Navratri fasting emphasizes the consumption of sattvic (pure) food items that promote purity, lightness, and positivity. 

Onions, along with garlic, are considered rajasic and tamasic in nature, which are believed to stimulate desires and restlessness, hence they are avoided during this time. 

 By abstaining from onions, individuals aim to cleanse their bodies and minds and maintain a sense of spiritual purity throughout the fasting period.

Onions have a strong and pungent flavor, which is believed to disrupt the balance of energies within the body, particularly during periods of fasting and spiritual practice. 

During Navratri fasting, when individuals may already be eating a simplified diet, avoiding foods that could potentially cause digestive discomfort is a common practice.