2.40 From purity follows a withdrawal from enchantment over one's own body as well as a cessation of desire for physical contact with others.
Purity is a fickle thing. It can easily be overdone. How pure can you be at anything? Yogis strive to be pure in all things, such as speech, diet, and practice. In my personal experience, purity is not obtainable and the act of striving is the practice itself.
Here is an example. How many of you have tried to eat well? Have you tried to become a vegetarian, or vegan or perhaps cut sugar out of your diet? I tried being a vegetarian for a while, thinking that my own purity would lead me to vaulted spiritual heights. When you change any habit, your body and mind will naturally crave the old ways until your physiology adjusts. So, when I became a vegetarian, I just ended up consuming large amounts of sugar to replace the protein that I was missing. I know about trying to replace protein with vegetables like garbanzo beans, but unless you don't mind farting up a storm, it is just not worth it.
Striving for purity is meant to show us the things we have been covering up in our lives through vice. Yogis will fast because it is said to be good for the body and eliminate toxins. Another interesting thing happens when you fast: all of your natural inclinations will be revieled to you when you crave food. When I have made attempts at fasting, different emotions would come into my mind such as anger, frustration and depression. If I paid attention, I could write down the thoughts that entered my mind for further review later.
A practice taught to me by Swami Prananda of the Temple of Kriya Yoga is a good habit for fasting, and does not require you to starve yourself. At the beginning of you meal, look for the portion that is most desirable to you and separate it. At the end of your meal, throw it away as an offering. It sounds odd and you might think you are wasting food, but you will see over time that thoughts will enter your mind, like they did mine, when you deprive yourself of your favorite part of your meal.
The bottom line is, purity is something to be striven for and not accomplished. Remember to eat a healthy diet and do not try to live off of sunlight, or like some people, drink you pee. That's just nasty.