What is a vegan diet?
The definition of veganism is: “The practice of abstaining from the use of animal products, particularly in diet, and an associated philosophy that rejects the commodity status of animals.”
It is similar to a “plant-based diet” but vegans eliminate all animal products, while plant-based diets do not necessarily eliminate animal products, but focus on eating mostly plants, such as fruits, vegetables, and nuts.
As for why a vegan diet might help someone lose weight, a lot of the foods consumed are low in calories, thus instantly putting someone into a calorie deficit if not managed correctly.
source: express.co.uk