We know that fatigue comes from being anemic, dehydrated, poor fitness in many ways. Rapid weight loss from GLP-1 receptor agonists like Wegovy or Zepbound can indirectly contribute to micronutrient deficiencies, including iron and vitamin B12. These vitamins aren’t directly affected by these medications, but nutritional deficits are due to reduced caloric intake and decreased consumption of nutrient-dense foods. A 2025 joint advisory from major nutrition and obesity societies emphasizes that GLP-1 RA-induced appetite suppression leads to caloric reductions of 16-39%, which can result in insufficient intakes of essential nutrients including iron, vitamin B12, calcium, magnesium, zinc, and vitamins A, D, E, K, B1, and C, particularly when energy intake falls below 1200 kcal/day for women or 1800 kcal/day for men, these changes are more likely.
Mechanisms of Nutrient Deficiency with Rapid Weight Loss
If you didn’t have any reason for poor absorption of food before you entered your weight loss journey you won’t be pushed into malabsorption. Bariatric surgery, which causes both reduced intake and malabsorption through anatomic changes, GLP-1 RAs effect to the gut is solely through appetite suppression and delayed gastric emptying. However, those consuming fewer calories will run risks of being anemic. Just reducing overall eating by a third means that you likely will not have proper intake protein, vitamin B6, vitamin A, calcium, and magnesium. Furthermore, it’s tough to know what nutrients you are missing as calculations can be complex.
Iron Deficiency Risk
Iron deficiency already is a risk to those adults who are either overweight or obese due to chronic inflammation increasing something called hepcidin levels and reducing iron absorption. During GLP-1 RA therapy, iron intake may decrease further as patients consume less red meat, fortified cereals, and other iron-rich foods.
Vitamin B12 Deficiency Risk
Vitamin B12 deficiency isn’t as common in those who are beginning a diet but can be seen before dieting. Unlike iron deficiency, which can develop within 6 months of reduced intake when on a GLP-1 diet, vitamin B12 deficiency typically takes several years to manifest because the body maintains substantial hepatic stores (2-5 mg) that can last 3-5 years even with zero intake. B12 absorption is aided by intrinsic factor from gastric parietal cells and occurs in the terminal ileum—both unaffected by these medications. But again, intake becomes a factor over time. We have now entered the 5th year of dieting with GLP-1s, so these conditions are now being seen, and unless you are scrutinized carefully you can succumb to these deficiencies.
Clinical Implications
The 2025 joint advisory recommends baseline screening for common deficiencies (vitamin D, iron, vitamin B12), etc before initiating GLP-1 therapy, with follow-up testing when weight loss is extreme or symptoms suggest deficiency.[3] Signs of nutrient deficiency include fatigue beyond expected levels, excessive hair loss, skin changes, muscle weakness, poor wound healing, and unusual bruising. These are the organizations who endorse this recommendation.
- The Obesity Medicine Association (OMA)
- The American Society for Metabolic and Bariatric Surgery (ASMBS)
- The American Association of Clinical Endocrinology (AACE)
- The Endocrine Society (contributing expert input and endorsement)
Dietary strategies to prevent deficiencies include emphasizing nutrient-dense foods (fruits, vegetables, whole grains, lean proteins, nuts, seeds) while avoiding refined carbohydrates and ultraprocessed foods. Factor in exercise when calculating calorie needs. Proactive supplementation with a multivitamin-mineral tablet, vitamin D, calcium, and B12 should be considered, particularly if you are eating very low calorie intakes.
