Specific Carbohydrate Diet: The Definitive Guide to SCD Diet
The Specific Carbohydrate diet (aka SCD diet) has been touted as a potential treatment for several gut health issues, including IBD and SIBO. In this guide, we're going to first look at what the diet actually involves, including a list of the legal and illegal foods. We'll analyze why the diet may work. And then we'll continue with the research and see where the diet may not work. This is the ultimate guide to the SCD Diet! Let's go.
Table of Contents
What is the SCD diet?
Sydney Haas MD proposed the Specific Carbohydrate diet (SCD) in the 1920s to treat celiac disease. S
The diet was then popularized by Elaine Gottschall’s book, Breaking the Vicious Cycle, in the late 1980s. The book describes how to implement the SCD and claims its benefit as a treatment for various gastrointestinal diseases.
The premise of the diet is that foods containing specific types of carbohydrates are more difficult to digest. This results in undigested carbohydrates arriving in the colon, causing fermentation and overgrowth of bacteria and yeast.
Gottschall proposes that fermentation of specific carbohydrate types shifts the microbiome profile towards a more pro-inflammatory state. The result is intestinal injury and inflammation.
- The Specific Carbohydrate Diet (SCD) was proposed in the 1920s to treat celiac disease.
- The diet was popularized in the book, Breaking the Vicious Cycle, authored by Elaine Gottschall in the 1980s.
- The diet restricts certain carbohydrates, which are proposed to promote the overgrowth of inflammatory bacteria and yeast, resulting in intestinal injury and inflammation.
Defining Carbohydrate Types
Carbohydrates can be described by the complexity of their chemical structures -- essentially the number of connected “sugar” or starch units. Monosaccharides are the most simple form of carbohydrates consisting of a single sugar unit. Disaccharides are slightly more complex and consist of two sugar units chemically bound together. Polysaccharides are even more complex, with multiple sugar or starch units bound together. S
Legal and Illegal Foods
Foods containing monosaccharides are given the green light on the SCD, while foods containing disaccharides and polysaccharides are restricted. Gottschall makes a few slight exceptions to these rules by allowing for fiber from fruits, vegetables, and nuts to be consumed. Pg. 30 Other exceptions include certain polysaccharide-containing dried beans such as lentils, navy beans, split peas, kidney beans, and lima beans. After 3 months on the diet, these dried beans are permitted in the event they’ve been soaked for 10-12 hours with the soaking water discarded before cooking. Some exceptions include bean flours, pinto beans, chickpeas, soybeans, and bean sprouts which are not permissible at any time on the diet.
The SCD is broken down into ‘legal’ and ‘illegal’ foods.
- Unprocessed meats, most fresh vegetables and fruits, all fats and oils, aged cheeses, some beans and legumes that are prepared as specified in the book, and a homemade yogurt fermented for 24 hours to reduce the lactose content.
- Foods that have been determined to be ‘legal’ are foods claimed to not ferment in the colon or that are easily absorbed.
- The idea is to reduce bacterial growth and the triggering of gastrointestinal distress.
- Milk and most milk products, grains, soft cheeses, some beans/legumes, canned vegetables, seaweed, starchy tubers such as potatoes or sweet potatoes, and non-honey sweeteners.
- Illegal foods are labeled as such due to their chemical structure (i.e., they contain disaccharides and/or polysaccharides).
How to do the SCD diet
Step 1: The Introduction Diet (1-5 days)
It is suggested to follow the introduction diet for up to five days when diarrhea or cramping symptoms are severe. In other cases, one or two days on the intro diet are said to be sufficient. S
Sample SCD Intro Diet: Appropriate Foods
Homemade Chicken Broth
Chicken, Poultry, and Fish
Broiled Hamburger Patties
Homemade SCD cultured yogurt
Step 2: The Intermediate Phase
After the introduction diet, the SCD diet suggests proceeding “gradually and sensibly.” It’s noted that the pace of food introduction will vary from person to person, depending on tolerance. The only guidance available for initiating a more diversified SCD is as follows:
- Foods that have previously caused a severe allergic reaction should be permanently eliminated.
