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There's an unfounded claim that eating cooked carrots worsens constipation, while the constipation is currently ongoing, and that, thus, it is not recommended to eat them at that time. I cannot find the proof or explanation behind it. It is stated in articles around the internet and, personally, doctors have also informed me of the same. I therefore ask: what's the scientific explanation and base for carrots worsening currently ongoing constipation?

Finally, here are three helpful resources on the matter, as constipation advice is a controversial topic. The article Does Fiber Relieve or Cause Constipation? A Critical Look, a godsend in a sea of misinformation, explains how fiber is and is not helpful during constipation. The article Do Bananas Cause or Relieve Constipation? is close to what's needed here. The resource Fiber Content of Foods (really not the best) shows the amount of fiber in different foods, allowing to contextualize and compare them, and it essentially shows that carrots really aren't too different from other foods in this particular trait.

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Cooking in general decreases the insoluble dietary fiber contents and increases the soluble fraction. I guess this is the theoretical basis for the claim(s) regarding cooked carrots (and other cooked vegetables), but whether the change is [clinically] significant enough to affect constipation is another matter...

Legumes, leafy vegetables, roots and tubers, gourds and other vegetables were analyzed for total (TDF), soluble (SDF) and insoluble (IDF) dietary fiber contents, both before and after cooking either by a conventional open-pan method or by pressure cooker. Data revealed a significant increase in SDF fraction with a concomitant decrease in the IDF fraction upon cooking by both the methods employed. Although the decrease in IDF matched the increase in SDF values in some cases, it was found to be more in vegetables categorized as 'other'.

WebMD says this about the differential effects of IDF vs SDF:

Is all fiber the same?
No, some fibers are soluble in water and others are insoluble. Soluble fiber slows digestion and helps you absorb nutrients from food. Insoluble fiber draws water into and adds bulk to your stool, helping the stool pass more quickly through the intestines.

On the other hand, the Academy of Nutrition and Dietetics says something more nuanced (and also more confusing):

There are two types of fiber: soluble and insoluble. Soluble fiber becomes "sticky" when it gets wet. Oats, which are rich in soluble fiber, are a great example of this. Insoluble fiber does not absorb much water, so it doesn't change when liquid is added to it.

Think of what celery would look like if you dropped it into a glass of water. It doesn't absorb liquid or become sticky. That's insoluble fiber.

For both diarrhea and constipation, you want to get more soluble fiber, such as oats, bran, and barley. For constipation only, you can add in some insoluble fiber as well—fruits and vegetables are good sources. Many people find that simply taking a daily fiber supplement, which is made up mostly of soluble fiber, will lessen both diarrhea and constipation.

So it's not totally clear from this that soluble fiber is necessarily worse gram-for-gram (than the insoluble one) for constipation. (According to Wikipedia, this Academy, although well established has had a somewhat controversial link to the processed food industry...)

Looking through a 2012 meta-analysis it doesn't look like there are enough empirical studies on the differential effects of IDF and SDF on constipation. Soluble fiber indeed seems to work "well enough" in some studies, as the Academy quote above says.

There's a 2017 academic review article that promises to elucidate the "enduring misconceptions" on effects of fiber... Regarding the laxative (i.e. anti-constipation effect) it says this:

In the large bowel, there are only two mechanisms that drive a laxative effect: large/coarse insoluble fiber particles (eg, wheat bran) mechanically irritate the gut mucosa stimulating water and mucous secretion, and the high water-holding capacity of gel-forming soluble fiber (eg, psyllium) resists dehydration. Both mechanisms require that the fiber resist fermentation and remain relatively intact throughout the large bowel (ie, the fiber must be present in stool), and both mechanisms lead to increased stool water content, resulting in bulky/soft/easy-to-pass stools. Soluble fermentable fibers (eg, inulin, fructooligosaccharide, and wheat dextrin) do not provide a laxative effect, and some fibers can be constipating (eg, wheat dextrin and fine/smooth insoluble wheat bran particles). When making recommendations for a fiber supplement, it is essential to recognize which fibers possess the physical characteristics required to provide a beneficial health effect, and which fiber supplements are supported by reproducible, rigorous evidence of one or more clinically meaningful health benefits.

The only conclusion from there is some soluble fibers are beneficial (for reducing constipation) and some are not, but it's unclear to me which of those are found in cooked carrots (and in what proportion).

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