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[–] 7759659? ago 

My guess is that they don't regain it all... or have a decrease in muscle. I highly doubt gut flora will make your BMR decrease by 800.

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[–] AxonCaradoc 0 points 3 points (+3|-0) ago 

Obesity creates a lot of fuckeries within the regulatory mechanism/signaling associated with metabolism, as does long term dieting/poor composition management.

Take leptin for example, this is the 'you're full fatass' hormone, it is actually secreted from fat cells. So, more fat, more leptin...and your brain becomes desensitized to it (there is a paper floating around that explains this pretty well if you google obesity, leptin, blood brain barrier). There is research going on in trying to find a way to apply leptin therapies but it's prohibitively expensive.

Long term low carb dieting tends to cause decreases (Although I'd say not clinically signicant) in thyroid output, your body produces T4 which is converted to T3 (the active form, hence it sometimes being supplemented)...can't remember if obesity attacks this directly but I've seen a handful of blood screens of thyroid function in obese subjects that were entirely normal (albeit these were 'fat powerlifter' types, so metabolically healthy if not aesthetically ideal).

The type of weight loss exercise regimens the uber-obese are put on tend to be devoid of resistance training and adequate protein intake which shy of anabolics are the only factors you can control to preserve metabolically active tissue (muscle) when the body in is an overall systematically catabolic state. You end up with a dieted down individual with less muscle mass than the average and wonky hormones.

Generally, depending on how obese an individual may be, they should try to keep body weight changes to 20-25% reductions per year for the changes to be sustainable...this might mean anywhere between 1-3 years of active dieting/lifestyle management to come out the other end of the process as an athletic individual.

However in our instant gratification world most people want to drop 100lbs in 3 months and then wonder why their hair is falling our, joints ache and simply smelling a doughnut results in weight gain (via eating the whole box because no moderation/self control has been learnt).