With the help of new research, our understanding of all nutrients increases over time. However, it would be hard to find a nutrient that we have learned more about in the past ten years as fiber. Everywhere you look, you will find debates about how the term "fiber" should best be defined, and about which substances should and shouldn't be classified as types of fiber. We want to provide you with full cutting-edge information about these debates, but at the same time, we want to give you simple recommendations about how to get optimal fiber benefits in your meal plan. So before explaining the new information that scientists have been discovering about fiber, our basic WHFoods Recommendations for fiber are as follows.
Food groups richest in fiber include Beans & Legumes, Vegetables, Fruits, Grains, and Nut & Seeds. Because different proportions of insoluble/soluble fiber, viscous/non-viscous fiber, and fermentable fiber can be found within each of these different food groups, we recommend that you rely on all five groups to provide you with optimal fiber intake. You could approach this first fiber goal on a weekly basis, or you could work these five food groups into your meal plan within a shorter period of time if you preferred.
A second goal would be meeting a minimal recommended intake level for fiber. Our recommendation at WHFoods is the same as the Food and Drug Administration's Daily Value (DV) of 25 grams. For us, this fiber level should be considered as a minimal versus optimal level. Public health organizations have not set upper limits on their fiber recommendations and we are glad that they haven't because we have seen health benefits being associated with higher levels of fiber intake, and we have not seen evidence of problems with fiber intake even three to four times the DV level in healthy persons who are routinely and consistently getting these levels of fiber from whole, natural foods. Our Healthiest Way of Eating Plan, for example, averages 52 grams of daily fiber, and we feel confident in the potential health benefits associated with that amount, even though it is double the Daily Value.
But before we get to current debates about fiber, let's start with some agreed-upon facts about fiber that have not changed and remain key features of this nutrient.
First, fiber has always been recognized as a plant-based nutrient. To make a simple comparison, plants have fiber in the same way that animals have muscles and bones. Fiber allows plants to maintain their shape and structure. And even though fungi (including mushrooms) are classified by scientists as belonging to their own separate category of living things, we would want to include them alongside of plants in this fiber discussion since they often contain chitooligosaccharides (CHOS) which most researchers consider a type of fiber. Although not widely popular in the U.S., there are other non-plant foods that contain CHOS and would be considered by many researchers as non-plant sources of fiber. Insects would be the largest single group here, followed by crustaceans like shrimp, crab, and lobster. While these last three crustaceans can be popular foods in the U.S., their CHOS content is not usually consumed because it is found in their outer shells which typically aren't consumed. At WHFoods, the bottom line here is simple: for most U.S. eaters, plant foods (and fungi like mushrooms) are going to be your exclusive whole, natural food sources of fiber.
Second, fiber does not digest in the same way that most other nutrients digest. Most nutrients undergo full digestion as they get chewed, chemically transformed in our stomachs, and then combined with enzymes and digestive fluids in our small intestine. This combination of chewing plus digestive fluids plus digestive enzymes is usually sufficient to allow our bodies to digest and absorb nutrients from food. In the case of fiber, however, the above processes do not substantially alter the fiber's basic nature. Fiber passes all the way through our stomach and small intestine and then proceeds on to our large intestine, still largely recognizable as dietary fiber. Our chewing and digestive fluids and enzymes are not enough to dramatically change it. It is only in the last portion of our digestive tract—our large intestine—that fiber can undergo a major transformation; if this transformation occurs, it is not brought about by human enzymes or human digestive fluids but rather by trillions of bacteria living in our large intestine.
Given these two basic distinctions that are fully embraced by all researchers, how has fiber become such a complicated nutrient to define? First, most scientists like to define nutrients according to their chemical structure. Their confidence in the nourishment provided to us by a vitamin like vitamin C is closely related to their ability to define vitamin C in chemical terms (i.e., a 6-carbon molecule with the chemical formula C6H8O6). The fact that vitamin C functions as an antioxidant is not regarded as a defining characteristic of vitamin C, since many nutrients function as antioxidants. So scientists tend to rely on definitions involving chemical structure. But researchers also know that a simple chemical formula is not possible when it comes to fiber. From a chemistry standpoint, fiber is structurally diverse, and this diversity has made fiber more difficult than other nutrients for researchers to define and universally adopt.
