A study of 5571 adults found that people who have higher blood levels of vitamin B12 are more likely to die prematurely (JAMA Netw Open, 2020;3(1):e1919274). Compared to those with lower B12 blood levels, people with higher B12 levels (>455.41 pg/mL) were likely to:
• be fatter
• have higher blood pressure, higher cholesterol, and higher blood sugar
• die prematurely of any cause
This study does not show that having high blood levels of B12 shortens lives. It is more likely that people with high B12 levels are doing something that both raises B12 levels and shortens lives.

Biologically active B12 is found only in animals; it is not found in plants. You would expect people who eat the most meat to have the highest blood levels of vitamin B12, and eating meat daily has been associated with increased risk for premature death (Amer J of Clin Nutr, May 2019;109(5):1462-1471). Vegetarians have lower levels of vitamin B12 because they do not eat animal products and are at reduced risk for heart attacks and premature death (Am J Prev Med, Jun 2015;48(6):e11-26). However, vegetarians are at increased risk for vitamin B12 deficiency (Eur J of Clin Nutr, March 26, 2014;68:541-548), which can cause forgetfulness, confusion, tiredness, muscle weakness and various other symptoms. All people who eat little or no meat should have their B12 levels checked, and those who are deficient need to take B12 pills or eat more animal products. See Vitamin B12: One Pill You May Need