The mixture and balance of vata, pitta, and kapha in each individual is decided at conception based on the parents' genetics, nutrition, lifestyle, and present emotions.
The doshas that predominate in the parents unite to produce the constitution of the new life they are generating, as stated in the introduction.
If the father is pitta dominant and the mother is vata dominant, and the pitta factor is greater than the vata factor, and the union occurs on a hot summer night after eating a spicy dinner, the baby born will have a pitta dominant constitution. If both parents are kapha, eat a kaphagenic diet, and make love during kapha season, their child will have a kapha dosha predominance.
Few people are born with a constitution in which all three doshas are equally present, giving them the best chance of good health and a long life, but the majority of us have one or two dominating doshas.
NOTE: Just because Ayurveda advises that balancing your doshas is important for health doesn't imply you should aim to have equal levels of vata, pitta, and kapha. Rather, it entails sustaining the equilibrium with which you were born.
Your prakruti, which means "nature," is the unique and specific mixture of the three doshas at creation. It's your psychobiological personality type.
Prakruti does not alter during the course of a person's life. For example, your prakruti may be largely pitta, with secondary vata and a smidgeon of kapha. (V2P3K1 is another way to write it.)
Maintaining balance for you requires maintaining this proportion. It would not be beneficial for you if your vata or kapha increased, resulting in an equal amount of the doshas.
The equilibrium of the doshas in our mind-body system fluctuates when conditions change—due to weather, food choices, exhaustion, stress, emotional state, exercise or lack thereof. Our vikruti refers to the changing status of the doshas as it relates to our current condition of health.
Your present doshic state will be the same as your prakruti if your health is superb. However, there will most certainly be a disparity, and it is the difference between the two that determines the healing path. Your goal will be to regain the equilibrium that your prakruti has revealed.
You may also want to read more about Ayurveda and Holistic Healing here.
