To truly understand the Taoist diet you have to first understand a little bit about Taoist beliefs. Taoism comes from the core of East Asian and Chinese culture and has roots as deep as 2000 years, although it has only spread to the west in more modern times as people begin to reject materialism for deeper spiritual understanding.
Taoists are egalous humble people that emphasize compassion, humility and moderation - the latter of which is stressed through their minimalistic eating habits.
Although not known for their rule breaking because of their caring non active views, Taoism focuses on the human connection with nature and therefore they do not believe in the rigid and orderly ways of modern society, preferring to follow the natural flow of the Universe. The common Taoist term Yin and Yang reflects to the positive and negative energies of the Universe.
The five colors blind eyes.
The five tones deafen ears.
The five tastes blur tongues.
Fast horses and breathing hunts make minds wild and crazy.
Things rare and expensive make people lose their way.
That's why a sage tends to the belly, not the eye,
always ignores that and changes this.
- Tao Te Ching, Part 12
Historically, the Taoist diet has basically of mainly fresh fruits and vegetables, with little meat and no grain - as they thought during the digestive process demon like creatures would have released from the rotting grain and attempt to eat them from inside out. During more contemporary times, the diet has changed to be primarily based around the acceptance of whole grains, as well as the fresh fruits and vegetables of tradition.
The Taoist diet relates the five basic flavors with an element of nature: sweet (earth), salty (water), sour (wood), bitter (fire), spicy (metal). They believe that becoming greedy and putting one flavor on a pedestal above another causes you not to taste at all, so it is important to balance the differences in order to reach internal harmony.
Taoism is all about the natural, and humans being part of nature. One of the most important beliefs is to 'eat only food' - meaning to avoid unnatural man made substances that the body can not process and may contain unbalanced flavors, such as artificial additives, drugs etc. Heavily processed foods that contain little or no nutritional value, such as white flour, sugar and fast food are also considered inedible. These are not things that the body is designed to consume and do not grow from the earth, so are not really natural 'foods' fit for human consumption.
Historical
In much of the classic Taoist literature, a lot of mention is made of the sagacious men of old - or, people who existed in pre-history. Several of the texts talk about them existing only on breath, and not consuming food at all. They lived as they were born and only gained sustenance from the qi or Yin Yang from the Universe.
This practice, known as "Bigu" is sometimes employed within some of the Taoist hermit traditions and mythological ideas, but it's not something that's practical or even safe for modern people, living in normal society to try. Taoists believe that the human state has altered and the ancient state has since fallen, meaning it is perfectly acceptable to eat foods.
The earliest Taoists are believed to have had a diet that reflected this notice of sagacious and enlightened masters from before history - and also before the development of agriculture. Thus in the earliest traditions, grains were not to be consumed by Taoists.
The reasons for this could be many - from health concerns, to a reverence for some mythological, pre-agricultural past, and even other social factors. The minimalist approach is often used to explain it, stating that Taoists live off more than food alone and subconsciously gain energy from the cosmic.
However, as alluded to earlier the reason provided in many of the early texts for not eating grain is to not arouse the "Three Worms".
-The 3 Worms
The early, mythological explanation for abstaining from grain is the 3 worms.
These are literally 3 demonic worms that were said to live in the intestines of human beings that were responsible for the decomposition of your body after death.
Of course, as their goal is to devour your body, it's in their best interest that you die as quickly as possible.
Before death the 3 worms would live in a person's intestines, feeding off the rotten bio-matter being digested.
Therefore, as your intestines digested the grain, the 3 worms would eat the waste that was produced. As they fed on the grains, they would grow stronger, and later be able to feed off the rest of your body, causing you to die more quickly.
Since longevity for continued cultivation is one of the primary goals of many Taoist practices, the object of the diet was to "starve off" the 3 worms, by lowering your intake of grain, or eliminating it completely.
From a modern perspective, it could be that the earliest Daoist simply noted a correlation between caloric intake, and aging, or ill health.
Assuming that a cell has a finite number of possible divisions during it's life cycle, it would be necessary to dramatically slow down the metabolic process in order to slow down the process of cell division.
Another previously mentioned possibility is just the reverence for a pre-civilized, pre-agricultural period in time, where men were married, nor were they engaged in the social activities and games of a surplus food producing culture.
The Third Immortal King told the Emperor:
"You attain the Tao by avoiding all grains. You will never again have to follow the rhythm of the moon and plant or harvest.
