Very often religious beliefs may affect somebody's choice of diet. For example, kosher or halal restrictions on the slaughter of meat, and the forbidding of the consumption of certain foods, notably pork. Psychological emotional states may also affect somebody's decisions on his or her diet.
Food faddism is very common in New Age spirituality - compare Breatharianism.
Some of the scientific community say that food faddism is a product of humans' ignorance about basic scientific dietary facts.