why can a person’s weight change +/- 5-10 pounds a day when they definitely aren’t eating 5-10 pounds of food/water a day. Where does this extra mass/wight come from?
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Don’t underestimate the weight of food and especially water. A half gallon of water (the standard-even-if-derided recommended daily amount) weighs over 4 pounds. A hearty meal can weigh 1-2 pounds on top of that.
Not all of this material stays in the body long-term because we expel it as waste, but our body’s capacity for storing water especially goes beyond the size of our bladders. Our cells literally get dehydrated overnight and will swell up with water over the course of the day.
It cannot. Conservation of mass applies – if you weigh 100kg in the morning and don’t ingest anything, you cannot weigh >100kg at the end of the day. You might have a measuring instrument variation, since home scales are not precise, but as you clearly point out, no extra mass can come from nowhere.
Without consuming a similar weight in food/putting on a similar weight in clothes there is little chance of any living person on earth ever putting an additional 10 pounds on the scale within just one day.
Losing 10 pounds is a bit more plausible, with breathing and especially *sweating* being a huge net-loss in mass for the body, but for the average human this type of fluid loss without replenishment would raise some serious health-related questions.
People can only gain as much as they intake. Drink 8 glasses of water in a day (no need to), that’s 4 pounds you’ve gained until you pee, sweat, and breath it out. If a person appears to gain weight without eating or drinking, it’s an issue with the scale.
People’s weight varies each day depending on their inputs, outputs, and retention (you do hold on to some, even incorporate some into storage in the body).
You can’t magically gain weight.
That being said short term fluctuations come from a few factors:
* Dehydration. Symptoms of dehydration start at 3–4% of water loss. If you weigh 100kg and 60% of your body is water that’s 1.8kg. So you can be 1.8kg “lighter” from dehydration without really noticing.
* Measurement error of the scale. However, most scales are surprisingly accurate with less than 300g (absolute) measurement error. In your case what matters most is how reproducible your measurements are. Just measure yourself a few times in a row and note the difference.
* Clothes. Clothes can be surprisingly heavy. A pair of jeans weighs almost half a kg. Same for a sweater.
* Bladder, stomach and intestine content. The bladder can contain around 600ml of water. Poop can weight half a kilogram.
In the end, if you want to track your body weight it’s best to measure it naked, in the morning, after you’ve gone to the bathroom and before drinking or eating anything. This should give pretty consistent weight.
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