The modern diet, with its many processed foods, lacks sufficient fiber:

https://www.scientificamerican.com/article/fiber-famished-gut-microbes-linked-to-poor-health1
https://www.leafscience.org/gut-microbes-support-neurogenesis-and-longevity-hormone-production

Resistant starch helps to influence appetite, as your biome is involved in brain signaling. source. This includes fibers beta-glucan, inulin, and glucomannan. They produce short chain fatty acids SCFA's, the primary being acetate, propionate, and butyrate. Fiber helps with weight regulation2: “The observed changes in energy intake and body weight occur both when the fiber is from naturally high-fiber foods and when it is from a fiber supplement”. For now, I am going to go by the hypothesis that supplemental fiber has the same benefits, or at least has most of the benefits of constantly eating foods full of fiber. (update: 1 2 3)

I'm taking glucomannan, because of all the fiber types I know of, it takes the fewest pills to get a serving. Glucomannan “can absorb up to 50 times its weight in water, making it one of the most viscous dietary fibers known [6]. Therefore, glucomannan is taken in smaller doses than other types of fiber supplements.” source I've also tried taking inulin in gummy form, not because these gummies have the best ingredients, but because they are yummy and I like eating them.

Update: Inulin gives me gas. Apparently I'm not the only one 1 2. Inulin fiber molecular chains are shorter and get completely broken down by bacteria in the large intestine. Glucomannan is harder for bacteria to break down, and I can take double or triple the recommended dose without bad effect. Update2: I found a study showing increased gas production from inulin, compared to glucomannan.

I sprinkle a bit of psyllium husk fiber into my smoothies, but not too much as I find the flavor decreases the more fiber I add! 3 I also like to add celery to my smoothies. Yes, I do try to eat more vegetables, and fermented foods, but I'm likely not going to dramatically change my diet anytime soon, so fiber supplements for me. I take glucomannan fiber with my food, especially with any simple carbs or high glycemic food.

Serving size is relative however. Is the one listed on the bottle relevant? A google search produces claims of healthy being a 10:1 or 5:1 ratio of carbohydrates to fiber. Another website claims that low glycemic foods have a 1:1 ratio. So if you have a 20oz milkshake, you are having about 100 grams of carbohydrates. That would mean 10 grams of fiber on the low side, or about 16 pills of glucomannan. Let's fudge that number a bit, considering glucomannan has a higher absorption ability than other fibers. It's still quite a few pills, more than I expected before doing this estimate. Still feasible. Studies on humans supplementing fiber, typically use 4 grams per day, and that has an effect by itself, but I wager taking more won't hurt, especially if you're having a milkshake. Update: I've been happily consuming 10 pills (6 grams) before high glycemic meals. Another 6 grams to go with dessert. Update2: Seems people in other studies have taken 10 grams per day.

When I say that the fiber will “absorb” the milkshake, I mean that the soluble fiber “gels” the milkshake. This gel slows down absorption of the sugars by the digestive system, making the milkshake have a lower glycemic index. Swallowing the pills before, or with the first few sips of the milkshake, rather than mixing it into the milkshake, makes for a more pleasurable experience. A gelled milkshake will be tasteless by comparison.

Another important factor, is that having fiber as the first part of a meal is better. For example, having your salad before eating the main course. Take your fiber supplement before eating, rather than after. This is sort of a deduction from an experiment highlighted in the examine article: “the study showed that both blood sugar and insulin levels were lower after meals that started with protein and veggies before carbs, compared to eating carbs first”. I've also read elsewhere that fiber slows down stomach emptying. However, if you forget, taking fiber supplements after your meal is still worthwhile, better than not taking it at all. It takes roughly 2 hours for a meal to fully exit the stomach, so the fiber will mix with the remaining food in the stomach.

I wondered why fiber supplements recommend taking the soluble fiber, anywhere from 15 minutes to an hour before a meal. Does it take that long to dissolve and be able to engulf the rest of the food? Why that long? From Billy Mitchell on Quora: “It increases the likelyhood that you’ll eat less by triggering an endocrine response from your intestinal tract promoting satiety, which takes time to happen. You wouldn’t get the response in time had you taken it right before a meal.”

Based on the idea of slowing down stomach emptying, and reducing the insulin response on high glycemic foods by slowing their absorption, I am guessing the best time to take the fiber supplements is right around 5 to 15 minutes before a meal. If it is longer than that, won't the fiber be increasingly in the intestine rather than the stomach? Don't you want the food and the fiber to be initially mixed before entering the small intestine? I don't know, but I'm guessing that would be beneficial. To help with the endocrine response and feeling full, I try to slow down my eating and enjoy each bit of my food. I figure the satiety will happen at the correct time with a slower pace.

