Actually, it's after ANY large amount of weight lost due to calorie restricted diets, even WFPB no S-O-S ones. Some of the Star McDougallers frequently mention how they have to eat less and less starch, less calories, first to lose those last few pounds, then to maintain their lower weight. Some Stars lost over 100 pounds, regained, and lost them again.
Doctors, especially obesity experts, have known about this happening for DECADES, which is why they now push for weight loss surgery and weight loss meds.
After ‘The Biggest Loser,’ Their BodiesFought to Regain Weight
Contestants lost hundreds of pounds during Season 8, but
gained them back. A study of their struggles helps explain
why so many people fail to keep off the weight they lose.
And this is the Obesity Journal article about the study referred to in the above article.