Second, while some of the health benefits of fiber are unrelated to its bacterial breakdown in the large intestine, many important benefits from this nutrient depend on its transformation by bacteria. As you might imagine, with literally trillions of bacteria in our large intestine and as many as 1,000 different species, interactions between any substance and bacteria can be difficult to predict. So it can be difficult to conclusively determine if a substance should be classified as a fiber based on any function that it might serve following transformation by bacteria.
However, there are also some important disadvantages to this simple classification system. First, it is possible for fibers to have soluble and insoluble components. In this case, the distinction tends to be less helpful. Second, the vast majority of foods providing fiber not only contain both soluble and insoluble types, but they often contain both in nutritionally significant amounts. So this distinction isn't always helpful in making food choices. Third, these two categories are often insufficient to account for important health benefits. For example, within the soluble fiber group can be found both viscous and non-viscous soluble fiber. Viscous soluble fibers are gel-forming and much more closely associated with cardiovascular and blood-sugar regulating benefits than non-viscous soluble fibers. So as you can see, even though this distinction between soluble and insoluble makes sense and has value, it has seemed less helpful over time as researchers learn more and more above fiber and try to determine the best way to obtain optimal benefits from fiber-containing foods.
In 2005, the National Academy of Sciences (NAS) updated its classification system for fiber to include three basic categories: Dietary Fiber, Functional Fiber, and Total Fiber. Since the Total Fiber category was simply a combination of the first two categories, we will focus on those first two. The NAS described Dietary Fiber as fiber naturally found in plants in an intact form. By contrast, they described Functional Fiber as fiber that could naturally be found in plants, but could also be synthesized. In addition, they noted that while Functional Fiber could naturally be found in plants, it should not be placed in this category unless it has been isolated from its natural plant context through food processing. This contrast between Dietary Fiber and Functional Fiber makes sense to us if the goal is to look comprehensively at fiber with an approach that includes processed foods and dietary supplements. There are a large number of foods to which fiber has been added during processing, as well as numerous dietary supplements containing fiber.
However, a complication with this fiber classification system involves the inclusion of most fiber types in both categories. For example, some resistant starch falls into the Dietary Fiber category, and other resistant starch falls into the Functional Fiber category. The same is true for other fiber types, including oligosaccharides, pectins, gums, and lignin. So this classification can be confusing in this regard. In addition, since we only focus on whole natural foods at WHFoods, Functional Fiber (as defined by the NAS) would not be present in any of our foods, since it requires isolated fiber rather than intact, whole food fiber.
Fermentable Carbohydrate: You can find this term being used more and more often to refer to fiber. It is a term that simply refers to the ability of fiber to pass through our stomach and small intestine without being broken apart by digestive fluids and digestive enzymes, and to make it all the way to our large intestine where bacteria can "ferment" it into other substances. "Ferment" in this context simply means metabolize it into other forms. The category of fermentable carbohydrate is a great way to focus on the role of fiber in supporting balanced bacterial populations in our large intestine, but it does leave aside the other many valuable health-support roles of this nutrient.
The EFSA system contains four basic categories of fiber based on its chemistry. From our perspective, this approach does a good job of explaining the full range of substances that can be considered as types of fiber. We would also note that all of the fiber types in the EFSA system can be found in whole, natural foods. We use this classification system for thinking about the fiber content of our foods, and have found it helpful in putting fiber research studies into context. The chart below summarizes the EFSA categories and specific types of fiber belonging to each category.
|Non-Starch Polysaccharides (NSP)||Resistant Oligosaccharides||Resistant Starches||Lignin*|
|celluloses||fructo-oligosaccharides (FOS)||numerous dextrins|
|pectins||chito-oligosaccharides (CHOS)||some high-amylose starches|
|hydrocolloids (gums, mucilages, beta-glucans)|
*There is only one basic molecule with a single uniform structure called "lignin," even though its three basic components (three different cinnamyl alcohols) vary in their proportion.)
