The search for a DF definition, methods and role

The DF hypothesis spurred a quest for methods and a definition that would adequately characterize this material in food and emulate conditions in the digestive tract. It also launched many research projects looking at its health benefits. The only existing DF method, the CF method, was found wanting because its strong solvents and harsh conditions failed to approximate DF’s fate in the digestive tract [22–25].

Interest in DF’s physiological role stimulated both a re-examination of older intervention studies and new research, especially epidemiological research, which tried to characterize the relationship of DF to various health conditions. Existing studies, most with isolated fibers, indicated DF’s beneficial effects on laxation, gut health, nutrient availability, enzyme activity, and cholesterol or atherosclerosis reduction [26–28].

Current DF definitions and methods

The 2009 Codex definition resulted from a nearly two-decade process undertaken by CODEX Alimentarius Commission (CAC), whose mission it is to provide global guidance and harmonization about matters regarding food and international trade [1, 2]. (Table 2 gives a brief overview of the CAC.) The definition and its two footnotes read as follows:

Footnote 1 states, “when derived from a plant origin, dietary fibre may include fractions of lignin and/or other compounds associated with polysaccharides in the plant cell walls. These compounds also may be measured by certain analytical method(s) for dietary fibre.

The two footnotes require comment. Footnote 1 aligns with most definitions and recognizes ‘associated substances’ and lignin when part of, but not isolated from, the DF complex.

The CODEX definition and footnote 2

Footnote 2 allows national authorities the option of including digestion-resistant oligomers with DP 3–9, thus enabling different operative definitions of DF. This is counter to CODEX’s mission, which is to facilitate international harmonization for food labeling, food composition tables, and interpretation of research. For example, results of studies including DP 3–9 could differ from those excluding them. DF values for foods such as wheat and onions that contain digestion-resistant oligomers with DP 3–9 or resistant starch (RS) could differ on food labels and in food composition tables. Thus, epidemiological research using food composition tables might also differ if some countries include these materials and others do not.

The exclusion of resistant CHOs with DP < 10 is an artifact of early DF methods and is not helpful for two reasons. First, an initial alcohol wash elutes short-chained fibrous materials. The loss of this short-chain, non-digestible material is not based on differences in physiological impact or data showing different health outcomes, but merely differences in solubility. Second, the alcohol separation has been shown to lack a precise cutoff at DP = 10, and may elute materials with DP > 10 [25]. Thus, the arbitrary cut-off at DP 10 does NOT have a sound basis either analytically or physiologically.

Resistant short-chain oligomers (DP 3–9) should be DF because they fit the definition in the following ways: 1) they are neither digested nor absorbed by the enzymes in the small intestine, 2) they are fermentable in the large intestine, 3) they aid laxation in the large bowel, and 4) they may increase mineral absorption [30–32]. If these materials are not included as DF, they would be cast into a ‘no-man’s land’ of being neither a digestible CHO nor DF [33]. DF experts from around the world attending the 2010 International Vahouny Dietary Fiber conference voted overwhelmingly for the inclusion of all indigestible CHO oligomers and polymers with DPs of 3 or higher [34].

DF intakes

There is a concerning gap between recommendations and intake worldwide [11–13, 49]. For example in North America, the average daily consumption of adults is 15 g/d, which means that those in the lowest quintiles of intake ingest below half the recommended levels [50–52]. Evaluation of NHANES 2003–2006 data show that under 5% of the US population ingests the recommended intake [11, 12].

A few initiatives have been successful in reducing the DF gap, such as programs from the Grains & Legumes Nutrition Council (formerly GoGrains) in Australia and New Zealand [53]. However, in most countries intake has not increased, despite a consistent body of evidence associating DF with reduced health risks [54–64] and messages from health promotion organizations to increase DF intake for both prevention and management of disease [13, 65–67]. Food intake surveys in the U.S. show that intake has remained roughly the same for over nearly two decades [11, 12, 50]. The DF gap even occurs in children in many countries and manifests itself as chronic constipation [68, 69]. Inadequate intake of DF in childhood is thought to be associated with health risks including obesity in later life [70].

