Once when the Buddha was living at Savatthi, King Pasenadi of Kosala ate a whole bucketful of food, and then approached the Buddha, engorged and panting, and sat down to one side. The Buddha, discerning that King Pasenadi was engorged and panting, took the occasion to utter this verse:
Now at that time the brahman youth Sudassana was standing nearby, and King Pasenadi of Kosala addressed him: "Come now, my dear Sudassana, and having thoroughly mastered this verse in the presence of the Buddha, recite it whenever food is brought to me. And I will set up for you a permanent offering of a hundred kahaapanas every day." "So be it, your majesty," the brahman youth Sudassana replied to the king.
Then King Pasenadi of Kosala gradually settled down to [eating] no more than a cup-full of rice. At a later time, when his body had become quite slim, King Pasenadi stroked his limbs with his hand and took the occasion to utter this utterance:
Who would have thought weight-loss could be so easy! In this brief exchange the Buddha is suggesting that over-eating is the root of obesity, which hastens the aging process and threatens one's life, and that this only occurs when mindfulness is weak or absent. If we eat slowly and with a great deal of attention, it can more easily become apparent (if we are truthful with ourselves) when an adequate amount of food has been consumed. Interestingly, he seems to be saying that wisdom will provide what is needed to refrain from further eating, rather than the modern conventional view that it requires will-power or self restraint.
Always one to play on words, the Buddha says that all our afflictions (literally, all our unpleasant feelings), and not just our bodies, will "become more slender." Perhaps this is what Pasenadi is referring to when he says the Buddha's teaching has not only helped him slim down his body (the immediate benefit), but the general increase of mindfulness and diminishing of greed will help with all aspects of the spiritual life (and thus with his rebirth in the future).
The commentary to this text informs us that the king did not engage Sudassana to utter the verse throughout the entire meal, but only once he had started eating. The idea is not to cultivate an aversion to food, for food itself is not an evil. As with so much else in the Buddha's teaching, it is a matter of understanding cause and effect, and of using food skillfully as a tool for awakening rather than allowing oneself to be caught by the latent tendencies of attachment, aversion and confusion that might be evoked by our relationship to food.
Notice the language of the last line of the Buddha's verse. The word for life (aayu) is the same one as in the Indian medical tradition of Ayurveda (=knowledge of life), and is regarded as something that can be squandered or carefully guarded. When approached with care, the preservation of life also slows down the aging process. The image is not one of conquering illness or death (for this comes only from full awakening), but of treating the precious resource of one's own vitality with wisdom.