The vegan diet is a new fad that will come and go like all the rest.
Veganism is a philosophy and way of living which seeks to exclude - as far as is possible and practicable - all forms of animal exploitation and cruelty for any reason, including medicine, food, clothing, entertainment or for any other purpose. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals. In short, veganism is a philosophical position and not a diet.
The philosophy of veganism has a long and varied lineage. Donald Watson defined the term 'vegan' as paraphrased above nearly a hundred years ago in conjunction with The Vegan Society. Before that, many notable thinkers and writers such as Dr. William Lambe and Percy Bysshe Shelley objected to eggs and dairy on moral grounds. Buddhism has a great history of compassion for non-human beings that includes many teachings in opposition to the consumption of meat. Pythagoras also objected to the slaughter of animals for his table, giving rise to the term 'Pythagorean diet', which was used by many ethical vegetarians until the 1800s, when the term 'vegetarian' itself was coined. So it is clear that a plant-based diet is merely one manifestation of an ancient and compassionate philosophy that values the lives of non-human beings and seeks to do them no harm.