How many times have you successfully lost weight -- only to gain it again further down the track?
Some people struggle with this for most of their lives. This is "yo-yo" dieting - a continuing pattern of gaining and losing weight. Sometimes it is called "weight cycling".
To quote one research paper:
Weight cycling may also have negative psychological and behavioral consequences; studies have reported increased risk for psychopathology, life dissatisfaction, and binge eating.
When your diet includes bouts of starvation, missing meals, or very low calories - you are setting yourself up for the yo-yo effect.
While it seems that good results can be achieved quickly, sooner or later, the body gets the hint that less calories are coming in. The body adapts, and decides that energy must be conserved - and metabolism (the way the body burns food for energy) begins to slow down.
Often this can be a 'plateau' - the weight loss that was initially achieved begins to slow down or stop.
Many people hit the wall at this point, and, struggling to maintain the diet, the weight begins to come back on. And what's even worse, is that for some people, the lowered metabolism means they end up with more weight than what they started at!
This is one of the problems with dieting (as a sudden fad) - the body's metabolism becomes less efficient with each dieting episode. The severe calorie restriction also causes loss of muscle tone (catabolism).
Less muscle once again means a slower metabolism.
The commons signs of this are flabby upper arms (due to loss of tricep muscle tone and subsequent fat gain).
It is a mental and emotional blow to go through all that dieting, just to be back where you started.
Rather than suddenly dieting - hoping for a quick weight loss, it is better to take a more long term sustainable approach.