GLP-1 medications like Ozempic, Wegovy and Mounjaro mostly cause nutrient shortfalls by cutting how much you eat, not by blocking absorption. In a 2025 study of GLP-1 users, average intakes were low in fiber, calcium, iron, magnesium, potassium and vitamin D, with vitamin D at about a fifth of the daily target.
B12 is a separate story. Food intake of B12 was fine in that study, but if you also take metformin, your B12 risk goes up.
Key takeaways
- Less food, fewer nutrients. A 2025 joint advisory says the risk rises when intake drops below about 1,200 calories a day for women or 1,800 for men.
- Most often low: vitamin D, calcium, iron, magnesium, potassium and fiber. Protein was short for more than half of users.
- B12: mainly a metformin issue, not a GLP-1 issue.
- No standard supplement dose exists. Get labs, then agree on doses with your doctor.
Does GLP-1 affect nutrient absorption?
GLP-1 drugs slow how fast your stomach empties and turn down appetite. The result most people notice is eating much smaller meals, and skipping some.
What the research has measured so far is intake. The 2025 study below used three-day food records, not blood tests, and the 2025 advisory from four US obesity and nutrition societies ties deficiency risk to low calorie intake. Neither shows that the drugs stop your gut from absorbing vitamins. The practical fix is to get more nutrients into fewer bites.
What a 2025 food-record study found
Researchers asked 69 US adults who had been on a GLP-1 drug for at least a month to log three days of eating (Johnson and colleagues, Frontiers in Nutrition, 2025). The study was small and funded by a supplement retailer, GNC, so treat it as a first look.
| Nutrient | Average intake | Reference used |
|---|---|---|
| Fiber | 14.5 g | 28 g |
| Calcium | 863 mg | 1,300 mg |
| Iron | 12.1 mg | 18 mg |
| Magnesium | 266 mg | 420 mg |
| Potassium | 2,186 mg | 4,700 mg |
| Choline | 305 mg | 550 mg |
| Vitamin D | 4 mcg (160 IU) | 20 mcg (800 IU) |
| Vitamin C | 51 mg | 90 mg |
| Vitamin A | 560 mcg RAE | 900 mcg RAE |
| Vitamin E | 9.6 mg | 15 mg |
| Vitamin B12 | 4.9 mcg | 2.4 mcg |
Between 88% and 99% of participants were below the target for vitamin D, potassium, choline, magnesium and iron. Protein averaged 77 g a day, and only 43% reached 1.2 g per kg of body weight.
Two things the study did not find: B12 intake was above the target, and intake did not get worse the longer people had been on the drug.
Nutrients most at risk on GLP-1 medications
The 2025 advisory (American College of Lifestyle Medicine, American Society for Nutrition, Obesity Medicine Association and The Obesity Society) lists iron, calcium, magnesium, zinc and vitamins A, D, E, K, B1, B12 and C as nutrients to watch. Daily Values below are the FDA label targets.
| Nutrient | Daily Value | Easy food sources |
|---|---|---|
| Vitamin D | 20 mcg (800 IU) | Salmon, canned sardines, fortified milk and yogurt |
| Calcium | 1,300 mg | Greek yogurt, milk, cheese, fortified soy milk, tofu set with calcium |
| Iron | 18 mg | Lean beef, lentils, fortified cereal, spinach with a vitamin C food |
| Magnesium | 420 mg | Pumpkin seeds, almonds, beans, oats |
| Potassium | 4,700 mg | Potatoes, beans, bananas, yogurt |
| Zinc | 11 mg | Beef, shellfish, pumpkin seeds, chickpeas |
| Vitamin B12 | 2.4 mcg | Fish, meat, eggs, dairy, fortified foods |
| Fiber | 28 g | Beans, berries, oats, vegetables, chia |
People at higher risk include anyone who has had bariatric surgery, has celiac disease or another condition that affects absorption, has had a nutrient deficiency before, or followed a very low-calorie diet before starting. The advisory says extra assessment or lab tests may be needed before treatment in those cases.
