Fire & Essence
The use of a candle as an analogy is actually quite accurate according to Chinese medical theory. Life is seen in TCM as a series of warm transformations. The root yang of the entire body is called the ming men zhi huo or the fire of the gate of life. This life fire is responsible for all activities and transformations in the body. We live only as long as this fire of life burns within us and we are stone cold dead when it burns out irrevocably.

This life fire is associated with or has its material basis in the Chinese idea of the kidneys. In Chinese medicine, the kidneys are the fundamental, first organ. They are called the xian tian zhi ben or former heaven root. This means they are the prenatal foundation of the organism, both its form and function. The original source of function is the life fire described above. Whereas the most essential material basis or pure substance is referred to as the jing essence or shen jing, kidney essence.

In Chinese medicine, there are two types of jing essence. There is xian tian zhi jing or former heaven essence. This is innate at birth. We are born with a finite amount of this former heaven essence. It is our endowment from our parents and the universe at large and it is stored in the kidneys. This former heaven essence is supplemented by what is called hou tian zhi jing or latter heaven essence. This latter heaven essence is manufactured out of the air we breathe and the food and drink we consume. Nutritive essence derived from food is transformed into qi and blood. Qi empowers function and blood nourishes form. As we move through each day, our activities consume both qi and blood. If, when we go to sleep at night, we have manufactured more qi and blood than we have used that day, this excess is transformed into acquired or latter heaven essence. Some of this latter heaven essence is stored in each of the five major organs of Chinese medicine -- the heart, lungs, spleen, liver, and kidneys. However the major portion of this acquired essence is stored in the kidneys which then form the Fort Knox of the body.

Every metabolic activity, every transformation within the organism requires both some life fire and some jing essence to act as catalyst and substrate respectively. If there were no acquired essence, we would be just like a candle. We would only be born with so much fuel and that would be used up fairly quickly. However, because latter heaven essence, derived from our diet, supplements our innate former heaven essence stored in our kidneys, this former heaven essence is capable of lasting a lifetime.

Longevity, Diet & Lifestyle
Chinese medical theory believes that the human organism is built to live 100 years. According to the first chapter of the Nei Jing, the premier classic of Chinse medicine, most people have enough jing essence to last 5 score years. Barring accidental death or infectious disease, we are designed to last 100 years as long as former heaven essence is not squandered by excessive consumption and as long as latter heaven or acquired essence is manufactured and stored to bolster and slow the use of former heaven essence. Since latter heaven essence is manufactured from the food and drink we ingest, it is no wonder that Chinese medical theory places such great importance on proper diet and promoting good digestion. Likewise, since acquired essence is stored in the kidneys at night when we sleep, it is no wonder why proper rest and sleep are important as well.