Answer:
Allowing only one or two food types.
Step-by-step explanation:
Fad diets are popular diets for a specific moment, they are like "fashion diets" because people tend to do them due to the marketing that brands use frequently about specific products. These diets are always use with a marketing purpose to mislead people towards dramatic weight loss, but agencies and companies don't offer scientific proofs to costumers about it.
So, the fact that a fad diet is not professional standard, and it's not proven that's good to have a healthy body, becomes a dangerous food trend, because people will follow this "healthy food habits" without knowing consequences as malnutrition and unbalance nutrition, which affects health in long-term.
At last, an only one or two food types is a fad diet, because it will lead to a malnutrition, not giving what they body needs to function well. People always have to consider a professional help when losing weight, because not eating enough is also a serious problem.