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Why, when someone eats food, and he/she directly checks his body weight on balance, why it seems to be no changes in body weight and there wasn't any execration, I want to know where did food weight went?

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    $\begingroup$ How accurate is your scale? How big is the meal? $\endgroup$ – John Dvorak Nov 9 at 10:14
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  • $\begingroup$ This question is wrong. If you eat some food, that extra mass goes straight into your belly and has to show up on the scale. Conservation of mass. $\endgroup$ – user1258361 Nov 13 at 17:03
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Compared to their body weight, humans don't really eat that much

Where I live, the typical serving at a restaurant is 150g or 200g of meat, plus about the same amount of side dish. Even if your test subject is a big eater, that's just 300 + ~200g of biomass. Eating that much food without drinking is difficult, so you allow your subject one beer, or about 500g of liquids.

Even with generous assumptions, an intake of more than half a kilogram of food in one go is unusual.

If you live in a country where this specific figure is higher than that, please adjust the value accordingly, and the next figure in the same ratio.

I could perform a scientific measurement of an average human mass based on a sample size of two people, but the more rigorous way to do it is to open a random web page on the topic and use the number stated there, up to 80kg for a healthy male.

Therefore, a person's weight after one meal will typically increase by, Fermi estimate, 1% of their body weight. Compare that to the common shrew, which eats 125% of its body weight per day. Source: Google

A scale I have available at home is graded at 5kg, so an increase of 0.5 kg is difficult to notice. The only scientifically valid test would be to have two different people read the scale reading and only communicate their values after the experiment. A single person taking two readings is likely to introduce a bias.

A typical digital scale, however, should be able to pick up the difference.

Be sure the "no excretion" criterion is met. A human body is able to dispose of just as much mass in one go.

The other answer beat me to this figure, but the amount of perspiration cited by Wikipedia is up to 2-4 liters per hour. So, if you've just been working out or if your AC is broken, You can actually sweat faster than you eat. That's the maximum rate of perspiration, though, and far from the typical situation.

Finding the typical rates is much more difficult. Every article I've checked boils down to "how much you sweat varies wildly, and if you think you sweat too much, go find a doctor".

For perspiration, Science Alert suggests 2.3 lb (1 kg) of carbon dioxide per day as the usual amount. If your meal takes 15 minutes, that amounts to about 10 grams of mass loss due to breathing while eating.

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To make a proper evaluation of the immediate effect of food on your weight you need to:

  • Have a sensitive (optimally: digital) and properly working scale that shows the same value when you weigh yourself few times in a row
  • Wear the same clothes during the pre- and post-meal measurement
  • Not vomit, urinate, defecate or sweat significantly between the 2 measurements

How much weight you can GAIN by having a meal?

An average main meal for an adult consisting of a soup, potatoes, steak, sauce, salad and compote could weigh, let's say 500 grams (0.5 kg). Such a meal should contribute about 500 grams to your weight (let's say in 30 minutes between your pre- and post-meal measurement), because no significant weight of food is lost during digestion and absorption of nutrients and because no significant metabolism that "burns" the nutrients occurs within 30 minutes of starting a meal.

How much weight you can LOSE during a meal?

If there is no vomiting, defecation and urination between the pre- and post-meal weight measurements, you can lose a significant amount of weight only by sweating. In one study, the sweat rate of athletes during training was 1.2 liters/hour in average, but up to 5.7 liters/hour (see Fig 1). In another study, young males sitting in the sauna at 95 °C 2 x for 10 minutes with a 5 minute break resulted in 0.2-1.1 kg loss of weight (check Table 1, the BML column). So, it might be theoretically possible that a person with a very high sweat rate would lose 500 mL of sweat (and hence 500 g of body weight) just by sweating when eating for 30 minutes in a hot environment. This seems to be the only possible explanation for "no weight gain after a meal" when the measurements have been done properly.

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