Maha Periyava has mentioned a lot about Upavasam. Especially Ekadasi upavasam.
Mahan has said ” Fast on Ekadasi and eat moderately on other days” We will NEVER get into trouble if we meticulously follow his order. Try fasting on Ekadasi. Then you will understand the benefits of it. As our stomach Is conditioned to be small, the next day you cannot eat your normal amount of food and if you continue in the coming days to maintain the same amount of food intake, that is the best think to do for a healthy life style. Have to stop at first burp. Eat only when hungry. Eat slowly. Have a balanced food of moderate amount and enjoy it. Don’t eat anything in between meals except drinks. Drink water to thirst. If you practice like this, you won’t have the urge to eat this and that. This will help in spiritual advancement.
On Ekadasi day one should be all the time thinking about divinity and Satsangam. That is essential. Should not sleep during day time. By practicing with passion and determination, one will see the change in the mindset and attitude. Take only udhrini jalam during Sandyavandanam on that day if possible.
When have to drive long, should not fast. Have to use discretion on these days. The method of Girivalam, Gayathri and meditation can be done. We have to pray to Him to give me strength to continue this for ever. We shall consider all His devotees as out brothers and sisters. Bhakthi is essential but Bhakthi without determination (vairagyam) is useless”