Why do monks have only one meal a day?

[tingtong]

Sacrifices that a monk need to give in is highly respected.. the ability of not thinking of food and the same time only consume food once a day.. is rather impossible for lay man.. it is greatly a statement that reminds me not to waste food..

[Kim Hyun Jae]

Taking a meal a day is literally training the body and mind not to be attached to food, that were caused by our desire to satisfy our stomach and taste. When we only take a meal a day, it's meant to do away with self - gratification to please their bodies. It is also as reminder to take only what the body need and not have food wastage nor give burden to donors of dana.

When we don't get to choose the food we like, during dana, and we can eat only what's given is a practice to be humble and grateful for what's being given against what we wish to Get and Have, that further feed our desire for more. It's a method to train the mind to be detached to worldly habits like indulgence in food and good taste.

[RedLantern]

The Buddha, as is well known, emphasized moderation, the Middle Way that avoids extremes, in all things.

A meal a day highlights one's attachments to food and to good flavor; thus it helps the practitioner to distinguish how much of his or her craving for food is need, and therefore normal and necessary, and how much is greed, and therefore a hindrance to liberation. Over-eating and under-eating both defeat the purpose of food, which is to nourish the body and keep us healthy so that we can work to benefit the world.

[cookie]

In my opinion even for lay practitioners, we should learn to cut down on the attachments on food. First step is to cut down on eating meat. Next to cut down on the desires for particular types of food. We should learn to eat as a necessity and to stop making judgements on the food we eat, ie, it is delicious, it is not delicious, that is better and this is worse. This reinforces our attachments to so called good tasting food. Any form of attachment is bad. It will bring our next rebirth to the lower realms if this attachment seed trigger during the moment of our death. Besides subdueing our minds on how food should taste, we should also cut down on the amounts we eat. Over eating is a predominant problem in many developed and developing nations worldwide. This results in us over farming our food. The whole eco system is disturbed and heading for destruction. There is also way too much wastage in our food. All these problems can be stemmed from the mere act of us following the Buddha's guidelines of eating only once a day; just sufficient to keep our bodies alive in whatever circumstances we may be in.

[MoMo]

This topic should be best answered by someone who had actually observing for a long period of time to experience its benefits of cutting our attachment and perceptions on how our food should be consume. It is said that one will enjoy great freedom of simplicity from living alms foods offered by laities to devote all energy and time into meditation.
I once heard a story how challenging the change of lifestyle from multiple meals daily to single meal a day was to a novice monk.
“When I was a novice monk at the north-eastern province of Thailand, very morning we had go on alms round to the local village to receive alms from local villagers whom are mainly some poor farmer.
We usually enter the village on a single files leads by the abbot or senior monks. On one occasion, there seem to be some laities from the city by the way of their dressing in the crowds. With their present could also mean that there could be some difference kind of food we would receive from the local villagers which they consume daily. As usual we receive some congee pack in small plastic bag along with other dishes in my begging bowl. When it comes to the turn of city folks making their offering, one kid was holding a tub of ice-cream as his offering of the day. My 9 months of mindfulness on the word “Budho” had change into “ice-cream” without notice! As he was placing a scoopful of ice-cream into my begging-bow scream: “ No…”  but he put it in any way! On the returning journey, the only thought I had was worrying that my scoop of ice-cream would melt by the heat from the pack of congee.
Upon arrival at the monastery, usually the abbot or senior would perform prayer and got to eat first followed by the novice. But on that day the abbot decided to give a talk on the vinaya rules of taking alms from lay people for bhikku and it was unusual long totalling 45 mins. I had great respect for the abbot and enjoy his teaching pretty much but this time heart was rumbling with grudges as “why do you have to give such a long one at this time…”. When he finally fished with the talk and had his share of alms, it was us the novice turn to eat and I look at my puddle of ice-cream and realized my mental state and in shame mixed everything in the bow and eat it!
When I finally fished with my meal, I was told by someone the abbot wants to have a “special” audience with me….” 

[lotus1]

Besides monks, the lay person or yogis would also take the "not taking of food after noon", i.e the 6th precepts under the Eight Precepts (attha-sila) at certain days or period to experience the monkhood and train theirselves from attachment. For lay person, they normally take the eight precepts during intensive retreat period to support meditation practice.

The Eight Precepts:

1. Panatipata veramani sikkhapadam samadiyami
    I undertake the precept to refrain from destroying living creatures.
2. Adinnadana veramani sikkhapadam samadiyami
    I undertake the precept to refrain from taking that which is not given.
3. Abrahmacariya veramani sikkhapadam samadiyami
    I undertake the precept to refrain from sexual activity.
4. Musavada veramani sikkhapadam samadiyami
    I undertake the precept to refrain from incorrect speech.
5. Suramerayamajja pamadatthana veramani sikkhapadam samadiyami
    I undertake the precept to refrain from intoxicating drinks and drugs which lead to carelessness.
6. Vikalabhojana veramani sikkhapadam samadiyami
    I undertake the precept to refrain from eating at the forbidden time (i.e., after noon).
7. Nacca-gita-vadita-visukkadassana mala-gandha-vilepana-dharana-mandana-vibhusanathana veramani sikkhapadam samadiyami
    I undertake the precept to refrain from dancing, singing, music, going to see entertainments, wearing garlands, using perfumes, and beautifying the body with cosmetics.
8. Uccasayana-mahasayana veramani sikkhapadam samadiyami
    I undertake the precept to refrain from lying on a high or luxurious sleeping place.

[yontenjamyang]

Basically monks in certain tradition have only one meal a day as part of their practice not to be attach to food. Also, the environment for these traditions in hotter climate countries like Thailand, Cambodia, Myanmar and Laos allows this to be practiced. For traditions where the climate is colder, monks are allowed more than one meals. Otherwise the monks will not survived. Nevertheless, the non-attachment to food is still emphasized.

Also, monks in Thailand for example, go for alms every morning to allow the community to provide Dana or alms as a way to gain merit and it is well know that the monks do not cook their food. The monks in the colder countries like China and Tibet do not go out for alms and there do cook their their own food. Dana or alms are brought into the temple or monastery because the climate does not allow the monks to go on alms' rounds.

So the reasons is the Vinaya is practical according to the environment at the same time keeping its objective of non attachment.

[Midakpa]

Food is one of the four necessities of life; the other three are clothing, shelter and medicine. For monks in the Theravada tradition, food intake is limited to the hours between dawn and noon. The practice of not eating in the afternoon is a very old tradition and is also observed by the samanera (novice who has taken the Ten Precepts), the dasasila mata nun (ordained Buddhist nun who has taken the Ten Precepts) and the lay devotee who has taken the eight precepts.

Eating one meal a day is a Pratimoksha vow and is part of the training of the monk. Sometimes the food received during the alms round is  meagre. This has the benefit of training the monk to be grateful for whatever is given by lay people and not be attached to food because a bhikkhu's life is supposed to be simple. This is why the Buddha laid down rules regarding "edibles": "There are these finer staple foods, i.e. ghee, fresh butter, oil. honey, sugar/molasses, fish, meat, fresh milk, and curds. Should any bhikkhu who is not ill, having asked for finer staple foods such as these for his own sake, then eat them, it is [an offence of Confession]".