- If diarrhea is present, raw vegetables and fruits should be avoided, except for ripe bananas and avocados.
- Avoidance of vegetables in the cabbage family is suggested until diarrhea has substantially cleared.
- Fruit should be ripe, peeled, and cooked, and introduced gradually
- Cooked carrots, string beans, tomatoes, and squash are said to be well-tolerated
- Making baked goods with nut flour can be initiated -- about one nut flour muffin or two nut flour slices of bread per day, increasing gradually if tolerated.
- Raw fruits and vegetables may be tried after starting a “few days” with cooked vegetables and ripe, peeled cooked fruits.
Step 3: Longterm Application
The diet is intended to be followed for 1 year during active disease flare-ups and another year once symptoms resolve. S Gottschall advocates for ‘fanatical adherence’ to the diet for at least 1 year after the last symptom or flare-up. Pg. 67, 70
During the reintroduction phase, she suggests trying one ‘illegal’ food per week. However, if symptoms occur, it’s recommended to return to only eating foods from the ‘legal’ food list. Pg. 70 Gottschall does not specify when to start the reintroduction of foods while following the SCD.
Foods allowed on the full SCD without the introductory or intermediate phase restrictions are detailed in our SCD Diet Food List.
Rationale for the diet
Like other dietary strategies suggested for SIBO, the goal is to eliminate carbohydrates from the diet that can be fermented by microbes in the intestines.
Gotschall also compares the use of the SCD to an elemental diet. She states that, like the elemental diet, the SCD reduces microbial growth while nourishing the patient. Pg. 20
This is an interesting and somewhat conflicting point for Gottschall to make, given that the primary carbohydrates found in elemental diets come from maltodextrin and cornstarch -- two polysaccharide-rich ingredients prohibited on the SCD.
- The diet primarily restricts disaccharides and polysaccharides but allows for the consumption of monosaccharides.
- Foods are broken down into “legal” and “illegal” categories.
- Legal foods include unprocessed meats, most fresh vegetables and fruits, fats and oils, aged cheeses, certain legumes, honey, and a homemade yogurt fermented for 24 hours to reduce the lactose content.
- Illegal foods include milk and most milk products, grains, soft cheeses, some beans/legumes, canned vegetables, seaweed, starchy tubers such as potatoes or sweet potatoes, and non-honey sweeteners.
- The diet starts with a highly restrictive introductory diet which gradually broadens based on tolerance to encompass more SCD-approved foods.
- The rationale for using the SCD for treating SIBO is to eliminate carbohydrates in the diet that microbes in the intestines can ferment.
- Gottschall compares the mechanism of the SCD to the elemental diet -- an interesting point because the elemental diet contains SCD-prohibited ingredients.
- The diet is followed for one year during active disease flare-ups and another year once symptoms resolve.
- One ‘illegal’ food per week is reintroduced at a time -- the specifics of when and how this should happen are not clear.
- If symptoms occur after reintroduction, the strict SCD is to be resumed.
Our concerns with the SCD Diet
Lack of Supporting Evidence
No studies have explicitly evaluated the use of the SCD to treat or manage SIBO.
Clinical studies on the SCD have primarily been in patients with inflammatory bowel disease (IBD), particularly pediatric IBD patients.S The SCD appears to have a positive and significant impact on disease activity scores in pediatric Crohn’s disease and ulcerative colitis. S The diet also appears to improve inflammatory markers in this population. S
As we understood Gottschall’s theory behind the SCD, it was apparent why this diet was proposed for SIBO. The notion of removing indigestible carbohydrates that stimulate the growth of inflammatory bacteria and yeasts would seem appealing for small intestinal bacterial overgrowth.
However, it has to be clarified that while Gottschall claimed that the diet works by starving out inflammatory intestinal microbes, science has yet to provide a firm answer as to why the diet has therapeutic value in IBD.