Our recommendation for obtaining fiber from all five major fiber-containing food groups (Beans & Legumes, Vegetables, Fruits, Grains, and Nut & Seeds) is largely based on distribution of the ESFA fiber types throughout these five groups.
As you sit reading this article, the food that you've eaten in the past day is slowing inching its way through your digestive tract. Some days, this may seem like a sprint, but in fact, it's more of a marathon. Between the small and large intestine, food and other waste products need to travel more than 25 feet to get to the end of the journey.
This process is controlled by very carefully choreographed muscle movement, which in turn is partly triggered by the presence of food inside of our digestive tract. So, in a real sense, the amount of food inside our intestine partly controls the rate at which our muscles move.
Insoluble fiber plays a unique role in this process. Because it cannot be broken down in the small intestine, and because it has the ability to attract water to it, insoluble fiber can help control the consistency of food in our digestive tract as well as the pace at which it passes through. (To put the result in everyday terms, it can help with "regularity.") However, viscous soluble fiber also has a role to play in this process, alongside of the role played by insoluble fiber. This second type of fiber helps control the "thickness" of your food as it gets digested. It's especially important when food is in our stomach because it can help pace and regulate the passage of food out of our stomach and along to our small intestine. This moment in the digestive process is called "gastric emptying," and soluble, viscous fiber is known to help slow gastric emptying, as well as helping keep it on a steady pace. Since rate of gastric emptying is related to our blood sugar and blood insulin balance, you can see how helpful soluble viscous fiber can be in steadying the amount of food that begins its passage through the intestinal tract at any one moment in time.
Numerous cardiovascular benefits are associated with fiber intake, but we will start with one of the best-documented benefits: improved control of blood cholesterol levels. Viscous soluble fiber is able to bind with cholesterol in the intestine and prevent its uptake into the body by allowing it to be eliminated in the stool. The most viscous fibers—including the beta-glucans found in barley, oats, sea vegetables, shiitake mushrooms, and other foods, as well as the pectins found in the skins of cherries, grapes, berries, citrus fruits, and other foods—have all been shown to have blood cholesterol-lowering effects. Soluble fibers, and especially viscous soluble fibers, appear to be the best type for blood cholesterol benefits. It's important to note that in addition to the foods rich in pectins and beta-glucans listed above, there are other foods also provide substantial amounts of soluble fiber, including soluble fiber with varying degrees of viscosity. In this extended list would be included numerous other fruits and vegetables. Choosing fiber-rich fruits and vegetables is usually going to helpful in getting the cholesterol-lowering effects of soluble fiber.
Improved cholesterol levels, however, are not the only cardiovascular benefit provided by food fiber. Blood pressure reduction has also been associated with fiber intake, as has reduced overall risk of high blood pressure (hypertension). Overall risk of cardiovascular diseases—and risk of specific diseases including atherosclerosis and coronary heart disease—has been shown to decrease in association with healthy fiber intake.
A number of research teams have been able to demonstrate that the addition of high-fiber foods to a regular meal not already rich in fiber can improve blood sugar control if done on a consistent basis over a period of weeks (and in some cases months). In addition, studies on blood sugar problems in whole populations have shown better blood sugar control when healthy levels of fiber are consumed. In other words, fiber-rich meals can help most of us regulate our blood sugars in a healthier way. Interestingly, some studies also show that the blood sugar benefits of a fiber-rich meal can often extend throughout the day, even after a second or third meal is eaten. As an example, a fiber-rich breakfast might be able to help us steady our blood sugar levels even after eating lunch or dinner. So you can see how the meal-by-meal benefits of fiber-rich founds can overlap and help establish a steady, ongoing healthy basis for blood sugar balancing.
As mentioned earlier, part of the blood sugar-balancing benefits of fiber come from the special ability of soluble, viscous fiber to impart a slow release of food from the stomach (called gastric emptying). But blood sugar benefits from soluble fiber go even further. High-fiber meals significantly increase production of a hormone called glucogon-like peptide 1 (GLP-1), a hormone known to reduce blood sugar levels. Interestingly, one research group has also suggested that this GLP-1 response gets triggered not directly by the presence of soluble fiber, but by the products of fiber fermentation by bacteria in the large intestine. So you can see how the blood sugar benefits of fiber may involve several different factors. It is also important to note that risk of adult onset, type 2 diabetes and insulin resistance have also been found to decrease with healthy fiber intake.
