I have heard that Buddha was eating similar to one meal a day. Is this true? Would this not lead to severe constipation? Was it common for ascetics to take medicines to help purge the food waste from their bodies?
According to the Theravada Bhikkhu Patimokkha (monk's rules), monks can eat multiple meals between dawn and noon. They may not eat food after noon, even when they are ill.
They can however drink fruit juices for the rest of the day but cannot keep it overnight (if there is risk of fermentation). Whole fruit is considered food, but fruit juices are of a different category.
Tonics (e.g. ghee, fresh butter, oil, honey, or sugar/ molasses) and medicines may be consumed at any time of the day, if they need it.
The Great Standards allow justified adaptation of the minor rules based on local, modern or special situations. For e.g. I speculate that a monk advised by a doctor to eat after noon, due to genuine medical reasons, may be exempted from the above-mentioned rule.
From The Bhikkhus' Rules: A Guide for Laypeople by Bhikkhu Ariyesako:
Food intake is limited to the hours between dawn and noon. The practice of not eating in the afternoon is a very old tradition mentioned in the earliest Suttas. It is also included in the Ten Precepts of the novice ...
'Food' (yaavakaalika) here refers to things like cooked grains; sweets made from flour, beans, etc.; fish; meat; fresh milk and sour milk;... fruits, tubers and all 'main course' foods. ...
When these staple foods go beyond their time limit (i.e., after noon) a bhikkhu will incur an offence if he consumes them. The original story shows the complications that can arise from leaving the monastery at the wrong time.
... It is also noteworthy that an ill bhikkhu has no exemption from this rule so he likewise should not take food in the afternoon.
The above sections have dealt with food (yaavakaalika) but as has been already mentioned fruit juices are considered under a different category. ... Although bhikkhus should not eat fruit — which is food — after midday, they can drink the 'fruit juice' any time throughout the day. However, they cannot store fruit juice beyond that single day. This is called yaamakaalika and is a juice-drink made from crushed fruit, which is then carefully strained of any pulp or particles. (The Vinayamukha (EV) Commentary suggests that it could not be stored beyond the next dawn because sugar mixed in with the fruit juice might lead to slight fermentation.)
When offering fruit juice it is important that it is well strained so that no pulp or fruit particles remain, for the fruit itself counts as food and so cannot be consumed in the afternoon.
There is now the category of 'tonic-medicines' (sattaahakaalika). These can be consumed at any time but cannot be stored longer than seven days (after they are offered).
Medicines that may be consumed without time limitation are called yaavajiivika.