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?"
In his analysis, Robbins points out that we're losing weight all the time in ways we never think about – ways that have nothing to do with eating healthily or going to the gym. Admittedly, a couple of 5 km runs he took over the weekend saw him displace over a kilogram in sweat. But there were still several kilograms of unaccounted weight loss: 69 grams per hour (1.65 kg every 24 hours) that couldn't be otherwise explained.
"In fact, I really was evaporating into thin air. Humans breathe in oxygen, and breathe out carbon dioxide – oxygen plus a carbon atom. All those carbon atoms have to come from somewhere, and they add up pretty quickly – over the course of a day, with a good work out thrown in, someone my size breathes out maybe half a kilo of carbon," he says.
Robbins estimates that we lose about the same amount again in exhaled water vapour, and then yet again by leaking water from our skin. Once he added up the estimated losses from these sources over the weekend, the riddle of his mystery weight loss over the weekend was solved.
"It also reveals another surprising truth; that when it comes to ditching mass from your body the anus really does bring up the rear end," Robbins says. "My penis, lungs and skin all managed to outperform my posterior when it came to taking out the trash."
Robbins's conclusion is that in light of all the things our bodies are doing that results in us losing weight, there's little or no point getting hung up on a number you don't like taken from one seemingly random weigh-in taken at one point during the day.
"None of this is massively surprising of course, but what I think it shows is just how unreliable any single measurement of weight is," he says. "On any given day my weight varied by about four pounds [1.8 kg], with a dozen pounds [5.4 kg] passing in and out of the giant meat tube that is me at only vaguely predictable times. When you consider that a sensible weight loss target is maybe 0.25 lbs [110 grams] per day, you can see how on most days that's just going to be swallowed up in the noise."
The best way to weigh yourself then – especially if you're the kind who's going to be at all emotional about it, which is probably most of us – is to stand on the scales but not attribute any particular importance to the number you see. Instead, just jot it down and calculate long-term trends over time, as that's the closest we can get to seeing how our bodily weight is really changing.
"Weigh yourself every morning, but ignore the number that comes up on the scales," says Robbins. "Instead take the average of the last seven days (preferably ten or fourteen), and after several weeks look at how that average is changing over time. That's where the real truth lies."