Losing unnecessary weight is hard enough, but the real challenge is maintaining weight loss. This is because the body, and brain, has natural mechanisms to fight off against ANY change in weight. This is because the body gets used to whatever weight that you may have been for long periods of time. Such mechanisms include feeling much hungrier than usual as well as burning calories more efficiently to burn overall less calories. This means that if someone has been, for example, 200 pounds for many years and than suddenly loses 20 pounds, that person will feel hungrier and burn less calories than someone that has been 180 pounds for years. This can be summed up by your body getting "used to a certain body weight".
This is a very important fact to always keep in mind when trying to maintain any healthy weight loss. Thus you should expect to feel hungrier, and also expect that you would need to eat less calories than others at the same weight body who have been at that weight for a long time. But the good thing is the your body will gradually adapt to the new body weight, and thus gaining weight becomes harder!
This process also works in the reverse so if you want to gain more weight, expect it also to be difficult initially but gradually this will be come easier as your body gets used to the change.
Source: http://www.livescience.com/53942-weight-loss-biology.html
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