Theories have included shifts in the intestinal microbiome, increased intake of antioxidants, a decreased intake of food additives, or the idea that monotony in the type of carbohydrates consumed might be beneficial in IBD. S, S
The exact mechanism for why the SCD is beneficial in the pediatric IBD population is unknown. It is also unknown if underlying mechanisms of the SCD showing benefit in other diseases such as IBD would be helpful in the management of SIBO.
Issues with the Introduction Diet
We have significant concerns about nutritional adequacy in the introductory and intermediate phases of the SCD. The diet is already restrictive, so we questioned if these early restrictive measures had a scientific basis to support their use.
Our hunch was validated after we sought another dietitian’s expertise who routinely implements the SCD in a pediatric IBD clinical research setting. The individual we spoke with has worked as part of a multi-site clinical trial on the SCD called the PRODUCE study, which is set to be published later this year.
In her clinical experience, the diet’s introductory phases would frequently result in inadequate calorie intake and malnutrition, mainly because patients would remain on these phases for too long. She noted that evidence supporting the use of these early-diet strategies was lacking.
As part of the clinical trial in which she contributed, she reported that the introductory diet was broken down into a two-stage approach. Most individuals would start on the second phase, which was the full SCD diet, without additional restrictions found in the introductory or intermediate stages. On rare occasions, patients would start on a “beginners'' phase of the diet. This typically was reserved for people initiating the diet who had been on an all-liquid or smoothie-based diet beforehand.
The beginner’s phase for these individuals would involve a full SCD diet except that fruits and vegetables were cooked soft, peeled, and de-seeded. Broths would also be included in this beginners phase. The main caveat was that patients had to agree to abandon the beginner’s diet phase within one week and move on to the full SCD to avoid malnutrition risks.
Long Duration and Poorly Defined Reintroduction
One observation we had when looking over the SCD parameters was the lack of clarity regarding reintroducing foods. We also found the duration of the diet to be extremely long.
A total of two years or more on the diet could be possible between the year during the disease flare-up and the one year after. Adding to this, introducing one restricted food per week would really drag out this diet trial. This is an exceptionally long time to try any restrictive diet, but especially one that hasn’t even been studied for the treatment or management of SIBO.
Adding to our concerns are studies in mice that have shown “microbiome scarring” after prolonged dietary restriction of carbohydrates that feed the microbiome. S In these studies, after restricting and then reintroducing these dietary components, the mice’s microbiomes did not make a full recovery to baseline after diet reintroduction. When these trials were repeated over generations in mice, the microbiome depletion became irreversible. S
For that reason, we think of prolonged elimination diets without clear reintroduction parameters as the “Russian Roulette” of diet strategies. When following such a diet over the long term, no one knows what’s happening to the microbiome or which helpful or harmful microbes might get axed in the process.
We also don’t know if it’s possible to get those microbes back after returning to a regular diet. Adding to this, we have no way of knowing if the overall changes will be for better or worse. For this reason, we believe the SCD, with its prolonged duration and lack of defined reintroduction parameters, is a significant gamble when it comes to SIBO management.
We believe it is best to abandon the SCD diet within one month if no symptom improvement is experienced. We also think working with a registered dietitian to establish proper timelines for food testing and reintroduction. Each food trial should involve a structured protocol to identify dietary symptom triggers.
Impact on the Microbiome
Changes to the microbiome across different studies and in case reports of individuals adhering to the SCD have been inconsistent. Some people have experienced a decrease in microbiome diversity after following the diet. In contrast, others experienced an increase in microbiome diversity. S
The limited data available on how the SCD impacts the microbiome makes it challenging to state if the diet suppresses microbial growth in the intestines. We also only have data from stool samples, which can’t tell us how the diet impacts the small intestinal microbiome. This is an essential point because the management of SIBO relies on reducing fermentation and the growth of unwanted small intestinal bacteria.