“All fibers fit” to address the fiber gap

Adoption of CODEX-aligned definitions provides a strategy to help address the fiber gap by acknowledging the role of both intrinsic and added fibers. The ‘all fibers fit’ mantra means that healthy diets include the various fiber types [31]. Constructing a diet with a balance of DF types is analogous to constructing a diet with the correct amount and balance of the vitamins needed for optimal health. The similarities are outlined as follows. First, vitamin requirements are fulfilled only when all vitamins are present both in quantity and type. In like manner for DF, the optimal diet would have both right quantity of total DF and balance of fiber types. Second, vitamins inherently in food and those added through fortification and enrichment can work together to fulfill the requirements [71, 72]. The same can apply to DF in that added fibers can interact with intrinsic fibers to meet the requirement act synergistically actually to reap health benefits.

Variety is one of the important concepts of healthy diets. Just as a balanced diet incorporates adequate amounts of each vitamin and avoids excesses of any one kind, an optimal diet should incorporate the right amount and balance of fiber different types needed to perform DFs’ many functions. Thus a varied diet that uses fiber-fortified foods together with fiber-rich foods has multiple benefits. It ensures inclusion of associated substances that are trapped in the DF matrix of natural foods, allows health benefits from the synergy of DF types and functions, and the fiber gap while staying within a day’s calorie allotment.

NHANES data from 2001–2004 document that under 5% of consumers in most age and gender categories meet the adequate intake (AI) for fiber [12]. Less than 10% of the population meets the whole grain recommendation [80, 81]. Fruit and vegetable intakes are about half that recommended, and again the higher-fiber choices within each groups are selected less frequently [81]. Per capita consumption of legumes is extremely low at 25 g/d (0.1 cup/d or 20 calories/day) [81].

Addressing the ‘fiber gap’ not only requires a continuing push towards greater intake of fiber-rich foods and the use of more foods with added fibers, but also needs to include consumer education about fiber sources. Surveys show that consumers are looking for more DF, but also reveal that they have difficulty choosing foods or groups of foods that are DF sources, identifying isolated fibers on the ingredient statement, or estimating the DF content of foods and diets [82–84].

Statements by some health professionals and in corporate advertising campaigns suggest that added fibers are ‘fake’, and that only foods with natural fiber can fulfill DF requirements. While it is true that added fibers may not carry all the ‘co-passengers’ found in the food fiber matrix, isolated fibers do show documented health benefits [85]. In fact these benefits are so well documented that many countries allow health claims for isolated fibers such as wheat bran and β-glucan and its derivatives. In some cases isolated fibers and their derivatives are more concentrated both in terms of DF and trapped co-passengers enabling intake of a smaller amount of food for an equivalent physiological dose than would required from the whole food. For example, the addition of oat bran and β-glucan to a serving of oatmeal yields a cholesterol-lowering dose of DF with half the calories of an equivalent dose of oatmeal without added DF [86].

The synergy of fiber inherent in foods and that added to foods has been documented and needs to be emphasized [87–89]. For example, a meta-analysis of intervention studies suggests that type 2 diabetes risk reduction is reached either with isolated fiber or with DF from whole foods [90]. Improved blood lipids and weight management was shown to occur with isolated fiber added to a diet with a baseline of 20 g of DF. Discounting of the contribution of added fibers because of their lack of epidemiological evidence fails to consider studies such as this cited meta-analysis [90].

Teasing out the effects of isolated DFs from epidemiological studies is methodologically challenging because food databases used in the analysis of food frequency only report total DF and do not attribute DF source. Thus health impacts of added fibers may be not be captured. Further, food composition data may not reflect added fibers especially those that are short-chain oligomers and resistant starch, so their presence may be underestimated [91]. Thus, epidemiological studies attribute all health benefits to DF that are inherent in foods and fail to recognize the contribution of added fiber or its synergy in impacting health and disease. However, data from intervention studies clearly show that increases in total DF intake through fibers added to a baseline of diet rich in whole grains, fruits and vegetables, legumes and other fiber-rich foods synergistically improves health outcomes over those observed with lower fiber intakes [87–89].