Why Does a Starvation Diet Slow Metabolism?

We hear that starvation diets will slow your metabolism down and your body just adjusts for it and uses less calories. It's true. But how much, and how does it work?

It can lower your metabolism by as much as 40%, and if you return to normal eating, it doesn't just go back to normal. From a handout at Horltorfmed.com-

Starvation dieting can decrease resting metabolic rate by as much as 40% and food restriction at a level to maintain just a 10% reduction in body weight results in significantly decreased intracellular thyroid hormone levels and a diminished metabolic rate that does not return to normal even after a normal diet is resumed.

How does it work?

T3 (triiodothyronine) is the active thyroid hormone that affects metabolism, and a lot of other things. Your body can also produce an inactive mirror image of T3 known as reverse T3, or rT3. When you start to starve, your body knows to use rT3 instead of T3 to keep it going on less calories.

...(Reverse T3) is an evolutionary fall-back that was useful in times of famine or in hibernating animals to lower metabolism. Studies are showing that stress and dieting (especially yo-yo dieting) can set this hormone into action as well as chronic illness such as diabetes, chronic fatigue syndrome and fibromyalgia.

The production of reverse T3 is found to be a major method by which the body 'tries" to regain any lost weight with dieting. As soon as the body senses a reduction in calories, the production of reverse T3 is stimulated to lower metabolism. With chronic dieting or stress, the body often stays in this "starvation mode" with elevated levels of reverse T3 and decreased levels of T3, which is a major reason for the regaining of lost weight with dieting as well being the mechanism behind stress induced weight gain (it is not due to increased cortisol)...

Source...

This is why trying to diet by drastically reducing calories doesn't work. Not only does it not work, when you give up, it makes dieting afterwards even harder to do.