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According to USDA, medium grain raw brown rice has carbs and fiber at a ratio of 76g / 3.4g = 22.4.

When cooked, the ratio changes to 24g / 1.8g = 13.3.

Are these numbers wrong, and if not, why does this ratio change in favor of fiber? Mere dilution shouldn't change this ratio.

EDIT: The missing 0 hypothesis seems to be the most plausible explanation so far, but I'd like to find sources completely independent from USDA to be sure. All of the sources I found just copy USDA directly or indirectly.

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    If you boil 100 grams of rice do you have 100 grams of rice or does the rice "suck" the water increasing it's weight?
    – SZCZERZO KŁY
    Jul 25, 2019 at 12:01
  • @SZCZERZOKŁY of course, but the question is about the ratio
    – user18825
    Jul 25, 2019 at 16:11
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    Insoluble fiber can absorb water. I wonder if the 'cooked' number isn't the dry weight of the fiber, but the weight with the water. Protein wouldn't do the same thing, so it'd look like you gained fiber.
    – Joe
    Jul 25, 2019 at 22:54
  • @Joe "cooked" weight is cooked weight. The question is about the ratio though.
    – user18825
    Jul 26, 2019 at 0:40
  • @MaxB : I'm talking about the cooked weight of the fiber. If they consider it to be the weight with absorbed water, it's going to increase. (the fiber measurement is in grams, so it's a weight)
    – Joe
    Jul 26, 2019 at 16:06

2 Answers 2

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When you cook things using water you add mass that have 0 calories. Some food, like rice, absorb that water. Hence boiling 100 grams of rice increase final mass to around 300 grams.
Then again you measure the 100 grams carbs and it's 3 times less than your starting points because you never added additional carbs in the process.

The fiber amount is IMHO wrongly calculated because 3,4 gram divided by 3,16 (ratio of carbs in 100 grams before and after cooking) is 1,075. Probably someone rounded up the 75 wrong and had 1,8 instead of 1,08.

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  • The missing 0 hypothesis seems to be the most plausible explanation so far, but I'd like to find sources completely independent from USDA to be sure. All of the sources I found just copy USDA directly or indirectly.
    – user18825
    Jul 25, 2019 at 21:34
  • @MaxB I would say that the data it's rather solid across other sources rather than copied. I've checked in few different languages and they usually give between 3,3 or 3,5 grams of fiber.
    – SZCZERZO KŁY
    Jul 26, 2019 at 7:11
  • Do you have contradictory sources (That is, with the ratio for "cooked" being might higher than 13)? If so, please link them in your answer.
    – user18825
    Jul 26, 2019 at 8:24
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It's likely that the brown rice was boiled and then drained; some of the starch would leach out into the cooking water and be removed with it, while the insoluble fiber would remain in the rice.

Additionally, note that NAL's nutrition data was taken from various sources over a long period of time. It's possible that the data for raw and cooked rice was taken at different times, from different brown rice cultivars. (I doubt that this was the primary source of the difference, though.)

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  • Your first suggestion only works for cooking methods where excess water is used and the rice drained. Many people cook rice such that only the amount of water that will be absorbed is used. Do you have any information about the testing methods which would suggest whether its is a water discard method? Failing that I suspect your second suggestion is more robust.
    – Spagirl
    Jul 25, 2019 at 10:37
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    I also would never drain rice, but it is a common practice, particularly with brown rice. And I seriously doubt that a nearly 2x difference between the two ratios could be explained by testing a different cultivar or crop. (And they are very careful about assay technique.)
    – Sneftel
    Jul 25, 2019 at 11:04
  • @Spagirl, the same effect would also apply if the rice is washed before being cooked, which is a recommendation I have seen many times.
    – Peter Taylor
    Jul 26, 2019 at 6:37

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