When people have lost sufficient weight, often they are caught out with the transition to increasing their intake. Even those people with a steely resolve to 'eat healthily' can come unstuck, and as a result throw in the towel and return to bad habits.
The problem can be that they increase their food intake proportionately, so a little more fat, a little more carbohydrate, a little more protein, or in whatever ratio they have been eating.
This is a mistake; if we think about it, any fat loss has resulted from the use of stored body-fat (animal fat btw) for fuel. If they now wish to stop the dwindling of those reserves, the logical step is to replace its use calorie for calorie with ingested natural fat to retain the same macro-nutrient usage ratio that they obtained whilst dieting.