Narrative Review: Fiber
Fiber Is Essential
Carnivore/keto/low carb zealots often downplay the health benefits associated with fiber consumption. This review goes over some of the research pertaining to the benefits of fiber intake. Hopefully, you will see why fiber intake is essential for optimal health.
Dietary fiber is the edible part of plants that are resistant to digestion but are absorbed in the small intestine. There are two main categories of fiber, soluble and insoluble fiber. Soluble fiber can dissolve in water to form viscous gels which are largely fermented by bacteria in the large intestine. Insoluble fiber cannot be dissolved by water so fermentation is limited.
The recommended amount of dietary fiber is 14 grams for every 1,000 calories per day, or, about 25 grams for women and 38 grams for men each day for Americans. However, most do not eat the recommended amount of fiber outlined in the guidelines.
Fiber intake seems to be important for various reasons, including its inverse relationship with all cause mortality, but in this review I will specifically cover heart disease, weight loss, digestive health, insulin resistance, and cancer risk.
Heart Disease
Heart disease or cardiovascular disease is one of the leading causes of death in the United States if not the number one leading cause of death. Dietary fiber intake seems to be important for reducing serum cholesterol levels and blood pressure, high serum cholesterol and hypertension are consequential factors that can lead to further issues down the line.
An umbrella review of meta analyses (so a meta analysis of meta analyses) concluded higher fiber intake to possibly be preventative of cardiovascular disease (CVD) as it is persistently associated with decreased risk. Interestingly, this umbrella review outlines the effect of fiber intake on serum lipid levels reported by each of the meta analyses analyzed. Higher fiber intake seems to reduce total blood cholesterol and more importantly LDL cholesterol. LDL cholesterol is a known causal factor of atherosclerosis.
The mechanisms by which fiber seems to help reduce risk of cardiovascular disease seem to lie in itâs ability to lower blood cholesterol levels. Dietary fiber is fermented by gut bacteria which leads to short chain fatty acid production. Short-chain fatty acids are rapidly absorbed from the large intestine and can be oxidized for energy production. Absorption of short-chain fatty acids such as propionic acid has been shown to decrease cholesterol synthesis in the liver, leading to decreased blood cholesterol. Fiber intake also seems to increase bile acid synthesis which can lead to decreased cholesterol because cholesterol is converted to bile acids in the liver.
Both observational and intervention trials seem to point to the protective effects of fiber against cardiovascular disease. This possibly due to fibers ability to decrease total cholesterol (LDL cholesterol more specifically) and decreases in blood pressure.
Weight Loss
High fiber intake is notoriously associated with a decrease in weight. Foods higher in fiber, such as fruits and vegetables, tend to lead towards greater weight loss and reduced adiposity. Other foods such as whole grain containing carbohydrates are also associated with decreases in body weight in an almost dose-response manner. However, this review stresses caution as dietary fiber alone does not inherently cause weight loss outside of caloric restriction.
This relationship between dietary fiber and weight loss can be due to a number of factors which impact CICO. When soluble fiber is fermented in the large intestine this can lead to the production of GLP-1 and PYY, both hormones associated with satiety. Being satiated can lead to reductions in caloric intake (calories in) which can possibly lead to weight loss. Dietary fiber may decrease a diets metabolizable energy (ME), which is gross energy minus the energy lost in the feces, urine and combustible gases. Insoluble fiber can also induce an increased rate of passage through the GI tract thus leading to less nutrients being digested and absorbed which can also impact caloric intake.
Even fiber in supplemental form leads to insignificant reductions in weight if any reductions at all. Perhaps this relates to baseline intakes of fiber, maybe the supplemental dose is too small to see an effect, or maybe only in conjunction with other factors does fiber have an effect on weight loss. Nevertheless, we are less concerned with fiber in of itself or in supplementary form but as a constituent in ones overall dietary pattern related to weight loss.
Foods high in fiber tend to be more nutrient dense foods. Nutrient dense foods tend to contain less calories per gram, contain high amounts of nutrients, and are often contain higher amounts of water. Energy dense foods by contrast tend to have more calories per gram of food, less nutrients, and are often highly palatable.
A year long intervention instructed two different groups of women with obesity to reduce fat intake while increasing fruit and vegetable intake or to just reduce fat intake. The group which increased their fruit and vegetable intake lost more weight. More recently we see those who followed a low fat plant based diet consumed less calories compared to a animal based ketogenic diet when subjects consume how ever much food they want. These results can be due in part to the higher dietary fiber intake participants engaged in.
Foods high in fiber such as fruits, vegetables, and whole grain are strongly associated with weight loss. Weight loss has its own assortment of health benefits.
Digestive Health
A notable issue described by a few who go on low carb or no carb diets is constipation. The evidence seems to be scarce on this given issue as their is no scientific evidence on an all meat no carb diet (the carnivore diet), so we have to rely on anecdotes. Anecdotes are notoriously unreliable and unsystematic but in the absence of more robust evidence it will have to do.
