The human body is actually very energy efficient. It knows that it may not get enough nutrients throughout the day. What it does is to segregate all of the things that will be good for you. It will allow those that you do not need to be classified as wastes. If the human body is not efficient, this can become a problem.
People may get tired very easily even if they have just made a small amount of effort. One thing that you should remember is that your body is a type of machine. Like all the other machines, you need to maintain it properly and it will serve you well.