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6 Functional Medicine Labs That Can Help Diagnose The Root Cause of IBS-D

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6 Functional Medicine Labs That Can Help Diagnose The Root Cause of IBS-D

IBS is a diagnosis that puts a name to chronic abdominal pain and abnormal bowel movements. But, the diagnosis, unfortunately, doesn't help us understand why these symptoms occur in the first place. IBS is common, affecting up to 15% of the United States population, and is considerably associated with poorer quality of life. Many with this diagnosis are left feeling frustrated with a life-long prognosis of poor symptom control. This article will discuss a functional approach to IBS, which can provide solutions for those struggling to manage their IBS.


What is IBS-Diarrhea (IBS-D)?

IBS is characterized by recurrent abdominal pain associated with abnormal bowel movements.  It is known as a functional gastrointestinal disorder (FGID) because the gastrointestinal (GI) anatomy appears normal despite present symptoms.

IBS is subcategorized into IBS-Constipation (IBS-C), IBS-Diarrhea (IBS-D), and IBS-Mixed (IBS-M), dependent upon the predominant stool type. In IBS-D, at least 25% of abnormal bowel movements are loose and watery.

IBS-D Signs & Symptoms

Characteristic symptoms of IBS-D include:

  • Abdominal pain and cramping, usually improved after defecation
  • Diarrhea
  • Fecal urgency
  • Abdominal bloating
  • Mucus in the stool

Bloody stools, fever, and unintentional weight loss are uncharacteristic of IBS and may indicate a more serious ongoing GI condition.

What Causes IBS-D?

The exact cause of IBS is not understood, but it is generally perceived to result from the interplay of:

  • Abnormal gut motility
  • Immune system dysfunction
  • Imbalance in the intestines' resident bacterial, viral, and fungal colonies
  • Hypersensitivity of the central nervous system and its interpretation of pain signals

From a functional standpoint, many underlying factors can lead to one or more of the above mechanisms.

Intestinal Dysbiosis

Some people may develop IBS as a result of traveler's diarrhea. This post-infectious IBS (PI-IBS) almost always has a diarrheal component and is characterized by lingering IBS symptoms despite clearance of the inciting pathogen.

Intestinal dysbiosis, referring to an imbalance in the non-pathogenic intestinal microorganisms, is a characteristic feature of IBS. This 2020 meta-analysis concluded that patients with IBS have lower Lactobacillus and Bifidobacterium levels and higher E. coli and Enterobacter levels within the gut.

Small intestinal bacterial overgrowth (SIBO) is the overgrowth of bacteria in the small intestine.  Intestinal permeability and excess gas formation caused by SIBO translate to the many characteristic symptoms of IBS. Some studies indicate that nearly 80% of patients with IBS also have SIBO.

Intestinal parasitic infections are quite common, estimated by the CDC to affect millions of Americans annually. Common symptoms of parasitic infections include diarrhea, gas, bloating, and abdominal pain. Unfortunately, parasitic infections are underdiagnosed due to the wide variation in testing accuracy (4). However, sufficient evidence supports the increased prevalence of parasitic infections, especially Blastocystis, Cryptosporidium, and Giardia, within the IBS population (1-3).


Bile acid malabsorption is a common cause of chronic diarrhea and abdominal pain. When bile acids, molecules that aid in fat digestion and absorption, aren't adequately reabsorbed in the small intestine, excess concentrations in the colon cause excess fluid secretions, diarrhea, and abdominal cramping. Literature reveals that over a third of patients with IBS-D have elevated bile acids in their stool.

The pancreas plays a role in digestion by synthesizing and secreting various digestive enzymes.  Exocrine pancreatic insufficiency (EPI) refers to the underproduction of these enzymes, leading to maldigestion of macronutrients and loose, fatty stools. This recent 2022 study concluded that 5% of patients with IBS-D met the criteria for EPI.

The underproduction of hydrochloric acid (stomach acid or HCl) is referred to as hypochlorhydria. HCl supports the digestion of proteins in the stomach and acts as an antimicrobial agent, killing pathogens passing through the upper digestive tract. When stomach acid is deficient, digestive symptoms occur due to maldigestion and an increased risk of developing SIBO.

