At one time I followed a vegan diet and ate no animal products at all, primarily based on data that associated eating mammal meat and processed meats with increased risk for diabetes, heart attacks, certain cancers and premature death. However, a couple years ago I became very forgetful, so I got a complete evaluation for memory loss and found that my B12 level was low. That in itself did not prove that I was being harmed by lack of B12, but further tests showed that my homocysteine and methylmalonic acid levels were both high, which suggested that I was getting tissue damage from my low B12 levels.

Vitamin B12 is found only in animals and is not found in plants. I now follow a diet that is high in plants but also includes seafood such as salmon and sardines. (Salmon and sardines have short lives, so they do not store significant amounts of mercury).

Vegans and Vegetarians Are at Increased Risk for Broken Bones
I also recommend that people who follow any of the high-plant diets should also do some form of resistance exercises to protect their bones. The EPIC-Oxford study followed 29,380 meat eaters, 8,037 fish eaters, 15,499 vegetarians, and 1,982 vegans for an average of 17.6 years, and found a marked increase in fractures in all the non-meat-eater groups with the most fractures of all in the vegans (BMC Medicine, Dec 2020;18(353). Overall, compared to meat eaters, vegans had higher risks of total, hip, leg, and vertebral fractures, while fish eaters and vegetarians had higher risk of hip fractures. Other studies show that vegetarians have lower bone densities than meat-eaters (Am J Clin Nutr, 2018;107:909–20; Public Health Nutr, 2008;11:564–72), possibly because they weigh less and take in less calcium (Nutr Res, 2016;36:464–77) and protein (Nutrients, 2019;11:1–18).

Evaluating People Who Have Abnormal Levels of B12
Vitamin B12 deficiency in North America occurs in about six percent of people under 60 and 20 percent of those over 60 (BMJ, Sept 4, 2014;349:g5226). Vitamin B12 deficiency can cause signs of nerve damage anywhere in your body; see Vitamin B12: One Supplement You May Need. Normal B12 levels are between 200 and 900 pg/mL. A B12 level below 300 pg/mL requires further tests for homocysteine and methylmalonic acid. If either level is high, you are likely to be B12 deficient and may already be suffering from nerve damage (The American Journal of Clinical Nutrition, July 2003;78(1):7–21) and should be treated immediately, since some of the nerve damage may not be reversible. If your B12 level is higher than 900 pg/mL, you need to be checked for possible causes such as liver disease, diabetes, or leukemia.

My Recommendations
A high-plant diet can help to prevent disease and prolong lives, but it can increase risk for nerve damage from vitamin B12 deficiency and possibly bone damage from lack of calcium or protein. A program of resistance exercise will help to strengthen muscles and bones. People who eat little or no animal products should have their B12 level checked, and if it is low, they should take at least 25 mcg/day of vitamin B12 pills or other source of B12.