood and energy levels. Because vitamin D is stored in body fat, people with obesity often start out with lower circulating vitamin D levels, and weight loss does not automatically fix that. 

During weight loss, low vitamin D may matter even more because it can overlap with symptoms people already feel on GLP-1s, such as fatigue and low energy. 

What to watch for:

Feeling tired, weak muscles, low mood, or getting sick often

What to do:

Ask for a 25(OH)D blood test and discuss supplementation if levels are low.

Vitamin B12

Vitamin B12 is essential for red blood cell formation, nerve function, and DNA synthesis. When B12 is too low, the body cannot make healthy red blood cells efficiently, which can contribute to anemia, fatigue, shortness of breath, and brain fog.

B12 also matters for normal nerve signaling, which is why tingling or numbness can appear when levels drop.

Weight loss can increase B12 risk simply because people may eat less meat, fish, eggs, and dairy, especially if nausea or food aversion is present. Your body doesn't store a ton of it, and you need to keep replenishing it through food.

What to watch for:

Tiredness, brain fog, tingling in hands or feet, shortness of breath

What to do:

A daily multivitamin can help. Some people may need extra B12 or injections.

Iron

Iron is central to hemoglobin, the protein in red blood cells that carries oxygen around the body. When iron is low, oxygen delivery suffers, which is why iron deficiency often causes profound fatigue, dizziness, headaches, weakness, and sometimes hair loss. Iron deficiency can progress to iron deficiency anemia if it is not corrected.

This is especially relevant during weight loss because reduced food intake can lower total iron intake, and foods rich in iron may be eaten less often. Weight loss does not automatically cause iron deficiency, but it can raise risk if the diet becomes too small or too narrow.

What to watch for:

Extreme fatigue, pale skin, dizziness, headaches, hair loss

What to do:

Check ferritin, hemoglobin, and iron studies before taking iron on your own, because unnecessary iron can be harmful.

Calcium

Calcium is critical for bone strength, muscle contraction, nerve signaling, and normal heart rhythm. It is often discussed for bones, but it also matters for everyday muscle function and long-term fracture risk.

When intake is low for long periods, the body can pull calcium from bone to keep blood calcium stable, which is not ideal over time.

What to watch for:

Muscle cramps, weak nails, tooth issues, numbness or tingling.

What to do:

Eat foods like dairy or leafy greens, or consider a supplement if needed.

Zinc

Zinc supports your immune system, wound healing, and even your sense of taste and smell. Hair loss, which is a common complaint among GLP-1 users, is sometimes linked to low zinc rather than the medication itself.

If food intake becomes too low, zinc can quietly fall short because many zinc-rich foods are also protein-heavy, such as meat and seafood.

Zinc is very underrated and deserves attention because symptoms are often vague at first - people may notice more colds, slower healing, or changes in taste before they ever connect it to nutrition.

What to watch for:

Hair thinning, frequent infections, slow wound healing, and taste changes.

What to do:

Check whether your multivitamin contains zinc, and consider testing if symptoms persist.

Why this matters and what to do atbout it

The problem with deficiencies is that they often do not feel dramatic at the beginning. You may just feel “off,” and it is easy to blame stress, poor sleep, or the medication itself.

A good place to start is a daily multivitamin. Think of it as basic coverage. It won’t replace real food, but it helps fill the gaps when you’re eating much less.

On top of that, a meal replacement shake that contains a full range of vitamins and minerals can be really helpful. When you’re eating smaller portions, every meal needs to count nutritionally. That’s why products like Maeva are designed to give complete nutrition in an easy, low-effort form - especially when you don’t feel very hungry.

And if you notice any of these symptoms, it’s worth speaking to your doctor. A simple blood test can usually identify what’s going on, and most deficiencies are easy to fix once they’re caught.

References

1. Khosravi ZS, Kafeshani M, Tavasoli P, Zadeh AH, Entezari MH. Effect of Vitamin D Supplementation on Weight Loss, Glycemic Indices, and Lipid Profile in Obese and Overweight Women: A Clinical Trial Study. Int J Prev Med. 2018 Jul 20;9:63. doi: 10.4103/ijpvm.IJPVM_329_15. PMID: 30123437; PMCID: PMC6071442.
2. Mallard, Simonette R et al. “Vitamin D status and weight loss: a systematic review and meta-analysis of randomized and nonrandomized controlled weight-loss trials.” The American journal of clinical nutrition vol. 104,4 (2016): 1151-1159. doi:10.3945/ajcn.116.136879
3. Urbina, Jorge et al. “Micronutrient and Nutritional Deficiencies Associated With GLP-1 Receptor Agonist Therapy: A Narrative Review.” Clinical obesity vol. 16,1 (2026): e70070. doi:10.1111/cob.70070
4. Cheng, Hoi Lun et al. “Impact of diet and weight loss on iron and zinc status in overweight and obese young women.” Asia Pacific journal of clinical nutrition vol. 22,4 (2013): 574-82. doi:10.6133/apjcn.2013.22.4.08
5. Lim, Karen H C et al. “Iron and zinc nutrition in the economically-developed world: a review.” Nutrients vol. 5,8 3184-211. 13 Aug. 2013, doi:10.3390/nu5083184