Food Hypersensitivities

Adverse food reactions, encompassing food allergy, sensitivity, and intolerance, have been reported by up to 65% of patients as an identifiable component of their IBS (5). There are different mechanisms behind why a person may respond poorly to a specific food:

  • Food Allergy: An allergic response is activated by IgE immune proteins, producing symptoms within seconds to minutes of being exposed to the trigger. (6)
  • Food Sensitivity: An immune response is activated by IgG proteins, leading to delayed symptoms that can occur up to three days after exposure. (6)
  • Food Intolerance: The body is unable to properly break down a portion of food, due to an enzyme deficiency (i.e., lactose intolerance) or a reaction to naturally occurring chemicals in a food (i.e., histamine intolerance). (5)

Hormonal Imbalances

Women are twice as likely to have IBS as men. Receptors on gastrointestinal cells are primed to react to female sex hormones, estrogen and progesterone. Fluctuations and imbalances in these hormones are associated with worsening IBS symptoms. (7)

Elevations in cortisol, the body's stress hormone, have also been noted among IBS patients. (7)

Functional Medicine Labs to Test for Root Cause of IBS-D

SIBO can be diagnosed via an at-home breath test. The Trio-Smart Breath test has the capacity to measure all three gas types associated with SIBO: hydrogen, methane, and hydrogen sulfide.  Typically, hydrogen and hydrogen sulfide SIBO subtypes are correlated with a diarrheal component.

A comprehensive stool analysis measures many intestinal biomarkers that offer insight into digestive function, inflammation, and the microbiome, proving to be a valuable and reliable tool in diagnosing intestinal infections, dysbiosis, and digestive enzyme insufficiencies.

Blood testing is available for food allergy and food sensitivity testing. These tests measure the immune response to common food triggers. Vibrant Wellness offers a combined panel that measures IgE proteins for food allergies and IgA/IgG proteins for food sensitivities. These blood tests do not pick up on food intolerances. A genetic test is available to assess an individual's susceptibility to gluten and lactose intolerance. There are also breath tests that diagnose fructose and lactose intolerance.

A comprehensive hormone panel measures sex hormones, stress hormones, and their metabolites. Understanding hormonal cascades provide insight into how hormones may contribute to gut function and vice versa.

Other Labs to Check

Before diagnosing IBS-D, practitioners should rule out celiac disease (CD) and inflammatory bowel disease (IBD). Your doctor may order an endoscopy or colonoscopy to image and biopsy the small and large intestines, which can diagnose CD and IBD. A less invasive serum Celiac panel can be ordered before an endoscopy to assess the likelihood of CD being present. A patient should consume gluten daily for four to six weeks prior to this blood test to decrease the chance of inaccuracies in test results. Additional inflammatory markers may also be elevated in the presence of either of these diseases, including fecal calprotectin and C-reactive protein (CRP).

A complete blood count (CBC) and comprehensive metabolic panel (CMP) are routine blood tests generally ordered in the initial evaluation of GI complaints. A CBC assesses the number and quality of red and white blood cells; abnormalities may be present in active intestinal infection, inflammation, or malabsorption. A CMP provides information regarding the health and function of the liver and gallbladder, along with measuring electrolytes, which can become imbalanced with chronic diarrhea.

The gut-thyroid axis is the well-documented notion that the health and function of the digestive tract and the thyroid gland are interdependent. Monitoring thyroid function with a comprehensive thyroid panel is essential to assess, as thyroid dysfunction can be both an underlying cause and a consequence of IBS.

Conventional Treatment for IBS-D

The goal of conventional treatment of IBS-D is to provide symptom relief. A typical conventional protocol may include the following:

  • Lifestyle modifications: sleep, exercise, and stress management
  • Dietary modifications: fiber supplementation and low-FODMAP diet
  • Medications: antidiarrheals, antispasmodics, antibiotics, and low-dose antidepressants

Functional Medicine Treatment for IBS-D

The successful treatment of IBS-D relies on the understanding that there is no universal treatment approach that works for everyone. Protocol customization dependent on an individual's history, labs, and preferences is critical in IBS treatment protocols. Commonly implemented components of a functional medicine IBS-D treatment plan are discussed below.