There are two key ways in which fiber helps to maintain colon health. "Colon" in this context simply means all of our large intestine. Both ways involve the bacteria that live in our large intestine.
A first way that we see fiber helping to maintain colon health is by directly acting as fuel for the growth of "friendly" bacteria. Some of the bacterial species in our gut are so specialized that they can digest specific subtypes of fiber. For example, some species of Bacteroides break down hemicellulose as their primary fuel source, while others in the same class (bacteria that share 96% of their DNA) break down pectins as their main fuel. The reason that bacteria can digest nutrients that humans cannot comes down to their production of very specialized enzymes. In other words, there is a very intimate and mutually supportive relationship between fiber in our diet and populations of bacteria in our large intestine. These bacteria do best when our fiber intake is best, and our colon stays healthiest when these bacterial populations are thriving and in balance.
The second way that fiber helps support colon health is also related to bacterial digestion of fiber. As food fibers are digested in the large intestine by bacteria, their metabolism can provide the short-chain fatty acids (SCFAs) that cells along the large intestinal wall use as a fuel source. SCFAs—and in particular, one SCFA called butyrate—are critical for colon health because they provide cells that form the lining of the colon with the fuel they need to carry out their metabolic activities. In addition to this key support of normal intestinal function, healthy intake of fiber has also been associated with reduced risk of colon cancer.
An important point to remember about WHFoods and fiber is the fact that nearly three-quarters of our foods rank as good, very good, or excellent sources of this nutrient! Except for our animal foods, you will find ranked sources of fiber in all of our food groups, and in our Herbs & Spices as well. Fiber is a showcase nutrient for most any plant-based meal plan, and our WHFoods meal plans are no exception.
For the sheer total amount of fiber available from any single food group, it would be difficult not to place Beans & Legumes at the top of the list. If you measure our WHFoods strictly on the percent of the Dietary Reference Intake/Daily Value that they provide, all of our top 10 fiber-rich foods except one (barley) belong to the Beans & Legume food group. Of course, this approach does not factor in nutrient-richness and the number of calories provided by each bean serving. But even when calories and our full rating system approach is used for evaluation, Beans & Legumes still account for half of our top 10 foods.
A one-cup serving of most beans will contain approximately 10-20 grams of fiber and often provide at least half of our daily minimum recommended amount. In addition, beans contain both soluble and insoluble fiber, often at a ratio of about 1 gram of soluble to 3 grams of insoluble.
Our nutrient-richness rating system approach also places three vegetable greens—collard, turnip, and beet greens—in our top five fiber-rich foods. While you will not get 10-20 grams of fiber per serving, you will still get about 5 grams of fiber per serving and at a much lower cost in terms of calories. To give you one simple comparison, 15 grams of fiber from pinto beans will cost you about 240 calories out of your daily calorie total, and 15 grams of fiber from collard greens will cost you about 63 calories. Of course, both of these outstanding foods provide you with a wide variety of nutrients that extend far beyond fiber.
We also see whole grain foods contributing sizable amounts of dietary fiber. Grains tend to contain about five to ten grams of fiber per 1 cup serving. This food group tends to favor insoluble fiber, often with a ratio of insoluble to soluble fiber of 4:1 or sometimes greater. Oats are an exception here, with a somewhat evenly balanced ratio of the two fiber types.
Collard greens, turnip greens, and beet greens are by no means alone in providing rich amounts of fiber within the vegetables group. Other standout vegetables included in our top 25 fiber-rich foods are green peas, winter squash, broccoli, spinach, Brussels sprouts, green beans, and cabbage. Fruits also appear on our top 25 list. Within the fruit group, raspberries, pears, and cranberries are our top fiber-rich foods.
