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Why Do Individual Weighing Results Vary Throughout The Day?

A person who weighs himself regularly enough will have noticed the discrepancy in the results. What causes the numbers to go up and down?!

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Science writer Martin Robbins provides explanations to the apparent inconsistency in the numbers in an article on The Guardian. According to him, individual measurements, when taken into consideration in isolation, will not yield significant results; reliability cannot be achieved in this way, given the variation that occurs in the human body.

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

Robbins himself carried out a little experiment. He weighed himself for three days in a row for every waking hour. He also noted down the weight of everything he consumed of foods and drinks as well as the matter egested by his body.

“I estimated the, er, other stuff – I do have some dignity,” he says.

By the end of three and half days (lasting from 6pm on Friday night to 9am on Tuesday morning), the readings were as follows:

  • Weight of food consumed: 3.58 kg
  • Weight of drink consumed: 11.28 kg
    (Total = 14.86 kg)

After the completion of the experiment, it was found that he had lost 1.86 kg. It appears that his body got rid of around 16.72 kg during the three-day 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?”

Robbins explains that we are constantly losing weight, not only by eating healthily or exercising, but rather, in ways we would not have thought of.

He says that apart from one kilogram apparently lost when running 5 km over the weekend, other unaccounted weight loss was noticed. For instance, he could not initially find explanations to the loss of 69 grams per hour.

“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.

He argues that people lose the similar amounts when exhaling water vapour, and by way of sweating, which seemingly explain his results.

“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.”

He concludes by saying how the individual measurements we take can be extremely unreliable.

“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.”

On the other hand, taking into account several groups of readings might be helpful.

“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.”

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