However, we do have evidence pertaining to fiber intake and constipation. Fiber intake seems to increase stool frequency (you poop more often) and softens stool consistency (softer poop is easier to pass out of the colon). This can indicate the importance of fiber intake for avoiding constipation.
Another common digestive issue is irritable bowel syndrome or IBS. IBS showcases consistent symptoms of abdominal bloating and pain whilst also having various subtypes. These subtypes refer to a personâs stool pattern like if the person is dealing with predominately constipation, diarrhea, both, or something else.
As mentioned before dietary fiber intake has an almost laxative effect allowing individuals to take a number 2 more frequently with favorable consistency, this can reduce some IBS symptoms. In addition, dietary fiber acts as a prebiotic which influences the gut microbiome. As remarked earlier the fermentation of dietary fiber leads to the production of short chain fatty acids which can decrease intestinal inflammation. This fermentation process is also related to the growth of beneficial bacteria in the gut. Decreased inflammation in the gut as well as beneficial bacteria could possibly explain the improvements in global symptoms of IBS, these global symptoms are the aforementioned abdominal bloating and pain.
Outside of constipation and IBS, dietary fiberâs impact on the gut microbiome is important. The bacteria composing itâs own environment in the large intestine has seen great interest over the passing years. Many interesting theories and hypotheses, but still numerous unknowns. What seems to be the case is dietary fiberâs role as prebiotic is important for microbiota diversification, favorable modification, and creation of short chain fatty acids (SCFAs). SCFAs possess key roles in regulating metabolism, immune system, and cell proliferation. Low intake of dietary fiber can lead to reduced microbial diversity, SCFA production, and can shift the gut microbial metabolism toward the utilization of less favorable substrates like dietary protein.
Insulin Resistance
Diabetes, in particular type two diabetes, is a major concern for western countries as rates of diabetes are rising in the developed world. Diabetes relates to the dysfunction of the hormone insulin, insulin seems to be important for cells utilizing glucose in the body. Without getting too much into the etiology of diabetes we can differentiate type two from type one diabetes merely from the fact type two relates to lifestyle factors more.
Individuals with type two diabetes have decreased insulin sensitivity (insulin resistance) and hyperglycemia (high blood sugar). Insulin resistance is basically the body not responding to insulin in an appropriate manner. This is important as insulin resistance is the precursor to type two diabetes.
One major associated factor for type two diabetes, therefore insulin resistance, is excessive amounts of body fat. This is also confirmed in the DIRECT trial as those who lost weight had their type two diabetes go into remission. Reversal of type two diabetes may be possible from dietary restriction and weight loss itself.
These findings are presented to bring context to the discussion. Other factors besides fiber intake are important for type two diabetes reversal and increasing insulin sensitivity. Nevertheless, dietary fiber can lead to decreased gastric emptying and decreased absorption of nutrients which can result in lower postprandial glucose and insulin levels.
Dietary fiber intake is associated with decreased risk of type two diabetes. This association persist even when correcting for age, height, bodyweight, smoking, fat intake, and exercise levels. There does not seem to be a distinction between soluble fiber and insoluble fiber as both seem to decrease risk of two type diabetes.
Cancer Risk
Dietary fiber seems to be protective against the infamous cancer as it can mitigate cancer risk even for those consuming high amounts of red and processed meat. However, the primary area of interest seems to be the relationship between fiber and colorectal cancer. Colorectal cancer is the third most commonly diagnosed form of cancer and the second leading cause of cancer death worldwide.
As established previous, dietary fiber impact has notable positive impact on the gut microbiome and GI health in general. When fiber intake is low gut microbial metabolism begins to use more unfavorable substrates such as amino acids. Protein fermentation by gut bacteria due to a lack of fiber occurs in the distal colon where colorectal cancer often appears. Perhaps this can shed some light into the relationship between red meat, processed meat, and colorectal cancer.
The research currently seems promising but it primarily consist of in vitro and animal based models. Nonetheless, the mechanistic data seems to corroborate with the epidemiological data. As individuals with the highest fiber intake had reduced risk of distal colon cancer and incidence of colorectal adenoma (a benign tumor which can turn cancerous). Other factors such as physical activity, energy intake, education level, and red/processed meat intake matter in addition to higher fiber intake.
A meta analysis of prospective cohort studies found a dose response relationship between fiber intake and colorectal cancer risk. Increased dietary fiber intake per day lead to decreased risk. Higher fiber intake seems to be important along with other considerations for lowering risk of colorectal cancer.
Discussion
High dietary fiber intake can reduce total cholesterol levels, reduce LDL-C levels, is inversely associated with obesity, helps against constipation, can reduce global symptoms of IBS, is beneficial for the gut microbiome, decreases risk of type 2 diabetes, helps regulate blood sugar, and reduces risk of colorectal cancer.
When people say âfiber is not essentialâ please tell them to shut the f*ck up. Dietary fiber is ESSENTIAL for optimal long term health as it is protective against many commonly feared chronic diseases.