Body weight equals "what goes in" (the food you eat) minus "what goes out" (the energy you burn).

Think of it as an hourglass. Filling the top with sand is like energy input. The sand emptying into the bottom is like energy output.

If you pour in sand faster than it goes out the other end by eating more food than you need, the hourglass fills up, and you will gain weight. If you balance the input and the output, then your body weight does not change. If you increase the amount of energy that you burn, your body weight goes down.

Now, let's take a look at how the body increases size and strength. The three keys to increasing strength are:

Training - A challenging exercise program combined with a balanced weight-lifting program is the key to adding muscle. It is important to note that the general guideline for the appropriate age to begin weightlifting is high school. Nutrition - When you eat a well-balanced diet, your body has 100% of the building-blocks necessary to build muscle. Sleep/Rest - Your body produces the most growth hormone when you are sleeping. So, getting at least eight hours of sleep is one of the most important things you can do to help your body grow.

If an athlete trains the right way, eats a balanced diet, and gets enough rest — their body is fully equipped to add muscle naturally and safely. There is no safe shortcut to adding strength and size.