As mentioned earlier, it would be wrong to omit Nuts & Seeds as a fiber-rich group of foods. From this group we have flaxseeds in our top 25, but receiving rankings of a good source are sesame seeds and almonds.
Finally, it is important to remember that Herbs & Spices can also contain valuable amounts of fiber. Cinnamon would be a special standout here.
Let's build a sample diet that draws on a variety of food groups for outstanding fiber results. For breakfast, we'll start with granola with fresh fruit This choice will start the day with 9 grams of fiber. This combination will also provide us with valuable amounts of both soluble and insoluble fiber. For lunch, we'll have black bean chili, a recipe that contains nearly 26 grams of fiber per serving. We'll enjoy it with a pear to temper the heat from the spicy chili, and to provide another 5 grams of fiber.
For dinner, we'll make baked miso salmon. For side dishes, let's add 5-minute Brussels sprouts and warm quinoa salad. This meal will give us a variety of different fiber sources from the vegetables, grains, and herbs and spices. The 29 grams of fiber this meal will bring our fiber total to nearly 70 grams for the day!
|World's Healthiest Foods ranked as quality sources of|
|Navy Beans||1 cup||254.8||19.11||76||5.4||excellent|
|Collard Greens||1 cup||62.7||7.60||30||8.7||excellent|
|Turnip Greens||1 cup||28.8||5.04||20||12.6||excellent|
|Beet Greens||1 cup||38.9||4.18||17||7.7||excellent|
|Dried Peas||1 cup||231.3||16.27||65||5.1||very good|
|Lentils||1 cup||229.7||15.64||63||4.9||very good|
|Pinto Beans||1 cup||244.5||15.39||62||4.5||very good|
|Black Beans||1 cup||227.0||14.96||60||4.7||very good|
|Lima Beans||1 cup||216.2||13.16||53||4.4||very good|
|Tempeh||4 oz||222.3||12.00||48||3.9||very good|
|Kidney Beans||1 cup||224.8||11.33||45||3.6||very good|
|Barley||0.33 cup||217.1||10.61||42||3.5||very good|
|Wheat||1 cup||151.1||8.19||33||3.9||very good|
|Green Peas||1 cup||115.7||7.58||30||4.7||very good|
|Winter Squash||1 cup||75.8||5.74||23||5.4||very good|
|Pear||1 medium||101.5||5.52||22||3.9||very good|
|Broccoli||1 cup||54.6||5.15||21||6.8||very good|
|Cranberries||1 cup||46.0||4.60||18||7.2||very good|
|Spinach||1 cup||41.4||4.32||17||7.5||very good|
|Brussels Sprouts||1 cup||56.2||4.06||16||5.2||very good|
|Green Beans||1 cup||43.8||4.00||16||6.6||very good|
|Cabbage||1 cup||43.5||3.90||16||6.5||very good|
|Flaxseeds||2 TBS||74.8||3.82||15||3.7||very good|
|Swiss Chard||1 cup||35.0||3.67||15||7.5||very good|
|Asparagus||1 cup||39.6||3.60||14||6.5||very good|
|Carrots||1 cup||50.0||3.42||14||4.9||very good|
|Oranges||1 medium||61.6||3.14||13||3.7||very good|
|Strawberries||1 cup||46.1||2.88||12||4.5||very good|
|Mustard Greens||1 cup||36.4||2.80||11||5.5||very good|
|Fennel||1 cup||27.0||2.70||11||7.2||very good|
|Cauliflower||1 cup||28.5||2.68||11||6.8||very good|
|Kale||1 cup||36.4||2.60||10||5.1||very good|
|Summer Squash||1 cup||36.0||2.52||10||5.0||very good|
|Eggplant||1 cup||34.6||2.47||10||5.1||very good|
|Tomatoes||1 cup||32.4||2.16||9||4.8||very good|
|Kiwifruit||1 2 inches||42.1||2.07||8||3.5||very good|
|Romaine Lettuce||2 cups||16.0||1.97||8||8.9||very good|
|Chili Peppers||2 tsp||15.2||1.88||8||8.9||very good|
|Bell Peppers||1 cup||28.5||1.85||7||4.7||very good|
|Bok Choy||1 cup||20.4||1.65||7||5.8||very good|
|Black Pepper||2 tsp||14.6||1.47||6||7.3||very good|
|Cloves||2 tsp||11.5||1.42||6||8.9||very good|
|Celery||1 cup||16.2||1.40||6||6.2||very good|
|Garbanzo Beans||1 cup||269.0||12.46||50||3.3||good|
|Sweet Potato||1 cup||180.0||6.60||26||2.6||good|
|Sesame Seeds||0.25 cup||206.3||4.25||17||1.5||good|
|Mushrooms, Shiitake||0.50 cup||40.6||1.52||6||2.7||good|
|Plum||1 2-1/8 inches||30.4||0.92||4||2.2||good|
Density>=7.6 AND DRI/DV>=10%
Density>=3.4 AND DRI/DV>=5%
Density>=1.5 AND DRI/DV>=2.5%
Fiber in foods does not get lost during storage. This is good news, since many important fiber-rich foods have a relatively long shelf life. Dried beans, for example, can often be safely stored for up to one year. As a reminder here, our individual food profiles provide exact storage times and steps not only for beans, but for each of our individual WHFoods.
