It's not easy being a body builder. They have to train hard, tan hard, and you've really got to know your hormone science. Yes, we did say science.
Very muscular body builders rely on hormones to make themselves look even more muscular. If their bodies have too much water, they'll be bloated, and their muscles won't be as defined. They could kiss that gold medal goodbye.
The most logical way to get rid of extra water is to avoid it for a few days, to excrete all possible water and not replace it by drinking anything. Body builders are often told to drink lots of water 8-10 days before a show, and limit themselves only about 8 hours beforehand. The reasoning is in our hormones.
To start to see what's going on, think about what would happen if a body builder really limits water intake a week before their competition. First, blood osmolarity rises (the same amount of salts are dissolved in less water), and the posterior pituitary gland releases antidiuretic hormone (ADH) that will circulate to the kidney and increase water reabsorption at the nephron's collecting duct. When more water is returned to the blood, water retention increases and urine volume decreases. First off, the body has saved and stored water, but secondly, the body builder can become severely dehydrated and risk severe complications.
What should the body builder do to win the competition? When they drink plenty of water, their blood osmolarity decreases, and ADH is blocked. Minimal amounts of water will be reabsorbed in the nephron and the urine will become dilute as lots of water is excreted. Another side effect of drinking lots of water is stimulating atrial natriuretic peptide (ANP) release. When the heart senses the increased blood volume (from drinking all that water), ANP will circulate to the glomerulus. This will the release of renin. When renin is blocked, aldosterone is also blocked, and the kidneys won't reabsorb more salts and water. Blood volume and pressure will be limited.
With their body's hormone primed and the kidneys convinced it will be producing dilute urine, a body builder will stop drinking water a few hours before a competition to ultimately confuse the body. For a while, lots of water will still be eliminated, the blood volume will decrease with less water coming in, and the body builder's muscles appear quite ripped. Just like that, our science-loving body builder wins the competition!
Some body builders don't follow these guidelines, but simply block ADH with drugs. Artificially blocking this hormone can cause severe dehydration, and some body builders end up getting pretty sick.
We should mention that you shouldn't try this at home. Body builders have support staff that help them monitor their blood composition, and help decrease the likelihood that they'll mess something up. Unless you have a support staff, keep drinking those eight ice-cold glasses of water a day.