I ask this question because I'm worried about my parents. My dad is in his early 70s and my mom will be 70 next year. They were both overweight a few years ago and my mom had been diagnosed with fatty liver disease. They have lost weight quickly through a commercial weight-loss plan* but whenever they gain it back (after a vacation or Christmas, for example) they go back on the weight loss plan to lose it again.
It's my understanding that frequent dieting results in loss of muscle mass, which slows metabolism, which makes it harder to lose weight the next time around. But I'm wondering if there's any scientific basis for this. Can anyone point me in the direction of some relevant studies?
(*The diet involves replacing several meals a day with weight-loss shakes and bars and so far as I can tell they are probably getting fewer than 1000 calories a day when on this diet, possibly as low as 800, which really worries me ... they are advised not to exercise or exert themselves in the first "phase" of the diet which makes it seem like they're probably on a starvation diet.)