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Why our weight fluctuates on a daily basis

Reuters/Carlos Barria
Our body is just trying to stay balanced.
Published Last updated This article is more than 2 years old.

It’s pretty great to hop on the scale in the morning to find that you’ve dropped weight from the night before. But don’t be disappointed if you weigh yourself again later in the day and find you’ve gained those same few pounds back.

The idea that our weight is a single number is kind of a misnomer. Throughout any given day, our weight fluctuates over about a 5lb. range as our bodies work to balance sodium with water.

“Your body wants to be in an equilibrium,” Katherine Zeratsky, a nutritionist with the Mayo Clinic, said.

We need some sodium (anywhere from 1,500 to 2,300 milligrams per day), which we get from food, in order to regulate things like blood pressure and for our cells to send signals. But we also need to make sure that we stay fully hydrated. Our bodies are made up of anywhere from 60% to 70% of water, which is a key component of parts of our blood, organs, and even our mucus

The kidneys scan and filter our blood so it can recirculate through our body and deliver oxygen. If we’ve eaten extra salt that day, or haven’t had enough water, our kidneys will spot it in our blood. “If we say have more sodium for one reason or another, our kidneys are saying ‘Hold onto some fluid,'” Zeratsky said.

Our bodies can pull fluid from a number of different sources. It can take some from within our cells, which contain tiny amounts of water, and from other fluid-filled areas like our lymph nodes. As we hold on to more fluid, some parts of our bodies swell just a bit.

When we’ve tipped the balance and have too much salt, we also feel thirsty. As we drink more water, it starts circulating in our blood and fills our cells to restore that balance. Holding onto extra water while the body tries to restore balance can contribute up to five additional pounds of our baseline weight, Zeratsky estimates.

Eventually, our kidneys will notice that the balance of salt to water has shifted in the other direction, and will send signals that we need to eliminate the extra fluid. A quick trip to the bathroom should do the trick.

The best way to avoid extra water weight is to constantly stay hydrated. Zeratsky recommended that most people should drink anywhere from about 64 ounces to 120 ounces of water a day, with the understanding that some of the water we get is through food, and that those of us who sweat more or have lost more water should shoot for the higher end of that spectrum.

It is possible to over-hydrate yourself, which can even be fatal in some cases. But the good news is, it’s pretty difficult to accidentally take in too much water. “Your body is pretty smart. Your kidneys will send it to your bladder and you’ll flush it.” 

There are other ways our weight can fluctuate, too. Zeratsky says that women often notice a change in their weight around their menstrual cycle as hormone fluctuations cause them to retain extra fluid. Additionally, if you eat a large meal, immediately afterward you’re going to be a little heavier than before your feast. On the other end, constipation can also contribute to holding onto a few pounds. Usually, though, these situations should relieve themselves in a day or so; most people can’t actually eat enough in a day to contribute to more permanent weight gain.

A little water weight fluctuation day to day is normal. If you’re worried about the number on the scale, a good way to watch for consistency is to weigh yourself at the same time every day; even if your weight goes up or down from there, you’ll have a general picture of how much you weigh at that time. “So much of the water balance is dependent on the climate, winter, summer, how active you are,” Zeratsky said. It all comes down to how your body is in balance.

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