While changes in fiber content can occur along with cooking, we do not see these changes as being problematic with whole, natural foods; relatively short cooking times; and everyday cooking methods. Beans are an interesting category of fiber-rich foods in this regard because they are typically only consumed in cooked form. We've look at fiber changes involved with overnight soaking of beans as well as stovetop boiling/simmering of beans and as a general summary, we believe you are still highly likely to get fantastic fiber-richness from cooked beans. All of the fiber data for our WHFoods beans and legumes has been calculated using their cooked form, and the same is true for all of our fiber data on grains.
Other processing steps can sometimes take a greater toll on fiber. Luckily, the canning of beans is not one of them. You'll be getting a similar level of total fiber in most commercially canned beans to the amount you will get if you cook simmer dry beans in your kitchen. However, because of the higher heats involved with canning and other processing factors, significant losses of other nutrients are definitely possible with commercial canning of beans. (Loss of molybdenum would be one example here.)
With the processing of grains, however, the impact on fiber can be much greater. The impact of grain processing on flour, for example, depends almost completely on the percent extraction used in production of the flour. While the terminology here can seem backwards from the meaning, you might expect "100% extraction" flour to mean that every single part of the original grain was included in the flour. Importantly, this 100% level includes all of the bran and all of the germ, which are the portions of the grain providing you with its fiber richness. Few breads or baked products are made with 100% extraction flour. The average extraction percentage for most U.S. breads and baked products is approximately 60%. This rate means that 60% of the original whole grain was incorporated into the flour, and 40% was discarded. Unfortunately, since most of the fiber-rich bran and fiber-rich germ from the grain was discarded, most of its fiber was discarded as well. To give you one practical example: it is not unusual to find "100% wheat bread" in the grocery store that has been made from 60% extraction flour and contains only 1 gram of fiber or less per slice.
While the cooking—and especially overcooking—of vegetables can result in significant loss of nutrients, fiber is not one of the nutrients greatly impacted by healthy cooking methods.
Based on the NHANES 2009-2010 survey of dietary intake, U.S. adults average only 16 grams of fiber intake per day. In fact, no age or gender group averages the Daily Value (DV) of 25 grams. For teenage girls, the average is about 12-13 grams, or the same as about 50% DV. A conclusion from this survey would be to point out that in the U.S. population as a whole, a person's risk of fiber deficiency is basically 100%. We just don't get anywhere close to enough fiber from our meal plans, even at the minimal DV level of 25 grams.
Many problematic practices contribute to this poor level of fiber intake. Included in the list would be routine consumption of heavily processed grain products, sparse consumption of fresh fruits and vegetables, and heavily reliance on animal foods. As explained in an earlier example, routine intake of whole, natural foods from the vegetables, fruits, beans and legumes, nuts and seeds, and grains food groups can easily boost fiber intake over the 25-gram level, and in the case of many of our meal plans, above the 50-gram level as well.
