By the way, here is an excerpt from the blog of an obesity researcher:

First, the data on yo-yo diets, otherwise known as weight cycling. Looking at the most recent and robust data, one set from than Cancer Prevention Study II Nutrition Cohort which followed 55,983 men and 66,655 women from 1992-2008, and the other set from the Nurses Health Study which followed 44,882 women from 1972-1994, neither demonstrated any relationship between weight cycling and mortality. Other studies have exonerated weight cycling from increasing the risk of hypertension, and type 2 diabetes, and there's a mixed bag of studies suggesting both protective and causal effects of weight cycling on various forms of cancer.

http://www.weightymatters.ca/2012/03/why-haes-may-never-go-mainstream.html

When you think about it, there's some sense to this. It's reasonably to hypothesize that the human body is adapted to weight cycling (at least to some degree).

However, if very few people keep the fat off, then the effects of regaining it also need to be considered.

Yes, I completely agree.