Moreover, with the diet having such varying effects from one person’s microbiome to the next, we don’t know which microbial species will be impacted and how that might change symptoms related to SIBO.
Does the SCD Restrict the Right Foods for SIBO?
Highlighting the SCD Starch Restriction
Carbohydrates restricted on the SCD include disaccharides such as lactose and polysaccharides, mainly in the form of starches. For healthy children and adults, most starch is broken down in the ileum, S which is the small intestine’s final segment. This means that starchy carbohydrates that aren’t already digested from salivary amylase from the mouth may travel the small intestine’s length.
While most dietary starch is absorbed and used for energy, it’s estimated that 5-10% of ingested starches are entirely resistant to digestion. S Generally, resistant starches which bypass digestion travel to the colon, where select gut microbes ferment them. S
It is possible that resistant starches, or starches that have not been digested and absorbed, could be fermented by small intestinal bacteria to generate symptoms.
However, from a symptom-generating standpoint, starches appear to have high digestive tolerance. For example, isolated resistant starch has been given in supplemental forms at doses ranging from 22-60 grams per day, with up to 100 grams per day having been demonstrated as tolerable. Pg. 227-228 Granted, these studies weren’t in SIBO sufferers.
However, from a symptom-generation standpoint, we would be less inclined to start with a starch-restriction approach simply because we have less reason to believe that starch would be the primary offending diet component.
Some starch-containing foods, such as wheat or certain legumes restricted on the diet, contain other fermentable fibers that could be irritants. We question if symptom improvement might be from limiting more potent irritants commonly found in certain starch-containing foods, such as fructans in wheat, for example.
When it comes to SIBO, we feel a more lax and broadened approach might be appropriate for certain starchy foods such as potatoes, corn, or rice. This approach is seen on the low FODMAP diet, which in our opinion, does a more thorough job when it comes to limiting and testing potent gut irritants.
- This is not the case when the SCD is used for IBD, where more strict adherence is likely needed.
If starches do cause symptoms, then it might be reasonable to adjust dietary intake of starches to the point of improved symptom control. Still, it may not be necessary to restrict to the degree suggested on the SCD.
Questionable SCD Restrictions and Permissible Foods for SIBO
We found the recommendations around permissible beans on the SCD to be somewhat arbitrary. From the SIBO management perspective, nearly all legumes and beans are potent sources of fermentable carbohydrates and prebiotics. A single 100g serving of common beans (great northern beans, kidney beans, black beans, navy beans, pinto beans), lentils, or chickpeas provides 60-75% of the suggested daily intake of prebiotic fibers. S
While generally, a prebiotic effect is a beneficial food trait, when it comes to SIBO management, we feel establishing more clear parameters around tolerable doses of beans may be more crucial than restricting or allowing specific bean types. The permissible beans on the SCD contain several fermentable carbohydrates and prebiotics that could be symptom-generating in high enough doses in SIBO.
Canned and processed meats are restricted on the SCD because they might contain added starch, whey powder, lactose, or sucrose. Pg. 71 While a case can be made against processed meats for several other health-related reasons, we question if the minuscule amounts of added carbohydrates in processed meats would be relevant for provoking SIBO symptoms.
When it comes to symptoms provoked by carbohydrates and bacterial fermentation, the type of carbohydrate and the dose matter most. We would be more likely to watch out for additions to processed meats like garlic or onions, which might have a more consequential effect on symptoms at lower doses.
The Allowed Foods
“The purpose of the SCD is to deprive the microbial world of the intestine of the food it needs to overpopulate. By using a diet which contains predominantly predigested carbohydrates, the individual with an intestinal problem can be maximally nourished without overstimulation of the intestinal microbial population.” Pg
The SCD removes some foods that potentially would “feed” the microbiome. However, we were surprised to find many allowable foods that would stimulate the intestinal microbiome's growth. Several permissible foods on the SCD contain compounds that are either broadly fermented by several intestinal microbes or other components that might worsen symptoms such as diarrhea.
