Final answer:
The ability to absorb vitamin B12 depends on intrinsic factor, and conditions such as atrophic gastritis can impair its production, leading to pernicious anemia. Although the exact percentage of those aged 51 and older with this condition is not specified, it is acknowledged that the risk increases with age.
Step-by-step explanation:
The ability to digest and absorb vitamin B12 is critical for producing mature red blood cells and for normal neurological functioning. Vitamin B12 absorption requires intrinsic factor, which is produced in the stomach. Conditions such as atrophic gastritis can severely impair the production of intrinsic factor, leading to inadequate B12 absorption, a condition known as pernicious anemia.
While the specific percentage of people aged 51 and older who lose the ability to digest B12 due to atrophic gastritis is not provided, it is known that this condition becomes more common with age. Those who have had surgical procedures such as total gastrectomy will require vitamin B12 injections to compensate for the loss of intrinsic factor production and to prevent pernicious anemia.