"Now, the people of mysterious antiquity, they reached old age because they remained in leisure and never ate any grains."
As the Dayou zhang (Verse of Great Existence) says:
The five grains are chisels cutting life away,
Making the five organs stink and shorten our spans.
Once entered into our stomach,
There's no more chance to live quite long.
To strive for complete avoidance of all death
Keep your intestines free of excrement! "
While many ancient Taoists practiced abstention from grain, this is not absolutely true. There are many accounts of Taoists who ate, or who literally begged for rice.
It may be that grain abstention was more of a purification process, or a sort of fast, leading up to important rituals, ceremonies or rites, like taking long medications, fasting, taking elixirs, and so on.
Modern
In more recent times, the typical diet has radically changed to focus on being PRIMARILY grain based, rather than practicing a total abstention from grain. Although there are some radical people who claim to never eat, they are often ridiculed by the media and later found out to be "starving" for attention rather than being a true ancient style Taoist.
The modern Taoist diet essentially follows the basic yin-yang and 5 elements theory, and treats heavily upon un-processed whole grains, fresh vegetables (particularlyly root vegetables) and very little meat.
It is important that vegetables are eaten in the right seasons and are either steamed or stir fried. Boiling takes out the natural goodness. Fruits tend to be dried or baked and eating tropical fruits is frowned upon as unbalancing the five flavors due to their strong, often citrus tastes. It is also important that they are seasonal, and free of any man made intervention.
Typically, all red and blue meats, including pork, rabbit, snails and the like, should be avoided. Poultry and game birds are OK to eat, as well as fish. However, fish and other seafood should only be eaten once a week because of their high Yin quantity. Some fish like salmon, shark, swordfish and mackerel, which are highly Yin should be completely avoided.
Consuming alcohol, caffeine and chewing / smoking tobacco is frowned upon because of their refined nature.
The Modern Taoist relations on moderation in their eating habits, and should try to avoid consuming anything too pungent (garlic, ginger, onions, etc,) and stay away from as many preservatives as possible.
Differences Between Taoist And Modern Western Diets
In the west, the life style and dietary habits have contributed to the dramatic rise in such problems as heart disease, obesity, stress, cancer, arthritis and so on.
The emphasis has moved away from the initial prevention (by eating naturally and healthily) towards drugs and surgery. An ounce of prevention is worth a pound of cure however. Why try to fight a disease once it has taken root, when with some simple guidelines we can avoid it in the first place?
Taoists believe that what is primary importance is natural health, not doctors and medicines, and this can best be achieved through eating natural foods. Remember that the body regenerates itself, the skin tissue and organs take about 2-3 years, even the bones are replaced after seven years, and it is all built from what goes into your mouth.
Nature can do its work but only if given the right tools. Ideally natural foods have been grown organically without the use of artificial fertilizers, chemicals or pesticides.
The Modern Taoist diet, in contrast to the modern western diet is:
Low Fat
High Energy
Vitamin and mineral enriched
Easy for the body to digest
Unrefined and processed
This means daily western items like bread and milk, which we think are perfectly healthy are considered almost toxic to strict Taoists. Instead rice and soya milk is used as a replacement and skimmed milk is generally accepted.
The "Ground Up" Approach
Qigong: returns to the set of Taoist exercises used to maintain and move with the qi (energy of the universe). Methods include meditation and focused physical movements. This helps to maintain physical and mental health.
Generally, in many types of Taoist Qigong, energy is drawn from the earth, upward. Similarly, the concept of "rooting" is the base of tai chi and many of the Chinese and Taoist martial arts, so historically, and within the Taoist context of power, from the ground up was thought of as the best way to get vital energy from food.
As with Taoist Qigong, Taoist diet also generally stresses a "ground up" approach to the consumption of vegetables. That is, that plants should be consumed in high percentage of total diet, especially those below ground (root vegetables) as opposed to those higher up, eg an apple.
The main reason for this was that the earth bound vegetables have more energy and the ability to deliver more qi to the body. Yams, all types of root crops, potatoes, carrots, turnips to name a few, were thought to deliver good earth energy, which helped the spleen (immune system) become stronger, and made jing qi more "rooted."
After the earth bound plants, greens like cabbages, bok choy, spinach, and so on, which were often pickled or preserved for winter use.
Next came the higher crops - peppers, tomatoes, eggplant, etc, which were used to deliver good energy, but in smaller proportions.