'' is onion good for spiritual progress"??

quote from yahoo answers
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Religiously and spiritually what are the reasons for not consuming onion and Garlic in Hinduism and Jainism.
Obviously the question is not from the angle of health or from any other angle

Garlic and onion both have many health benefits but that is not the point the questioner wanted; he wants only spiritual reasons for avoiding garlic and onion.
I am not going to quote references from Scriptures and Puranas as it has been elaborately given by other knowledgeable members.

I am going to support my answer from practical view point only.

About Jains not eating Garlic and onion
Jains do not eat Garlic or onion, that is a part of their religion;
The Jains have very strict dietary rules; Not only they are vegetarian, but they do not eat anything with seeds (this would prevent the plant from renewing its life) or any bulb for the same reason. They also believe that eating Garlic and onion increases the sexual desires.
Jains (a sect of Hindus in India) in their efforts to eat with the least amount of violence won’t eat plants that grow underground, including potatoes, because animals may be harmed in their harvest.

Now coming to some Hindu spiritual aspirants not eating Garlic and onion.

One possible reason is that as onions, ginger, or garlic, give off a strong odor and when a group seven or eight Sadhus/ Rishis meditate in forests or in a caves in a circle and if somebody ate those things, it would produce a strong, pungent odor that would affect other people meditation and their ability to become still inside, so it would seriously interfere with other people practice; That why they considered those foods taboo and made them off limits.

It is not that all Hindus avoid eating Garlic and onion, only those who are in the process of spiritual ascendance avoid eating Garlic. The goal of all spiritual sadhana, is actually to cleanse oneself of all the internal impurities. Kama, Krodha, Lobha, Moha, Mada, and all other impurities should be completely annihilated. Two very important niyamas are a must for this sadhaka to achieve moksha. Observance of celibacy is the first. This can be acquired gradually. Avoidance of onion and garlic, which are rajasic foods, is the second.' A Sadhaka should observe perfect discipline. He must be civil, polite, courteous, gentle, noble and gracious in his behavior
He must have perseverance, adamantine will, asinine patience and leech-like tenacity
in Sadhana. He must be perfectly self-controlled, pure and devoted to
the Guru.
Food plays a prominent place in Yoga-Sadhana. Food plays a prominent place in Yoga-Sadhana. An aspirant should be very careful in the selection of articles of Sattvic nature especially in the beginning of his Sadhana period. Later on when Siddhi is attained, drastic dietetic restrictions can be removed. Purity of food leads to purity of mind.

The discipline of food is very necessary for Yogic Sadhana. If the
tongue is controlled, all the other Indriyas are controlled.

Garlic and onion is a catalyst to reproduce the sex desire in us.
It should be pointed out that Garlic and onion are avoided by spiritual adherents because they stimulate the central nervous system, and can disturb vows of celibacy.
As well as producing offensive breath and body odor, these (alliaceous) plants induce aggravation, agitation, anxiety and aggression which a Spiritual Sadhaka must avoid.
For meditation, one needs to concentrate.

If there is blaring mike sound or strong odor both will disturb the mind, and concentration becomes very difficult.

Food items prepared mixing with Garlic and onion give off profound smell that would impair Deity worship, effectively disturb the sadhaka from performing his rituals.

Many Hindu homes avoid using Garlic and onion on in their food item on Ekadasi/ Dwadasi and other pooja days. Imagine that you are doing a daily prayer and if the neighbor cooks Karuvadu (dry fish) mixed with garlic and onion, the odor coming out from cooking would become unbearable for you to concentrate on your daily prayer.

Temple Prasadams never contain Garlic or onion.
In the temples where there is a crowd, some bad smell may emanate from someone; this will surely disturb meditating on the deity. To nullify the effect of bad odor, we find Scented Dhoops, Samprani, Camphor are lighted.

Reiki practitioners explain that garlic and onions are among the first substances to be expelled from a person’s system – along with tobacco, alcohol and pharmaceutical medications.

Saint Viswamintra losing to Ramba has a purpose; their union has produced Sakuntala and through her son Bharata, and our country got the name Bhāratavarṣa; and "Bhārata" today is the official name of the Republic of India
Similarly Parasara Muni’s affair with fish smelling ferry-woman is to give a gift to this world in the form of Vyasa, who later composed the Mahabharata.
God Bless you all'' unquote