B12 absorption older adults: Sugimoto et al., 2026
A 68-year-old man on atezolizumab (an immunotherapy drug) developed severe B12-deficiency anemia, hemoglobin dropped to 6.1 g/dL, traced to autoimmune gastritis triggered by the drug blocking immune checkpoints. This is a single case report, useful as a clinical flag but not as proof of causation or prevalence.
Key takeaways
- The patient's hemoglobin fell from normal to 6.1 g/dL after starting atezolizumab; B12 supplementation partially reversed it to 9.5 g/dL
- Immune checkpoint inhibitors can unmask or accelerate autoimmune conditions, including gastric parietal cell destruction that blocks B12 absorption
- The authors attributed the anemia to drug-induced autoimmune gastritis, but causation is inferred, this is a single case with no rechallenge or matched controls
- For reading lens: case reports are hypothesis generators, not evidence of prevalence or mechanism; they tell you what CAN happen, not how often it does
- B12 deficiency severe enough to cause anemia typically takes years to develop; the timeline here suggests the drug accelerated a pre-existing process
The study
Sugimoto and colleagues published this case report in The American Journal of Case Reports in 2026 (PubMed). A 68-year-old Japanese man with hepatocellular carcinoma and renal cell carcinoma received atezolizumab-containing chemotherapy. He developed severe megaloblastic anemia, hemoglobin 6.1 g/dL with signs of hemolysis. Lab work showed gastric parietal cell antibodies (positive), elevated gastrin, and negative Helicobacter pylori antibodies. Upper endoscopy revealed open-type (O-3) atrophic gastritis, loss of gastric folds and marked vascular visibility across nearly the entire stomach, consistent with autoimmune gastritis. No biopsy was performed. The authors diagnosed pernicious anemia secondary to autoimmune gastritis triggered by atezolizumab. Treatment with mecobalamin (a B12 form) raised hemoglobin to 9.5 g/dL and resolved the hemolytic pattern.
How to read this study
What this case does well: The authors ruled out the standard differential for severe anemia, they checked for hemolysis markers, tested for H. pylori (the common bacterial cause of gastritis), measured antibodies to gastric parietal cells, and documented endoscopic findings consistent with autoimmune gastritis. They also showed temporal association (anemia appeared after atezolizumab) and biological plausibility (checkpoint inhibitors are known to unmask autoimmune conditions). The response to B12 supplementation supports their diagnosis.
What this case is missing: No rechallenge, they didn't stop and restart atezolizumab to confirm the anemia recurred, which would strengthen causation. No gastric biopsy to histologically confirm autoimmune gastritis. No pre-treatment B12 level, so we don't know if he was already deficient. No matched cohort to estimate how often this occurs in patients on checkpoint inhibitors. The timeline is murky, autoimmune gastritis develops slowly over years; the paper doesn't clarify whether atezolizumab triggered new disease or unmasked subclinical disease already present.
How I'd weight this paper: I treat this as a flag, not a proof of causation or prevalence. It tells us checkpoint inhibitor-induced pernicious anemia is biologically plausible and worth monitoring for, but it doesn't tell us how common it is (could be 1 in 10,000) or whether stopping the drug reverses it. If I saw three more well-documented cases in the next year, I'd weight it more heavily. As a single case, it's hypothesis-generating.
What they found
At presentation, the patient's hemoglobin was 6.1 g/dL (normal range for adult men: 13.5-17.5 g/dL). Lab markers showed signs of hemolysis, lactate dehydrogenase (LDH) elevated, haptoglobin low. His mean corpuscular volume (MCV) was elevated at 115 fL (normal 80-100 fL), consistent with megaloblastic anemia. Gastric parietal cell antibodies tested positive. Gastrin was elevated, a finding consistent with loss of acid-producing parietal cells, which normally suppress gastrin release. H. pylori antibodies were negative, ruling out the most common infectious cause of gastritis. Endoscopy showed O-3 atrophic gastritis (open type, severe), with loss of gastric folds and prominent vascular visibility across almost the entire stomach. After starting mecobalamin supplementation, hemoglobin rose to 9.5 g/dL and hemolysis markers normalized. The authors attributed the anemia to pernicious anemia triggered by atezolizumab-induced autoimmune gastritis.