IBS-D Diet

Because adverse food reactions are so common in patients with IBS, dietary therapy is almost always a component of an IBS treatment plan.

Fiber generally helps relieve IBS symptoms by improving stool consistency, supporting the growth of Lactobacillus and Bifidobacterium, and interacting with the nervous system to decrease pain signals. Most people do not meet the recommended minimum of 25g of dietary fiber daily. Non-fermentable fibers, such as psyllium, oats, and partially hydrolyzed guar gum, are usually best tolerated by those with IBS.

An elimination diet is strongly encouraged for IBS patients, especially if food reactions are suspected, or testing reveals positive results. Generally, an elimination diet is followed for four to six weeks and then followed by a rechallenging of foods back into the diet.

Herbs & Supplements for IBS-D

Carminative and spasmolytic: herbs are those that help to break up intestinal gas and soothe muscular contractions. The most popular in the context of IBS-D is peppermint. This meta-analysis concluded that enteric-coated peppermint oil is a safe and effective therapy for IBS management.

STW5 (IberogastⓇ): is a safe and effective herbal formula for treating functional gastrointestinal disorder, including IBS. Symptom relief is attributed to this formula's spasmolytic and anti-inflammatory effects. (8, 9)

Probiotics: 2-8 billion CFU, total probiotics per day for minimum 3 months is a common diagnosis for probiotic therapy. Probiotics are live microorganisms that support the gut by balancing the microbiome. Two meta-analyses have shown Bifidobacterium and Lactobacillus probiotics to be safe and effective in treating IBS (10, 11). Research also supports using Saccharomyces boulardii, a beneficial yeast, to successfully treat several diarrheal diseases, including IBS-D (12-14). Bacillus coagulas, Lactobacillus acidophilus, and Bifidobacterium bifidum have been shown to decrease stool frequency in IBS-D patients.

Antimicrobials: may be needed to remove unwanted microorganisms from the digestive tract.  Rifaximin is a prescription antibiotic that is commonly implemented in the treatment of IBS-D and SIBO. A functional practitioner may also utilize antimicrobial herbs in combination with or as an alternative to antibiotics. Herbal antimicrobial formulas have been shown to be just as effective as Rifaximin in treating SIBO. The GI Effects test is unique in its ability to show which prescriptive and natural agents were most effective at inhibiting the growth of the bacteria to the patients microbiome.

Digestive Enzymes: Various digestive enzyme formulas are available to provide digestive enzyme support for the proper digestion of foods if an enzyme insufficiency is present.

Pectin: at 24 grams total per day for a minimum 6 weeks showed a reduction in stool frequency, duration of symptoms, frequency of vomiting, need for oral rehydration and need for intravenous fluid in children aged 5-12 months with persistent diarrhea.

IBS-D patients supplemented with pectin improved symptoms as shown by quality of life, composite symptom score; as well as improved immune function as shown by normalized IL-10/IL-12 ratio and improved fecal bacteria composition with an increased bifidobacteria and decreased clostridium spp.

Lifestyle Changes for IBS-D

How we eat can be just as important as what we eat when it comes to digestive symptoms.  Healthy eating habits include:

  • Practice mindfulness during meals: sit down to eat; eat meals slowly and chew food thoroughly
  • Listen to hunger signals: eat when you are hungry, and stop eating when you are full
  • Implementing mindfulness-based therapies to manage stress, such as meditation and talk therapy, improves the quality of life in patients living with IBS.
  • Given this essential role that the vagus nerve plays in the gut-brain axis, stimulating the vagus nerve has been show to reduce stress induce IBS flares.
  • Any moderate physical activity, ranging from 20 to 60 minutes three times weekly, has been associated with reducing IBS symptom severity. Exercise enhances the number of beneficial microbial species, enriches the microflora diversity, and improve the development of commensal bacteria. All these effects are beneficial for the host, improving its health status.


IBS-D is a prevalent and disruptive functional gastrointestinal disorder. Because the underlying cause is elusive, conventional therapies often fail to achieve complete remission of symptoms. The utilization of functional testing and individualized, integrative treatment plans can be highly successful in targeting underlying factors contributing to intestinal dysfunction and in providing symptom relief and resolution.

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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Lab Tests in This Article


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