According to Vastu Astro Guru- Manoj Jain, A good bath cleans and refreshes your body but it can be relaxing for the mind as well. Bathing is also known as “Snanam” in Sanskrit and is defined in almost all Vedas, Upanishads, Sutras, etc. in many ways.
The early morning 4 O’ clock is considered as the most auspicious time in the Hindu Religion. It is known as “Pratha Kala” or “Brahma Muhoortham” or the most auspicious time. Most of the “Rishis” (Saints) in Olden days used to take bath in this hour and attained the sainthood. “Parasara Smirithi” explains this very well as “rusheenaam rusheetha nityam prathas snanath na sam sayaha” i.e Saints always take bath well before early morning and start their daily rituals.
Manoj Jain tells that "The Kurma Purana" says that without taking the pratah-snana [bath before sunrise] one remains impure and cannot perform any of the daily activities a civilized person must perform such as Japa, homa, and deity worship.
In the opinion of Vastu experts, if a person eats without having bathed, he is said to be eating only filth, for everything he touches becomes as impure as he. "The Padma Purana" declares that one who does not bathe in the morning is a sinner fit to suffer on lower planets. Pratah-snana is compulsory for all, except those who are ill.
In Vedic culture, bathing is considered a sacred act to be accompanied by meditation on the Lord and recitation of prayer. The scriptures describe the benefits of taking a cold bath early in the morning. Such a bath can purify even a sinner, for it has the power to wash away all external and internal contamination. Whereas a warm water bath cleanses physically, cold water revitalizes the subtle body, removing the influence of sleep and dreams as well as of evil-minded persons. The cold bath also gives strength, sensitivity, longevity, effulgence, and purity. Taking an early morning cold bath increases one’s knowledge and determination and affords peace of mind. It removes unhappiness, lamentation, degradation, and bad thoughts. In short, it counteracts all the ill effects of sin.
In the Hindu, tradition water stands for rejuvenation, prosperity and the male-female principle. Lord Vishnu is the lord of the water and his consort Lakshmi the goddess of prosperity. Therefore, bathing in a river can activate the forces of Vishnu and Lakshmi in our life.
These are some scientific reasons given by Vastu Astro Guru - Manoj Jain why one should have to take a bath before eating.