[I think for me, the most compelling bit of evidence for 25-50g per day of fermentable, prebiotic fiber is the fact that human breast milk contains 15-25g of HMOs (fermentable, prebiotic fiber) per 700 calories. Recognizing that a baby's immune system is weak, they obviously maybe need more immune stimulating fibers than an adult. But relatively speaking, it's clear to me that recommending adults get more than 25g per day is a no-brainer.
A baby, from birth until he, or she, is weaned, ingests about 15-25g of fermentable fibers per day. Recommending an adult get at least that much, via resistant starches, inulin, and a whole host of other fibers makes perfect sense to me.]
Most recent fiber studies:
Literally hot off the press are these two papers:
Fiber consumption and all-cause, cardiovascular, and cancer mortalities: A systematic review and meta-analysis of cohort studies (Liu, 2015)
A database was developed based on 25 eligible studies and 42 cohorts, including 1,752,848 individuals with an average 12.4 years of follow-up. The range of follow-up period was from 1 to 40 years.
And the result?
The present study showed that high dietary fiber intake is associated with low all-cause mortality and CVD, cancer, IHD mortality. The mortality rate was lower by 17% for CVD, 11% for all-cause mortality, 9% for cancer, 20% for CHD mortal- ity, and 34% for IHD mortality for each 10 g/day increment of total fiber. We believe that the results presented represent most of the information available on dietary fiber and mortality.
This paper is good in that it just looks at "fiber" and doesn't get too anal about the type of fiber. The author feels that after looking at the fiber intakes of almost 2 million people and following them for up to 40 years, she can comfortably say that for each 10g of fiber you eat, you ward off serious disease by an average of 11%. So our 20-50g/day is starting to look pretty smart!
This second paper, Prebiotic intake reduces the waking cortisol response and alters emotional bias in healthy volunteers (Schmidt, Dec 2014), looks at the levels of stress markers (cortisol, etc..) after consuming FOS or GOS. In terms of mental well-being, they compared GOS to a healthy person taking prescription drugs!
Participants receiving B-GOS supplements showed increased attentional vigilance to positive versus negative stimuli on the dot-probe task. Our effects are similar to those seen following administration of pharmacological agents such as the selective serotonin reuptake inhibitor citalopram [antidepressant 'Celexa']or the benzodiazepine diazepam ['Valium'] in healthy individuals. These effects have been interpreted as showing an early anxiolytic-like profile, where threatening stimuli are less likely to be attended to.
In other words, GOS mellows you the hell out! Or is it just gut bugs doing what they do when well-fed?
These study participants took only 5.5g of supplemental FOS or GOS at breakfast for 3 weeks. The study found GOS (as Bimuno) to be more effective in stress reduction than FOS. This could be a dose/adaptation issue, but compelling nonetheless.
Towards a better definition of prebiotics
In 2008, the original definition of "prebiotic" was tweaked a bit by the ISAPP to reflect current understanding:
Dietary prebiotic is a selectively fermented ingredient that results in specific changes, in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health.
Much more is known now than in 1994 when Roberfroid was looking at inulin and bifidobacteria.
This picture illustrates some of the beneficial actions of prebiotics:
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And this chart:
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Where to from here?
It seems that the focus of "fibers" shifted to "prebiotics" in the '90's when it was realized that humans had very animal-like guts. Many rebelled! Konstantin Monastryrky wrote his Fiber Menace in 2008 and many leaders in the diet world stayed stuck in the '70's with their advice on fiber..."Roughage! Bah!"
Into our bowel-regularity-worshipping society there has come a substance that ensures regularity. It’s called fiber. It is sold everywhere in diverse forms. All manner of ‘experts’ from our doctors to our grandparents encourage us to consume plenty of fiber... And, despite numerous studies showing that fiber doesn’t really do squat for us healthwise, everyone continues to recommend it. (Dr. Michael Eades, 2006)
In the 2010's, it seems everyone is talking about gut health and the true value of our hind-gut is finally coming to light. The focus will probably shift once again when people realize that in order to have 'good guts' when need to feed our gut bugs.
Lots of books and blogs are dedicated to gut health, but it is rare they discuss prebiotics beyond "fiber." Many recognize the importance of of microbial populations, but focus almost completely on probiotics and fermented foods.
