Serious Eats - seriouseats.com

Come on people, the reason 'empty calories' and 'junk food' are normally a bad thing is that, when you eat them, you're not usefully contributing to the key elements of a balanced diet, so you will end up needing to consume more food in order to get the recommended daily allowances to keep your body running in good condition.

If you live in a third world country such as India, much of your diet consists of the empty calories in white rice, and you'll probably end up with the diseases and early deaths that malnutrition brings. At the other extreme, the likely result in a Westerner is that you'll be eating more calories than you're burning, so you'll put on weight.

Once in shape, swimmers generally don't have a huge mass of muscle to build or maintain, so their requirements for proteins, vitamins, and so on are only 1-2 times those of 'normal' people [ref]. But they have a massively larger need for calories (i.e. 4-5 times), and of course the easiest way to get that massive dose of additional calories is through 'empty calories' - lots of fats and sugars.

If he were to keep the same diet once he stops swimming competitively he'd be breaking scales not records!