Former wrestler — and shameless self-promoter — Tim Ferriss (The 4-Hour Workweek) explains how to lose 30 pounds in 24 hours — that is, how to cut weight for a weight-classed sport with weigh-ins well ahead of competition:
If weigh-ins are hypothetically held at 9am Saturday morning, restrict additional salt intake beginning at Thursday dinner. No red meat or starchy carbohydrates (bread, rice, potatoes) should be consumed on Thursday night or on Friday, as both of these food product categories cause the disproportionate storage of water (3 grams of water per 1 gram of glycogen; creatine and fibrous tissue water retention in red meat). Drink your normal volume of liquids in the form of purified or distilled water until Friday morning, at which point water consumption, limited still to purified or distilled water, should be reduced to 1/3 your normal volume. If you don’t want to do the math, just drink 1/3 cup every time you would drink a full cup.On Friday night, following a early (5–6pm) and light dinner consisting primarily of vegetables, thermoregulatory work should begin and water consumption should be eliminated until weigh-ins. Non-prescription diuretics, discussed in the following section, would be consumed at breakfast, lunch, and dinner on Friday, in addition to upon waking on Saturday.
Here’s where it gets interesting:
The bathtub is the preferred tool for dehydration based on the outside humidity in total submersion, which is 100%. The higher the humidity, the less the evaporation, and the more your body must sweat to cool core body temperature. This is why athletes will sweat more in a steam room than in a dry sauna. Fill the bathtub with water that does not burn the hand but causes moderate pain if the hand is moved underwater. Your target weight by bedtime should be 2–3 lbs. more than your necessary competition weight, as you will evaporate that volume range of water during 6–9 hours of sleep.Set an alarm clock next to the bath for 10 minutes, and preferably have someone who will also alert you at the 10-minute mark. Submerge your entire body and head in the bathtub, entering which should take at least 2 minutes. For ease of entry and to minimize movement, sit cross-legged at the front of the bath and lay down slowly, putting your head underwater so that only your face is exposed to the air and pointing towards the ceiling. If you feel faint at any point or when you reach 10 minutes, exit the tub and run cold water over your scalp but no other areas; ideally, place an ice pack on your head and neck instead of using water. Towel off, but do not shower, as you will reabsorb water through the skin.
He also recommends two potassium-sparing non-prescription diuretics: dandelion root (Taraxicum officianalis) and caffeine:
Dandelion root has the highest vitamin A of any known plant (14,000 iu per 100 g of raw material) and a high choline content. Dandelion root is one of few commonly available plants that increases sodium chloride excretion by the renal (kidney) tubule while simultaneously exhibiting potassium-sparing properties. When sodium excretion is increased, the kidneys increase water excretion to maintain electrolyte and osmotic balance. Dosages for dehydration, based on a 4:1 extract, are 250–500 mg 3x daily with meals.Caffeine not only increases sodium chloride excretion but acts primarily by increasing renal blood flow and stimulating parietal cells to increase gastric secretions. The latter combines with dandelion’s effect of increased bile flow to not only increase water excretion but food elimination (gastric emptying). Dosages for dehydration are 200–400 mg caffeine (preferably caffeine anhydrous) 2–3x daily with meals. 200 mg is roughly equivalent to two cups of drip coffee, or one medium cup of french-pressed coffee.
Used in combination for a 200 lb. competitor, 250–500 mg of dandelion root would be taken with 200–400 mg of caffeine at all three Friday meals (remember that dinner is early, 5–6 pm), and upon waking 3 hours prior to weigh-in at 9 am. It is recommended that the athlete also supplement each meal with a non-prescription 99 mg potassium product.
There’s a whole nuther complicated routine for rehydrating after weigh-ins, but it basically involves drinking water with some sodium and carbs — and glycerol.