Doctors often recommend fiber to bowel disease patients. While not wholly without merit, this advice usually backfires.
There are three problems: helping bacteria feed and multiply may be undesirable; fiber, such as the brans of cereal grains, often contains toxic proteins; and, finally, whole grain fibers and other “roughage” scrape and injure the intestinal wall. (Dr. Paul Jaminet, 2010)
Unfortunately it seems that most new research into prebiotic fibers is all focused on the food industry. Manufacturers of processed food are always looking for ingredients that are cheaper and add some type of perceived benefit to the consumer. Does adding 1g of Partially Hydrogenated Guar Gum to low fat, pasteurized chocolate milk make it any healthier?
The National Fiber Council
There is no end to fiber supplements on store shelves, showing quite clearly that people are buying fiber. The National Fiber Council (yes, a real thing!), established in 2004, has a mission, "...to serve as a primary resource for credible information about the benefits of dietary fiber for consumers and health care professionals." The NFC recommends healthy adults should get 32g of fiber per day.
Reading through the NFC website is a bit painful. Some of their soundbites on fiber are correct, but they've 'dumbed' it down considerably. The wheat in their logo is questionable. Their entire website just seems to be an infomercial for buying fiber supplements. Their recommendation for 32g of fiber is quickly shown to be unattainable in the US diet. They include a scientific looking study paper that concludes:
Because of the undesirably low levels of dietary fiber intake in the US population, partnerships between fiber supplement manufacturers, food producers, and health authorities may be required to educate consumers about the health benefits of dietary fiber intakes from a variety of supplements and foods.
Unfortunately this astute conclusion is clouded a bit by the "Acknowledgement" section:
Funding. Preparation of this manuscript was funded, in part,by the National Fiber Council,which is supported by Procter & Gamble and by the High Carbohydrate, Fiber (HCF) Nutrition Research Foundation. Declaration of interest. JW Anderson serves as Chairman of the National Fiber Council (funded by Procter & Gamble),as a member of the Scientific Advisory Council, Breakfast Research Institute (funded by Quaker- Tropicana-Gatorade), and the International Scientific Advisory Board (funded by Sanitarium). He is a consultant to Cantox, Cargill, DSM Nutritional Products, Exponent, Kao, Soy Research Institute, and Unilever.
To further make things difficult, the Food and Drug Administration closely guards products from making claims about the healthful properties of fiber.
For instance, if a company wants to discuss soluble fiber's effects on coronary disease, they are only allowed to discuss it in terms of oat beta-glucans and psyllium and include these additional requirements:
The product must be:
- Low saturated fat
- Low cholesterol
- Low fat, and
- The food product must include one or more of the following whole oat or barley foods: Oat bran, rolled oats, whole oat flour, whole grain barely, or "Oatrim" or Psyllium husk.
It does not appear that the FDA even recognizes the term "prebiotic" and a quick look at their labeling approval process makes it look unlikely that there will be any legally recognized prebiotics on the market soon.
To further complicate things, researchers are now recognizing we have limitations in understanding the complex interactions between gut microbes and human physiology. A 2013 paper, The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism, shows:
Although much is known about the biochemistry of the conversion of carbohydrates into SCFAs by the bacteria composing the microbial community, there is a paucity of data on the production rates of SCFAs by the gut microbial community as a whole. This is largely due to the inability to sample the large intestine of man. Therefore, and as discussed in the previous section, the supply rate of SCFAs to the host remains enigmatic. There is a pressing need for measurement of true production rates of SCFAs, and the degree by which specific carbohydrates and microbiota influence the mass and composition of SCFAs.
In the '70's, researchers were pleading for more studies to determine if bacteria were an important aspect of the fiber puzzle. In 2013, researchers are a bit more focused on what we need to know (from the same link as above):
A number of questions need to be addressed:
1) What are the in vivo SCFA production and uptake fluxes under different conditions (i.e., with different fibers, with different microbiota, or in different disease models)?
2) How do these SCFAs then affect glucose and lipid fluxes via their dual role as substrates and regulators?
3) Can we quantify the role of different tissues and hormones?
4) Does the demand of the host for specific SCFAs drive a change in microbial metabolism?
5) At which timescales are different, apparently contradictory effects working?
