Abstract: This review is the first in a series of three articles considering how different types of dietary fibre may affect how the gut functions and gut health. This first review will focus on the impact of dietary fibre intake on the upper gastrointestinal tract (i.e. the mouth, oesophagus and stomach). While a larger body of evidence links fibre intake to bowel health and disease, it is apparent that the presence of fibre, whether as an added ingredient in foods, or as an integral part of the structure of plant foods, also plays key roles on oral and gastric secretions and upper gut motility. These actions are possibly modulated through fibre’s effects on the physicochemical properties of luminal contents in the gut.
The major physiological functions of the mouth, oesophagus and stomach are discussed and recent evidence relating dietary fibre intake to these actions is introduced. A summary of evidence linking habitual dietary fibre consumption to major mucosal diseases of the upper gastrointestinal tract is also provided.
Alex’s Notes: I am very fortunate to share with you the first article in a series of reviews that focus on the actions of dietary fiber on the gastrointestinal tract and critically consider the recent evidence in this field and highlight areas for future research, as well as highlight how consumption of more fiber-rich foods in the diet could relate to longer-term health outcomes within the gut. Speaking of fiber-rich foods, the authors rightly emphasize early on that
“Even with meticulous consideration of other confounding lifestyle factors (Kratz, Baars, & Guyenet, 2013), such evidence cannot separate the impact of dietary fibre intake from the intake of its major dietary sources (i.e. fruits, vegetables and cereal products) from other putatively beneficial components within these foods (Mellen, Liese, Tooze, Vitolins, Wagenknecht, & Herrington, 2007), or the broader effect that inclusion of high amounts of these foods within the diet may have to displace other less optimal food choices (Bogart et al., 2014 and Lazzeri et al., 2013).”
Alongside this, it is important to also note that “dietary fiber” includes a broad range of compounds with differing structures and physiochemical properties. As the first article in the series, the review at hand rightly focuses at the beginning of digestion with the mouth, esophagus, and stomach.
The mouth’s main role in food digestion is mastication (chewing) in order to grind food into a more homogenous and softer bolus for easy swallowing. Only minor macronutrient breakdown occurs here as a result of enzymes present within saliva (amylase and lipase for the nerds). Chewing of food has two implications that send signals to the brain: the amount of force needed to crush and grind, and the amount of (temporary) periodontal ligament deformity. There is no doubt that previous feelings of pleasure or enjoyment are associated with these eating experiences that ultimately result in preferences for specific food textures – of which dietary fiber plays an important role (at least in plants). Moreover, enhancing the fiber content of foods through manipulations that led to either increased hardness or increased gel rubberiness has been shown to increase chewing time.
Before food can be effectively swallowed, it must become soft and lubricated. Chewing accomplishes the former and saliva the latter as it effectively coats the mouth and esophagus for easy movement of the bolus. In fact, reduced production of saliva is one of a number of key factors that could drive dysphagia (a difficulty with swallowing food), and one of the main routes for dietary modulation to improve swallowing in dysphagic patients is addition of dietary fibers that change the rheological properties of food. Dietary fiber effectively increases saliva production, which in combination with an increased chewing time, leads to enhanced bolus formation and ease of consumption. Moreover, saliva contains a wide variety of antimicrobial factors that benefit oral health. The authors hypothesize that consumption of plant-based foods may directly help to remove bacterial plaques by increasing shear stress at the surface of teeth, or indirectly reduce bacterial numbers by increasing salivary flow.
After swallowing, the bolus enters the stomach where mechanical and chemical digestion occurs. Numerous studies demonstrate that increased gastric pressure during a meal leads to a reduced desire to continue eating (unless it is Thanksgiving), and the presence of dietary fiber within the bolus could form strong gels within the stomach acid that affects these processes. Moreover, fibrous foods are generally more voluminous, meaning that for a given caloric intake, one must eat more of these foods which would obviously expand the stomach to allow for a larger food intake. However, while intragastric volume may be an important driver of satiation and the lack of drive to continue eating, it is gastric emptying rates that are a main player in feelings of satiety and the want to eat again after a meal.
The current review summarizes numerous studies to come to several conclusions. First, factors other than intragastric viscosity such as nutrient content and rate of consumption play an important role in satiety. Second, the beneficial effects of fiber on post-meal energy intake are not retained over a relatively short time period. That is, the body becomes adjusted to changes in fiber intake so that the initial satiation effects are less pronounced with repeated exposure. The authors rightly point out that this highlights a need to examine longer-term changes on eating habits that accompany increased fiber intake. Third, evidence for stomach acid secretion as a result of mechanical and chemical stimulation exists, but there is a lack of studies that consider the impact of dietary fiber on stomach acid secretion. Some evidence does suggest that fiber impacts pepsin secretion and gastric mucus production in a manner similar to fiber’s impact on stomach acid, but these effects are only seen with unrealistically high fiber intakes (20% of diet).
Up to now, the review outlined the acute effects of dietary fiber consumption, and
“The final section will focus on the association between fibre intake and long-term health outcomes in relation to the upper gastrointestinal tract (particularly cancer). The vast majority of evidence linking fibre to such long-term outcomes is from observational studies in large populations. As previously stated, it is important to consider that the vast majority of fibre consumed by participants of such studies will be in the form of plant foods (particularly fruits, vegetables and grains). Separation of health benefits of dietary fibres from other dietary components in these foods is therefore not possible from even the most careful data interrogation.”
Starting again in the mouth, it was recently demonstrated that an eight week high-fiber diet led to significant improvement in many periodontal disease markers, but these values returned to baseline 24 weeks after the intervention period at finished. Observational evidence suggests that higher fiber diets reduce bone and teeth loss with aging. Head, mouth, and neck cancers showed minimal relationships to fiber intake.
Gastric reflux is an area of increasing concern as it has been linked to Barrett’s esophagus and throat cancers. Unfortunately, intake of highly fermentable carbohydrates can lead to increasedrisk of gastric reflux occurring. That said, one previous intervention study assessed the impact of a low-fat, high fruit and vegetable diet on biomarkers of cellular transformation within the esophageal tissue of 87 participants with Barrett’s esophagus and found no impact for the entire three year intervention period; although it must be noted that biomarkers did not deviate from baseline values to a high degree in either the intervention or control group. More recently, a meta-analysis noted a total of eight studies demonstrated a consistent inverse association with increasing fiber intake and esophageal cancer risk, while five studies showed no impact, and two studies suggested a significant inverse relationship with increasing fiber intake and Barrett’s esophagus risk.
To summarize part one of this review series,
“It appears as though most of the actions of dietary fibre on upper GI physiology are possible through actions on luminal bulking, increasing luminal viscosity and potential binding to other nutrients. It is difficult to predict whether effects of dietary fibre or fibre-rich food consumption at a single meal can appropriately model factors that may lead to disease progression. It is therefore recommended that such acute physiological data are rationally considered and discussed critically within the scientific literature. Further longer-term studies modelling the impact of fibre (and other dietary factor) intake are necessary before attempting to relate these findings to health outcomes.”
Nonetheless, the evidence suggests that increasing fiber intake through fiber-rich foods is consistently associated with reduced disease risk. Stay tuned for part 2, whenever it gets published.