Some world religions have dietary restrictions which are observed by their followers as part of their religious practice. Most of these religious dietary restrictions are laid out in the holy books of the religion, such as the Bible, Vedas, Qu'ran, Sutras, and Torah. Depending on the religion, dietary restrictions can be quite severe, as is the case with the Jainists, or relatively simple, in the instance of Presbyterians. There are a number of reasons for religious dietary restrictions to be put in place, but in most cases the restrictions can be disregarded in an emergency situation, as would be the case with a Jewish man stranded on desert island with only ham sandwiches to eat. He would be allowed to eat the prohibited ham, as it would be preferable to starving to death.
One of the most common reasons behind religious dietary restrictions is food safety. Many religions were founded in hot climates in an era long before refrigeration. As a result,some have restrictions on eating carrion, as it may be rotten, or notoriously unstable fish like shellfish. Some specific animals are restricted because they are believed to have unclean living habits, or because they require a great deal of energy to raise. In some cases, foods are restricted because the native population is allergic to them. In this sense, dietary restrictions protect the followers of the religion, allowing them to grow fruitful and multiply.
Another basis for religious dietary restrictions is religious beliefs. Buddhists and Hindus, for example, believe that they have a duty to reduce suffering in the world. For this reason, many followers of these religions belong to vegetarian sects, because animal-derived foods often involve suffering. In other instances, such as the sacred cow in Hinduism, a particular plant or animal is sacred or holy, and should not be eaten.
Many dietary restrictions are closely related to fasting. A number of religions around the world practice fasting as a way to increase spiritual purity and growth. In addition, it teaches followers self-will, and helps them to resist temptation. In times of food scarcity, the practice of fasting might also have helped to make life more bearable, as fasting in the name of God is easier to endure than fasting because of famine or poverty.
In some cases, religious dietary restrictions address specific issues, as is the case with gluttony, or intoxicants such as alcohol and coffee. In these instances, overconsumption of these products can be dangerous to someone's health, or to social well-being. As a result, priests counsel moderation in consuming these products to ensure that their followers lead sober, productive lives. While some of these religious dietary restrictions might seem excessive, such as the total ban on alcohol for devout Muslims, they make sense when one examines the context of where the religion originated. In the case of Islam, a ban on dehydrating substances makes sense for followers of a religion founded in a desert environment with limited water.