What the Research Shows

A 2026 review of 47 studies found vitamin D deficiency prevalence of 7.5% at 6 months escalating to 13.6% at 12 months, with dietary intake averaging only 20% of recommendations. GLP-1 RA users had 49% higher risk of insufficiency compared to SGLT2 inhibitor comparators. Baseline obesity itself is associated with lower vitamin D — the vitamin is fat-soluble and sequestered in adipose tissue. Rapid weight loss can paradoxically release sequestered vitamin D, creating transient fluctuations.

What the Research Shows

B vitamin deficiency rates reached 22.4% by 12 months of GLP-1 therapy. Thiamine (B1) is the most clinically urgent: case reports document Wernicke encephalopathy (acute neurological emergency) and beriberi in GLP-1 users — linked to severe caloric restriction combined with persistent vomiting. B12 risk is amplified when co-prescribed with metformin. Fatigue — the most commonly reported non-GI symptom — may reflect subclinical B vitamin insufficiency rather than the medication's direct effects.

What the Research Shows

GLP-1 RA users had 26–30% lower serum ferritin compared to SGLT2 inhibitor comparators, with over 60% consuming below estimated requirements. Anemia incidence reached 3.2% at 12 months. GLP-1 users significantly reduced consumption of red meat and other heme iron sources — the most bioavailable dietary iron. The cumulative effect creates a slow depletion pattern that drops ferritin well before hemoglobin and clinical anemia appear.

What the Research Shows

GI adverse events affect 40–70% of users during dose escalation. Vomiting depletes potassium; diarrhea depletes sodium, potassium, and magnesium; reduced intake limits replenishment. Magnesium depletion is particularly common and clinically silent — standard serum testing reflects only 1% of total body magnesium, meaning intracellular depletion occurs without abnormal blood tests. Muscle cramps, fatigue, and cardiac rhythm disturbances are the most common clinical manifestations.

What the Research Shows

Approximately 25–40% of total weight lost is lean mass. Most GLP-1 users consume only 0.6–0.8 g/kg/day — well below the 1.2–1.6 g/kg/day threshold recommended for lean mass preservation. Protein is the most satiating macronutrient, making it the hardest to consume when appetite is suppressed. Higher intake combined with resistance training can preserve 60%+ of lean mass. For adults 65+, sarcopenia risk is amplified — expert consensus recommends ≥1.2 g/kg/day minimum.

What the Research Shows

A meta-analysis found no increased fracture risk with GLP-1 RA use (pooled RR 0.80, suggesting possible protective effect). Over 60% of users consumed below recommended calcium intake — dairy reduction from appetite suppression and GI intolerance is common. GLP-1 receptors are expressed on osteoblasts, and preclinical evidence suggests direct bone-protective effects independent of weight change. The net effect appears neutral to mildly protective based on available data.

What the Research Shows

Constipation affects 10–16% of GLP-1 RA users across major trials. Most consume well below the recommended 25–30 g/day fiber — a direct consequence of reduced food volume. Fiber maintains gut motility, feeds beneficial bacteria, and supports normal bowel function. Emerging research suggests microbiome composition shifts associated with reduced dietary diversity, though clinical significance remains under investigation.

What the Research Shows

Bariatric surgery patients receive mandatory preoperative screening and lifelong postoperative monitoring per ASMBS/IFSO guidelines — including regular testing of iron, B12, vitamin D, calcium, thiamine, folate, and zinc. A 2025 analysis found a critical asymmetry: no equivalent consensus guidelines exist for GLP-1 therapy, despite comparable weight loss magnitudes (15–25%) and overlapping deficiency patterns. The 2026 Clinical Obesity review quantified the gap: 22.4% cumulative deficiency rate by 12 months — yet most prescribing guidelines contain no nutritional monitoring recommendations.