I don't know if one type of supplemental fiber is better for the gut flora than another. Update:
1 In one dynamic, pectin, glucomannan, and inulin performed equally well (on mice).
2 In other dynamics, glucomannan and inulin (soluble fibers), performed better than cellulose (on mice)
3 Glucomannan produces less gas than other fibers, as it is less easily digestible by intestinal flora.
4 Glucomannan and inulin are both comparable in effectiveness of increasing beneficial bacteria (The kind that happens to help with depression! No citation, but I've looked this up previously)

The word on the internet is that fiber can affect absorption of certain minerals. I started to fear that it could affect absorption of other supplements as well. However, it appears it isn't the fiber itself, but the phytic acid (aka phytate, inositol polyposhpate) content 1 2 3 of some types of fiber, including cereal fiber. The phytic acid, in some types of fiber affects absorption of certain metallic cations such as calcium, zinc, iron, and magnesium. However, phytic acid also has beneficial effects (see source). As far as I can tell, glucomannan does not include phytate 1 2.

The following opinion article from 2011 slants towards eating natural fiber: http://www.berkeleywellness.com/healthy-eating/food/article/faux-fiber-versus-real-thing. It presents the idea that foods naturally high in fiber come with a variety of fiber types, not just one homogenous fiber type found in supplementation. However, this experiment, where monkeys were fed a “western diet”, actually improved in health metrics while supplementing with glucomannan fiber. I now feel safe eating whatever I want, with fiber pills handy. I do love eating healthy food, and even prefer it, but it's ok for me to not be strict about it.

Another study on mice I read a long time ago, is that mice that never ate any sugar or other high glycemic food, but were injected with a glucose solution GOT DENTAL CAVITIES. This means it may be possible to avoid cavities by avoiding high glycemic food, or by taking soluble fiber with high glycemic foods. I hypothesize the fiber supplementation would render sugary foods to be non-high-glycemic, and thus not cause cavities. Sugar in your mouth may still may be a factor, but to what degree I'm not sure: I have to find that mouse study again, as maybe they used controls for comparison.

The site nutritionfacts.org states that whole grains that need to be chewed are better than the same powdered whole grains, because some pieces make it through to the large intestine undigested, leaving a meal for beneficial bacteria.

The only upper limit I could find is the following from Duke University, that recommends not too much above 50 grams, and gives an example of what it would take to eat 64 grams:
https://studentaffairs.duke.edu/studenthealth/nutrition-services/fiber

“Signs You’re Consuming Too Much Fiber:
Gastrointestinal distress, which may include bloating, gas, constipation, cramping and/or diarrhea.”

However, these conclusions are not backed by citations to research, and may well be the personal experience of one person. While helpful, the conclusions are more hypothesis than fact.

From the discussion above, fiber may help in regulating unhealthy insulin responses
https://mekineer.com/health/zero-calorie-sweeteners#insulin-response-to-lingual-stimulation
https://www.marksdailyapple.com/dairy-insulin

++++ Old Notes |

Former website healingcrow.com writes that supplementing inulin is sufficiently untested, and could lead to bad outcomes:
https://www.scdiet.net/healingcrow/HealingCrow/www.healingcrow.com/ferfun/conspiracy/conspiracy.html
This is the company Natren's position, as mentioned in the healing crow article: https://www.natren.com/prebiotics.html

copied from here

Best overview article: https://authoritynutrition.com/fiber-can-help-you-lose-weight/

Viscosity rather than quantity of dietary fibre predicts cholesterol-lowering effect in healthy individuals
“ The degree of viscosity appears to be inversely related to glycemic response, with the more viscous dietary fibers producing greater effects. These fibers form viscous solutions when mixed with the gastrointestinal (GI)5 tract contents, slowing gastric emptying and thickening small intestinal contents. This may reduce contact between food and digestive enzymes and interfere with diffusion of nutrients to absorptive surfaces, thus slowing the rate at which glucose molecules become available for absorption at the small-intestinal brush border ” jn.nutrition.org

The following two, HPMC and Methylcellose, are not digested by bacteria in the gut, and therefore do not produce gas. I have been taking MC for about a decade. It works well. However, I am going to try some others, because I understand that you want fermentable fibers, to keep up the beneficial gut bacteria.

- HV-HPMC (hypromellose):
http://www.ncbi.nlm.nih.gov/pubmed/23146593
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3545536

- Methylcellulose A4M:
the nominal viscosity of METHOCEL A4M is 4000 cPs measured as a 2% aqueous solution at 20C

This one I have yet to try out and fully research:
- Triphala:
http://ajcn.nutrition.org/content/79/4/529.full ++++