Why does weight loss come slowly following rapid loss during the initial three weeks of a rigorous diet?

 The body will behave as if it's being starved and the metabolic rates decreases.

Explanation:

Experimental Studies showed that individuals who used starvation diets for weight loss, consuming 50% of their body energy needs for three weeks, did have a significant reduction in their body weight entirely. It is also interesting to note that, if they lower their lean muscle mass by 5% it will reduce their lean muscle mass and organ size by 20%.

This leads to metabolism, resulting in a decrease in rate of metabolism causing a breakage of certain muscle stores so as to generate energy. Starvation diets is proven to give a far-reaching negative impacts on the body as starving to lose weight causes a decreases in the metabolic rate.