Background/Objectives: The bioavailability of crystalline vitamin B₁₂ (B₁₂) through active absorption is reported to have a maximum capacity of 1.5–2.5 µg per dose. A small passive bioavailability has also been suggested at high doses. The present study aimed to determine the dose-dependency of active B₁₂ absorption and to quantify its passive absorption at higher doses. Methods: The dose-dependency of crystalline B₁₂ bioavailability was determined in nine healthy adults, using oral [¹³C]-cyanocobalamin, in a cross-over design at doses of 2.5, 5, and 10 µg. The dose order was randomised, with a washout of one month. Literature data from was added to the present study data in a meta-analysis of the relation between B₁₂ bioavailability and dose, to evaluate its pattern at different doses. Results: Bioavailability, as a function of dose, was significantly different between 2.5, 5, and 10 µg doses of [¹³C]-cyanocobalamin at 50.9 ± 32.5%, 26.7 ± 22.3%, 15.4 ± 13.6%, respectively, (p < 0.01), while the absolute bioavailability trended upward, at 1.16 ± 0.74 µg, 1.22 ± 1.02 µg, and 1.39 ± 1.23 µg (p = 0.46). The meta-analysis showed two distinct phases of bioavailability. Up to a dose of 2.6 µg, there was a significant steep positive correlation, with a slope (bioavailability) of 43%/µg suggesting an active process with a maximum of 1.2 µg. At higher doses, the slope was 1%/µg, not significantly different from zero, possibly a passive process. Conclusions: The active bioavailability of crystalline B₁₂ is not dose-dependent, saturating at ~1.2 µg.

Purpose: To assess the presence and quantity of CD34+ hematopoietic stem cells in patients with Alzheimer’s disease (AD) through a meta-analysis. Methods: A systematic search of the databases identified the observational and interventional studies reporting baseline CD34+ cell counts in AD patients. The data on mean counts and the measures of variation were extracted. Standardized mean differences (SMDs) were calculated using common and random effects models to compare the CD34+ cell counts between the AD patients and controls. Heterogeneity among the studies was evaluated using tau², tau, and I² statistics. The risk of bias was assessed using the Newcastle–Ottawa Scale and the ROBINS-I tool. Patients: Five studies were included, comprising four observational studies and one open-label trial, with a total of 271 participants (139 AD patients and 132 controls). Results: The meta-analysis indicated an increase in CD34+ cell counts of the AD patients when compared to the controls. The common effects model showed a moderate SMD of 0.2964 (95% CI:0.0490–0.5437). However, the random effects model yielded a non-significant SMD of 0.2326 (95% CI: −0.4832–0.9484). Significant heterogeneity was observed among the studies (I² = 87.1%, p < 0.0001). Conclusion: AD patients may exhibit higher circulating CD34+ cell counts than the controls, but substantial heterogeneity and potential biases limit definitive conclusions.