Logic behind Ekadashi fasting-
एकादशी is the eleventh lunar day (tithi) of each of the two lunar phases (waxing-shukla pasha and waning-krishna paksha) which occur in a vedic calendar month.
Adult body contains up to 60% water. Moon being the closet to the earth on this day++

The gravitational force of the moon exerts its influence on the water in the human body as it does in the case of the oceans of the planet, causing biological high tide and low tide too. ++

The fortnightly Ekadashi fasting accompanied with healthy eating improves insulin responsiveness, lowers blood cholesterol and prolongs life span. It helps improve the mental stability of people suffering from anxiety and depression. ++

It also detoxifies the body, cleanses the blood and improves the functioning of kidneys and liver. It is amazing how the ancient Vedic Indians devised this method to keep ourselves fit and free from any negative influences!

observed by partial or complete fast. High protein and carbohydrate-containing foods such as beans and grains are not consumed by observant people during the fast as it is a day to cleanse the body. Instead, only fruit, vegetables, and milk products are eaten. ++

This period of abstinence starts from sunrise on the day of Ekādaśī to sunrise on the following day. Even if you dont follow fasting, avoid eating anything that absorbs a lot of water- like rice, etc.

Ekadashi fast is also mentioned in Vedic medical texts of Ayurveda and is mentioned in detail in many original treatises such as Charaka Samhita and Susruta Samhita.