What it means for the average man
For men not on immunotherapy drugs, this case has limited direct relevance. But it highlights a broader principle: B12 absorption depends on healthy stomach lining. Autoimmune gastritis, whether drug-triggered or age-related, destroys gastric parietal cells, which produce intrinsic factor, the protein required to absorb B12 from food. Without intrinsic factor, dietary B12 passes through unabsorbed, and stores deplete over 3-5 years. The actionable takeaway: if you're over 50 and develop unexplained anemia or neurological symptoms (numbness, balance problems), ask your doctor to check serum B12 and methylmalonic acid. Supplemental B12, especially sublingual or intramuscular forms that bypass the gut, can reverse deficiency before irreversible nerve damage occurs.
The caveats
This is a single case with no histological confirmation (no biopsy), no pre-treatment B12 measurement, and no rechallenge. Atezolizumab is a checkpoint inhibitor used for cancer, it lifts brakes on the immune system, which can unmask or accelerate autoimmune conditions. The general population doesn't face this drug-specific risk. The timeline is unclear, the authors suggest atezolizumab triggered the autoimmune gastritis, but autoimmune gastritis typically develops slowly over years. It's equally plausible the patient had subclinical gastritis before treatment, and the drug accelerated it to symptomatic pernicious anemia. Without matched controls or epidemiological data, we can't estimate how often this occurs in patients on checkpoint inhibitors. The case supports monitoring B12 levels in cancer patients on immunotherapy, but it doesn't establish prevalence or prove causation.
Frequently asked questions
Should I trust a single case report as evidence?
No, not as proof of causation or prevalence. Case reports tell you what CAN happen, not how often it does or whether the suspected cause is the real cause. They're hypothesis generators. This case flags checkpoint inhibitor-induced pernicious anemia as biologically plausible and worth tracking, but you'd need a cohort study or case series to estimate how common it is, and an RCT or rechallenge to prove causation. Use case reports as clinical alerts, not as evidence to change behavior.
What is pernicious anemia?
Pernicious anemia is B12-deficiency anemia caused by loss of intrinsic factor, a protein made by gastric parietal cells that binds to B12 and allows it to be absorbed in the small intestine. When autoimmune gastritis destroys parietal cells, intrinsic factor production stops, dietary B12 passes through unabsorbed, and anemia develops over 3-5 years as B12 stores deplete. The term "pernicious" is historical, it was fatal before B12 supplementation was discovered. Today it's treatable with high-dose oral, sublingual, or intramuscular B12, which bypasses the need for intrinsic factor.
Does B12 absorption decline with age even without autoimmune gastritis?
Yes. Gastric acid and pepsin are required to cleave B12 from food proteins before intrinsic factor can bind it. Acid production declines with age, about 10-30% of adults over 50 have some degree of atrophic gastritis or low stomach acid. Proton pump inhibitors (PPIs) and H2 blockers further suppress acid. Lower acid means less B12 released from food, which can lead to subclinical deficiency even when intrinsic factor is intact. Supplemental B12 (which doesn't require cleavage from food) bypasses this step and is absorbed normally in most older adults.
Should men over 50 supplement B12 preventively?
It's reasonable, especially if you take PPIs or have a history of gastritis. The RDA for adult men is 2.4 mcg/day, but absorption efficiency drops with age. Doses of 500-1000 mcg/day (common in B-complex or multivitamin formulas) saturate passive absorption pathways and ensure adequate intake even when stomach acid is low. Sublingual and intramuscular forms bypass the gut entirely. Testing serum B12 and methylmalonic acid (a sensitive marker of B12 function) every few years is the cleanest approach, supplement if you're borderline or deficient, skip it if you're replete.
Sources
- Sugimoto T, Kanemori G, Kanehira H, Kitao A, Nagatani Y. A Case of Pernicious Anemia Induced by Atezolizumab in Hepatocellular Carcinoma and Renal Cell Carcinoma. Am J Case Rep. 2026. PubMed
- Langan RC, Goodbred AJ. Vitamin B12 Deficiency: Recognition and Management. Am Fam Physician. 2017;96(6):384-389.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
