The science of why your weight jumps around so much from day to day

If you’ve ever gotten into the habit of weighing yourself every day, you’ll have noticed something a little strange about the numbers on your bathroom scales. They’re all over the place. From day to day, it almost like you’re weighing a different person. The numbers seem to yo-yo up and down irrespective of how much you eat, drink, or exercise. And if you’re actively trying to lose weight it, it’s not just confusing – it can be downright disheartening too. Nothing kills diet motivation and willpower quite like seeing those numbers go up when all your hard work and snack-sacrificing means they ought to be going down.

But according to Martin Robbins at The Guardian, it makes perfect sense that the numbers on your bathroom scale don’t make any sense. Why? Because there’s simply way too many things going on in your body all the time for individual measurements taken at any particular moment to be at all meaningful when viewed in isolation.

“Weight measurements are like opinion polls – individual results don’t tell you anything because there’s just too much random noise, error and variation,” he says. “It’s only when you have a few dozen that you can start to reliably pick out a trend.”

To get a better sense of all the ups and downs occurring in his weight, Robbins set himself an ambitious task: over a three-day long weekend, he weighed himself every waking hour to see what his body was up to. He also accurately recorded the specific weight of everything he ate and drank over the period, and even weighed the urine he passed. “I estimated the, er, other stuff – I do have some dignity,” he says.

At the end of three and a half days (from 6pm on Friday night to 9am on Tuesday morning), Robbins ingested a whopping 14.86 kg of consumables, consisting of 3.58 kg of food and 11.28 kg of drink. While that might sound like a lot – and it is – it’s not like he was all-out gorging himself the entire time. At the end of his experiment, he’d actually lost 1.86 kg, meaning his body had disposed of some 16.72 kg over the course of the weekend.

“7.4 kg of that was accounted for by urine, and an estimated 1.8 kg by, well, crap, but that still leaves a whopping 7.52 kg of mass that just vanished into thin air,” he says. “Where did it go?”

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3 Responses to The science of why your weight jumps around so much from day to day

  1. alfy says:

    Nearly fifteen Kg of food? I suspect Martin has probably farted off around 13 Kg of methane and hydrogen sulphide into his home. Ask his unfortunate wife and children about the experiment. He should try comparing his weight standing up on the scales with crouching down on them. There is about a Kg difference. It is the quickest way of losing weight I know.
    Had I been more athletic I would try weighing myself while (i) doing a headstand, (ii) standing on one leg, or (iii) doing a handstand. I have been told that it is about how far you are from the centre of the earth, I don’t believe it.

  2. Phil Krause says:

    Your middle one?

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