Beyond the problematic, everyday food practices listed above that greatly compromised fiber intake, other diet-related practices can impact fiber intake as well. Weight loss diets that encourage very liberal intake of animal foods and severe restriction on carbohydrates can often create too little fiber intake, since animal foods do not provide fiber and since most whole, natural fiber-rich foods also provide at least moderate amounts of total carbohydrate.
While outside the scope of this nutrient profile, it is worth noting that in treatment of some health conditions requiring special feeding methods and/or special diets, fiber intake is deliberately restricted to facilitate the healing process.
As researchers have learned more and more about the role of fiber in support of large intestine bacteria, they have also learned more about the relationship of fiber to other nutrients. As a general rule, researchers have traditionally dismissed the large intestine as a possible site for absorption of most nutrients, and they have focused instead on the small intestine when studying nutrient absorption. However, thanks to our present-day understanding of fiber as a key player in the health of our large intestine, we now know that metabolism of fiber by large intestine bacteria can result in absorption of certain nutrients from that area of the digestive tract. For example, the minerals calcium and magnesium are now included in the list of nutrients likely to be absorbed not only from the small intestine but from the large intestine as well, and in no small part due to the role played by dietary fiber. This recent research is causing new questions to be asked about the relationships between fiber and other nutrients. At least in the case of calcium and magnesium, it might be that case that increased intake of fiber in a whole, natural foods diet can increase availability of these two minerals as well.
As a general rule, research studies on fiber intake do not show problematic interactions with other nutrients. We've seen some studies showing small drops (less than 5%) in absorption of select nutrients when fiber intake was significantly increased through a combination of whole, natural foods plus processed food components (like oat bran), but high levels of fiber intake from whole, natural foods in whole population studies of diet and health generally correspond to high levels of nutrient intake overall.
As mentioned earlier, the National Academy of Sciences (NAS) did not set any Tolerable Upper Intake Level (UL) for fiber when its fiber recommendations were updated in 2005. From studies of diets worldwide, we also know that routine dietary intake can average 75-100 grams per day when whole, natural plant foods account for the vast majority of dietary intake. The big picture here appears to be a ringing endorsement of all fiber amounts that correspond to healthy intake of whole, natural foods, even when those foods are particularly fiber-rich.
Incorporating fiber-rich foods into your personal diet, however, can take some patience and some practice. From our perspective, it is often a mistake to try and improve your fiber intake by adding processed food components like purified brans or germs. If you do take this approach, we recommend "baby steps" in the addition of these processed fibers. Cramping, gas, bloating, and other problems are fairly common when processed fibers are added too quickly to a meal plan. (For many persons, "too quickly" might mean as little as one teaspoon per day for one week, and then an increase to two teaspoons the following week.) While these problems might not be considered to involve dietary toxicity risk, we believe they are worth pointing out in this section of a fiber profile. A far better approach—and the approach we recommend—is to gradually replace one or two commonly eaten low-fiber foods (for example, processed grains) or fiber-free foods (e.g., animal foods) on multiple days of the week with a fiber-rich food, and allow your digestive tract to gradually adapt to this increased-fiber intake. For example, it might take several weeks or even several months, for a person accustomed to consuming a 15 gram-per-day meal plan to gradually increase fiber-rich, whole, natural foods intake to a level two or three times that amount.
Finally, we would repeat our previous mention that certain health problems can call for restricted fiber intake. In these situations, high intake of fiber could be thought of as posing a risk of dietary toxicity risk. But in the vast majority of situations, high intake of dietary fiber not only poses no toxicity risk, but has been shown to make possible health benefits that may not be available at minimal intake levels.
In 2005, the National Academy of Sciences (NAS) updated its recommendations for fiber intake. These recommendations appear below.
The NAS chose not to establish a Tolerable Upper Intake Limit (UL) for fiber intake.
The Daily Value (DV) for fiber is 25 grams per day. The DV is the standard that you will see on food labels. The DV is also the standard that we adopted as our WHFoods recommended minimal daily intake level. However, our meal plans typically provide substantially higher amounts of fiber than the DV. As one example, our Healthiest Way of Eating Plan averages 52 grams of daily dietary fiber.