Does Ozempic cause B12 deficiency?
There is no good evidence that semaglutide itself drains B12. The drug with a proven B12 effect is metformin, which many people with type 2 diabetes take with Ozempic or Mounjaro.
In the Diabetes Prevention Program follow-up, which enrolled adults with prediabetes, low or borderline-low B12 showed up in 19.1% of people on metformin versus 9.5% on placebo after five years, and the odds of deficiency rose about 13% with each year of use. If you take both drugs, or you eat little meat, fish, eggs or dairy, ask your doctor to check B12.
Symptoms to watch for
Shortfalls build slowly, and early signs are easy to blame on the medication:
- Tiredness, weakness or feeling short of breath on stairs (iron, B12)
- Muscle cramps or twitches (magnesium, calcium, potassium)
- Constipation that won't settle (fiber, fluid, magnesium)
- Tingling or numbness in hands or feet (B12)
- Brittle nails or hair shedding (iron, protein, overall intake)
GLP-1 drugs cause some of these on their own. On the Wegovy label, fatigue was reported by 11% of adults on the drug versus 5% on placebo, and dizziness by 8% versus 4%. A blood test tells you which is which.
Hair loss on GLP-1 medications
Hair loss is listed on both major labels: 3% of adults on Wegovy versus 1% on placebo, and 4-5% on Zepbound versus 1%. Fast weight loss and low protein intake are both worth raising with your doctor if you notice shedding.
Should you take a multivitamin on a GLP-1?
A basic multivitamin-mineral is a reasonable safety net when you are eating far less, and the 2025 advisory says clinicians can consider one, or targeted vitamin D, calcium or B12. It deliberately gives no doses, because needs depend on your labs, diet and other medications.
Two cautions: more is not better for fat-soluble vitamins like D and A, and iron pills can worsen nausea and constipation. Read more in our guide to supplements on Ozempic.
Blood tests to ask your doctor about
The advisory recommends re-checking nutrient levels during treatment but sets no schedule. Tests worth asking about:
- Complete blood count (anemia)
- Ferritin (iron stores)
- Vitamin B12, especially with metformin
- 25-hydroxy vitamin D
- Basic metabolic panel (kidney function and electrolytes, useful if you have had vomiting or diarrhea)
How to get enough on a small appetite
- Protein first. The advisory suggests 1.2-1.6 g per kg of body weight a day while losing weight, or 80-120 g a day, and eating the protein part of the meal first. Use the protein calculator and see protein needs on Ozempic.
- Pick dense foods. Greek yogurt, eggs, salmon, beans and lentils cover several at-risk nutrients at once. Our list of what to eat on Ozempic has meal ideas.
- Add fiber slowly and drink enough. Dehydration from vomiting or diarrhea can hurt the kidneys, which is a label warning. The advisory names magnesium citrate as an option for constipation; see Ozempic constipation and fiber.
- Track a few days. Logging meals shows where the gaps are. Nutrify logs a meal from a photo, which helps when you are eating small amounts many times a day. It has a 7-day free trial, then a paid subscription.
Low protein also speeds muscle loss on these drugs. Ozempic muscle loss covers how to protect it.
This article is general information. Talk to your doctor before starting supplements, especially if you take other medications.
Sources
- Johnson B, Milstead M, Thomas O, et al. Investigating nutrient intake during use of glucagon-like peptide-1 receptor agonist: a cross-sectional study. Front Nutr, 2025.
- Mozaffarian D, et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint Advisory. Am J Clin Nutr, 2025.
- Aroda VR, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab, 2016.
- US FDA. Daily Value on the Nutrition and Supplement Facts Labels.
- WEGOVY (semaglutide) and ZEPBOUND (tirzepatide) prescribing information, adverse reactions tables, via openFDA drug labels.