The International Scientific Association for Probiotics and Prebiotics (ISAPP)
The ISAPP seems like a much more reputable organization that the National Fiber Council. Scrolling through their website, you'll see they have many corporate sponsors, but that's probably unavoidable. The ISAPP seems more geared towards probiotics, but they have also been looking at prebiotics and fiber. They are quite behind the times in their recommendations for prebiotics, as stated in their "Consumer Guideline" pamphlet:
Which prebiotics are in the marketplace? As the main reason for using a prebiotic is to provide benefits through gut bacteria, the most tested prebiotics are directed towards bifidobacteria and (to a lesser extent) lactobacilli. Future prebiotics may promote other beneficial gut organisms (for example, eubacteria, propionibacteria , faecalibacteria or roseburia). The most widely accepted prebiotics are FOS and GOS. To confirm prebiotic effects, well conducted human trials are required. Consumers should look for the labels FOS, inulin (a type of FOS ), GOS or TOS (trans GOS). There is a growing list of further prebiotics such as polydextrose, soybean oligosaccharides, isomalto - oligosaccharides, gluco - oligosaccharides, xylo - oligosaccharides. palatinose, gentio - oligosaccharides, some starch derivatives and sugar alcohols (such as lactitol, sorbitol and maltitol). However, the evidence for these, especially in humans, is not as well advanced as it currently is for FOS and GOS.
As one researcher said of dietary fiber, it also seems that defining a "perfect fiber" is hopeless. We can, however, use the available science and safely experiment with increasing the amounts of fiber in our diet to meaningful levels and use supplements to obtain the desired level of prebiotic fibers in our diet.
Even the ISAPP recognizes that getting all of your prebiotic fiber from real foods is difficult:
What foods can I find them in? Some dietary prebiotics occur naturally in foods such as leek, asparagus, chicory, Jerusalem artichoke, garlic, arti choke, onion, wheat , banana and oat, as well as soybean. However, it would take a large quantity of these foods for their active oligosaccharides to exert a useful prebiotic effect. A more realistic method involves fortifying popular foodstuffs with defined amounts of prebiotics. Thus, you will find that dietary prebiotic compounds are now added to many foods including yogurts, cereals, breads, biscuits, milk desserts, nutritional supplement bars, ice - creams, spreads, drinks, water, infant formula, as well as to some animal foods. They are also available as powdered, gum or liquid supplements.
The ISAPP doesn't trust us to seek out our own prebiotics, they feel we will only get them if they 'sneak them in' by adding prebiotics to snack foods!
Responders vs. Nonresponders
If you've read any of the papers from the '70', '80's, and '90's. You may have noticed that all of the early researchers were baffled when some human test subjects responded differently than others to the same type and dose of fiber. We now know that this has to do with the resident gut bacteria possessed by the test subjects. A 2010 paper, Responders and non-responders to probiotic interventions How can we improve the odds?, also talks about resonders and non-responders to prebiotics. The authors recommended that future studies on probiotics and prebiotics should focus on:
- Clearly define the end goal: this could be supporting a health claim or having the highest clinical effect and impact.
- Design the study to maximize the chance of a positive response by identifying precise parameters and defining the level of response that will be tested.
- Base the selection of the intervention on scientific investigations: which strain(s) and/or product formulation should be used and why.
- Carefully select the study cohort: use biological or genetic markers when available to stratify the patient population before enrollment and decide at what point intervention will provide the best outcome (for example, in acute phase of disease, or during remission, with or without use of pharmaceutical agents).
If you are desperately trying to find the right combination of prebiotics (or probiotics), keep in mind that the odds may be stacked against you. Your genetics, for instance, your ability to secrete fucose in your gut as a food source for certain micobes, can have a huge impact on your gut flora. Physical deformities that alter your rates of peristalsis, the movement of food through your GI tract, also impact gut flora.
Your personal history plays a role. Antibiotic use, exposure to toxins, sickness and disease. Also lifestyle factors...sleep, stress, and exercise impact your gut flora and how you may respond to interventions of prebiotics.
I'm afraid that for the time being we are on our own. You can expect more in-fighting between camps as we try to attract attention to our favorite microbes or favorite fibers.
The "perfect fiber" is the one(s) that you can comfortably add to your diet and gives you the positive results you seek.