The trouble is that vitamin B 12 basically exists only in animal foods. Although fermented bean products and some bacteria and algae foods also contain this nutrient, there is no human experiment to prove whether the vitamin B 12 contained in them is active. Reliable non-animal sources are only fortified foods and supplements (bacterial synthesis) added with vitamin B 12.
Vegetarians should at least keep eggs and milk if they don't eat fortified foods and supplements, which can largely prevent vitamin B 12 deficiency. Vegetarians in developed countries can easily buy fortified foods containing this nutrient, but fortified foods are not popular in developing countries, and people lack the awareness of supplementation. The most convenient way for vegetarians in China to take vitamin B 12 is to take supplements (the most common preparation specification is 25μg, and the recommended dosage for vegetarians is one tablet twice a day).
A reasonable vegetarian diet can bring a series of health benefits while ensuring a balanced nutrition, but it is difficult. It is recommended to choose an incomplete vegetarian diet, such as egg white (just don't eat meat), which is relatively easier to achieve a balanced nutrition. Even if you consider a vegetarian diet, it's best to start with egg yolk.
The key is to eat soybeans and their products regularly. Its nutritional value is similar to that of meat, eggs and milk. It is recommended to eat at least five times a week, and then eat more whole grains (recommended brown rice, millet, whole wheat and oats), dark green leafy vegetables (recommended sweet potato leaves, Chinese kale, Chinese cabbage, small rapeseed, Chinese cabbage, celery, parsley, spinach and amaranth) and fruits rich in vitamin C (citrus).
If you are eating out, such as students who live on campus, and the canteen lacks soy products, you can buy some soy products yourself, cut them into small pieces, scald them with boiling water, and add seasonings and cold dishes to eat. It is suggested to choose dried yuba and dried bean curd skin which are resistant to storage, but they need to be soaked in advance. It is more convenient to buy some dried tofu with low oil and salt directly from the supermarket.
In addition, it is suggested to buy some instant wheat germ to supplement nutrition, or mix it with milk, soybean milk, oatmeal, whole grain flour and sesame paste, or sprinkle it when steaming rice, cooking porridge and making pasta.
It is also recommended to buy some compound nutritional supplements in pharmacies (the key is B vitamins, especially B 12, followed by vitamins C, D, iron and zinc). However, "health products" do not need to buy nutritional supplements, because the state's supervision over drugs (including prices) is far greater than that of health products, so it is enough for "drugs" to buy nutritional supplements, which are cheap and of better quality.
For more vegetarian nutrition knowledge (including recommended intake of various foods and rich sources of key nutrients), please refer to the Vegetarian Dietary Guide: /p/220529 13.
Finally, it is suggested to pay attention to the topic of vegetarianism in "Zhihu" community, and Baidu Post Bar "Vegetable Food".