Athletes can purposely and quickly change their body weight by manipulating fluid levels.
The same trick can be used by low-carbohydrate
diet promoters.
If you are used to eating a lot of carbohydrates, and you suddenly go low-carb, you'll probably lose a few pounds of water in a few days.
(If you remember the "hydrate" part of
"carbohydrates", it'll help you remember that
carbohydrates help you retain water. 1 g of glycogen helps us store approximately 3-4 g of water.)
This can make it seem like a low-carb diet is the
"magic secret" to fast weight loss.
Unfortunately for dieters, they haven't lost fat.
But fortunately for athletes such as wrestlers or boxers, they can "make weight" without losing too much lean tissue.
Conversely, if you aren't used to eating a lot of
carbohydrates, and you have a high-carb meal or
two, you'll also often notice yourself being thirstier afterwards (plus retaining water).
Bodybuilders and physique athletes about to go onstage use this water retention effect to "plump up" their fluid-depleted muscles at the last minute. If they time it right, extracellular fluid will stay relatively low but intracellular fluid in muscles will go up. This makes them look both leaner and more muscular.
Many people losing weight notice that weight loss is not linear. Instead, it often seems to happen in bursts. People can often be surprised by sudden weight loss the day after a big meal. Some refer to this as "the whoosh".
"Whooshes" can seem like a paradox. How can we lose weight after eating more?
One possible explanation is that energy restriction can cause fluid retention, in part from changes in hormone levels (such as increased cortisol) or the different osmolarity of dissolved proteins and electrolytes.
Briefly eating more after a period of eating much
less changes the osmotic balance. Our body
rebalances our fluids.
End result: We lose water... and weight.
That's why weight fluctuation from changes in fluid balance is one reason the bathroom scale doesn't always give us the whole picture.
