Adaptive

Adapative thermogenesis is the actual term to describe that much maligned
metabolic adaptation experienced by those who loose weight quickly. Makes sense:
the individual is suddenly faced with less food, or needs to be more active, and
metabolism adapts through thermal activities. In effect, the thyroid gland is the main
player here. If it didn't, the individual would be very vulnerable to changes in environmental
conditions, and not much of a candidate for survival. So starving the person starves the
thyroid, with the effects that we observe. The ability to procreate is the last to go.

What can be done to counter this: the fitness community advises - after a show, for example -
reverse dieting. Rather than an abrupt return to normal food intake, the individual slowly adds
foods back to the diet. Metabolic function follows along.

One interesting research finding I stumbled upon had to do with the fate of those who had gone
through bariatric surgery. One might assume that given  very large amounts of weight loss, these
individuals would suffer the most form a damaged metabolism. Not at all: it is those who got
thin by dieting who are cursed. The bariatric patients exhibit a return to normal metabolism
because - through no will of their own - they have followed reverse dieting guidelines. From
eating just liquids, then a few solids, a bite more here, another there, they have reset their
metabolism.

 http://www.jtsstrength.com/articles/2014/07/11/need-know-metabolic-adaptation/

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 Where I would have questions would be with respect to the nature of the foods
ingested. If one suddenly cuts out carbs - which is what a body builder going to
competition might be doing - this might lead to the need for a quick metabolic
adjustment. Whereas a student going on a diet, and cutting out fast foods for a while,
might not see such a quick response because burgers and fries are high fat and protein.
Need to ponder this a bit more...

The Biggest Looser victims were not just cutting down carbs, but also doing more cardio
(which burns quick energy). The body might have adapted because it literally ran out
of cheap fuel. Ingested protein is - on a first pass - used for body building and repair, and broken
down to be used as fuel only if it is in excess of need.

In the same vein, our student dieter cutting out fats might well lack the elements necessary
to produce cholesterol, testosterone (which regulates metabolism) and other thyroid hormones.
So the metabolic slowdown for the student will be the result of another process.