Well, you've guessed it :-), I've found an excerpt in an article that explains how this could be the case. Hold on to your hats cos it does get a bit techy. I'm logging it hear so that if the idea holds water then I may write a laymans version in time
It's not actually the whole article just a few paragraphs so rather than send you there and tell you to scroll down to some random point I've taken the liberty of pasting it here. Please check out the original article at
One thing I’m wondering about is what actually signals the body to notch down metabolic rate. I have heard people say that it’s the calories ingested, but I suspect it’s not that simple. I wonder if consumption of muscle protein does this for example. Do you know what triggers it?As discussed in the second post, one of the effects of short-term fasting is a decrease in the thyroid hormone T3. We already know that T3 (and T4 to a lesser degree) is directly responsible for metabolism, so it’s not too hard to deduce what causes the metabolic rate to fall. T3 conversion falls, metabolism falls - simple as that. But why does T3 conversion fall?
Well, for that, we’ll look back up to the previous question and just say one simple word: cortisol. Cortisol blocks the conversion of T4 to active T3, along with promoting the production of a thyroid hormone called “reverse T3″. This mirror image of T3 has an empty iodine receptor; it binds to thyroid receptors in cells, but does not activate them. By binding to these receptor zones, it blocks the action of T3. It’s easy to see why chronic stress, and therefore chronically elevated cortisol levels, helps push one towards obesity. It’s not just the late nights at work with fast food for dinner (though that doesn’t help); the stress is actually causing a chronic depression of the metabolism, along with the immune system as seen above.