We were surprised to find several dietary sources of oligosaccharides allowed on the SCD. Common dietary oligosaccharides include fructooligosaccharides (FOS) and galactooligosaccharides (GOS).
No human enzymes can break down FOS or GOS, such as those found in onions, garlic, leeks, certain beans, or cashews allowed on the SCD. The result is that these fibers are malabsorbed in all of us. S They’re also highly fermentable fibers, which can lead to gas production, bloating, abdominal pain, and excessive flatulence in sensitive individuals. S
While monosaccharides are given the green light on the SCD, not all monosaccharides are innocuous. Fructose is a monosaccharide found in many fruits, honey, and some vegetables, all of which are permissible on the SCD. When fructose is present in higher concentrations in these foods, the result can be an osmotic effect where fructose draws water into the bowels. S This can lead to distension, diarrhea, pain, and bloating. S
Moreover, fructose is poorly absorbed across the small intestine’s length, which arguably might not be ideal for SIBO management in sensitive individuals. Unabsorbed fructose can also be fermented, resulting in the production of gases, such as hydrogen. S
Like fructose, polyols can have an osmotic effect when poorly absorbed, where they draw water into the intestines. Polyols are naturally found in several fruits and vegetables allowed on the SCD, such as stone fruits, cauliflower, or mushrooms. S
Potential Irritants and Fermentable Foods Allowed on the SCD
High FOS or GOS
(Peaches, plums, nectarines, apricots, cherries)
We're calling into question the premise that the SCD is designed to reduce intestinal microbes' fermentation, given that several foods on the diet are unrestricted and fermentable. While the SCD removes some of “nature’s laxatives,” such as lactose, it allows for several other natural dietary laxatives to remain in the diet.
This might be great for someone struggling with constipation but could require more caution for those struggling with diarrhea. This is notable because the symptom most correlated with SIBO is diarrhea. S
- The SCD appears promising in IBD but has not been studied for the treatment or management of SIBO.
- The SCD could help manage SIBO symptoms by reducing fermentable carbohydrates in the diet; however, the SCD should not be mistaken as a SIBO treatment.
- The SCD restricts only some fermentable dietary components, suggesting that bacterial fermentation and growth could still be possible on the diet.
- We believe the low FODMAP diet does a more thorough job of limiting fermentable compounds in the diet.
- The low FODMAP diet also has more clear parameters for reintroduction and methods to spot symptom-generating foods.
- We would suggest the low FODMAP diet be given a fair trial with a dietitian’s supervision for the management of SIBO symptoms before undertaking the SCD.
- Given that the SCD has not been explored in clinical trials for SIBO, we believe the diet's long duration (>1 year) is likely inappropriate for the management of SIBO and potentially harmful.
- If the diet is undertaken, we believe oversight by a Registered Dietitian to ensure nutritional adequacy is essential.
- We think skipping the introductory and intermediate phases of the diet is best due to a lack of scientific evidence supporting this approach, along with significant risks for malnutrition.
- If symptom improvement is not experienced within one month, we believe abandoning the SCD and returning to a regular diet is the best approach.
- The end goal of the SCD trial for SIBO should be to promote diversity in food intake while maintaining symptom control. For this reason, we do not suggest undertaking the SCD without clear reintroduction time-frames and symptom monitoring strategies. A qualified professional such as a registered dietitian can offer guidance on reintroduction and symptom monitoring.
An evidence hierarchy is followed to ensure conclusions are formed off of the most up-to-date and well-designed studies available. We aim to reference studies conducted within the past five years when possible.
- Systematic review or meta-analysis of randomized controlled trials
- Randomized controlled trials
- Controlled trials without randomization
- Case-control (retrospective) and cohort (prospective) studies
- A systematic review of descriptive, qualitative, or mixed-method studies
- A single descriptive, qualitative, or mixed-method study
- Studies without controls, case reports, and case series
- Animal research
- In vitro research