Resistant starch and polydextrose affect your body in different ways, according to a 2026 randomized controlled trial of 74 healthy adults. Resistant starch increased certain bile acids in the blood and boosted beneficial Akkermansia bacteria in the gut, while polydextrose changed how the body processes bile acids in stool without increasing blood levels. These findings suggest that different fiber types work through distinct biological mechanisms, meaning the type of fiber you choose may matter for your digestive health.
A new study compared how two types of dietary fiber affect your body’s bile acids—substances that help digest fat. Researchers gave healthy people either resistant starch, polydextrose, or both for 50 days and measured changes in their blood and stool. According to Gram Research analysis, resistant starch increased certain bile acids in the blood and changed which bacteria lived in the gut, while polydextrose had different effects on how the body processes bile acids. These findings suggest that different fibers work in unique ways to influence digestive health.
Key Statistics
A 2026 randomized controlled trial of 74 healthy adults found that resistant starch increased conjugated bile acids in blood, while polydextrose did not produce the same effect.
According to research reviewed by Gram, resistant starch supplementation increased taurine-conjugated bile acids in plasma and was associated with higher levels of Akkermansia bacteria in the gut.
A 50-day controlled intervention study showed that polydextrose reduced the transformation of primary bile acids to secondary bile acids in the stool, demonstrating a distinct mechanism from resistant starch.
The 2026 study found negative correlations between plasma primary conjugated bile acids and faecal secondary bile acids specifically in response to resistant starch, suggesting enhanced bile acid absorption.
The Quick Take
- What they studied: How two common dietary fibers (resistant starch and polydextrose) change the levels of bile acids in blood and stool, and how they affect the bacteria living in your gut.
- Who participated: 74 healthy adults took part in this 50-day study. They were randomly assigned to take resistant starch, polydextrose, both supplements, or a placebo (fake pill) without knowing which they received.
- Key finding: Resistant starch increased certain bile acids in the blood and was linked to more of a beneficial bacteria called Akkermansia, while polydextrose changed how the body processes bile acids in the stool but didn’t increase blood bile acids.
- What it means for you: Different fibers may help your digestion in different ways. This research suggests that choosing the right type of fiber for your body might matter more than just eating any fiber. However, more research is needed before making specific dietary changes based on these findings.
The Research Details
This was a randomized controlled trial, which is one of the strongest types of nutrition research. Researchers randomly assigned 74 healthy people to receive one of four treatments: resistant starch alone, polydextrose alone, both together, or a placebo. Nobody knew which treatment they were getting, which helps prevent bias. The study lasted 50 days, and participants took their assigned supplement every day.
Researchers measured bile acids—chemicals your body makes to digest fat—in both blood samples and stool samples. They used advanced laboratory techniques to identify and measure 20 different bile acids. They also analyzed the bacteria in participants’ guts using genetic testing to see which species were present and how abundant they were.
The researchers then looked for connections between the fiber supplements, changes in bile acids, and changes in gut bacteria. They used statistical methods to determine which changes were real and not just due to chance.
Understanding how different fibers affect bile acids and gut bacteria is important because bile acids influence how your body absorbs fats and cholesterol, and they may affect metabolism and inflammation. By comparing two fibers with different properties, researchers can understand whether all fibers work the same way or if specific types have unique benefits. This helps explain why some people might respond better to certain dietary changes than others.
This study has several strengths: it was randomized (reducing bias), double-blind (neither participants nor researchers knew who got what), and controlled (included a placebo group for comparison). The sample size of 74 is reasonable for this type of research. The researchers used advanced laboratory methods to measure bile acids accurately. However, the study only lasted 50 days, so we don’t know if these effects continue long-term. Also, all participants were healthy, so results may not apply to people with digestive diseases.
What the Results Show
Resistant starch had clear effects on bile acids in the blood. It increased several types of conjugated bile acids (bile acids attached to other molecules) and deoxycholic acid compared to placebo. These increases were statistically significant, meaning they were unlikely to be due to chance. Interestingly, polydextrose did not increase bile acids in the blood the same way.
The researchers found that resistant starch’s effects were connected to changes in gut bacteria. Specifically, people whose gut bacteria shifted to have more Akkermansia (a beneficial bacteria) showed the biggest increases in certain bile acids. This suggests that resistant starch works partly by changing which bacteria live in your gut, and those bacteria then influence bile acid levels.
Polydextrose had different effects. While it didn’t increase blood bile acids like resistant starch did, it changed how the body processes bile acids in the stool. Polydextrose reduced the transformation of primary bile acids into secondary bile acids, which is a different mechanism than resistant starch’s effects.
Neither fiber changed the total amount of bile acids in stool, but both changed the proportions of different types. Resistant starch decreased the percentage of primary bile acids in stool, while polydextrose increased it.
The study found negative correlations between certain blood bile acids and stool bile acids when people took resistant starch. This means that as blood bile acids went up, stool bile acids went down, suggesting the body was absorbing more bile acids from the intestines. Polydextrose showed different microbial associations, meaning it affected different types of bacteria than resistant starch did. These secondary findings support the idea that the two fibers work through distinct biological pathways.
Previous research suggested that dietary fiber influences bile acid metabolism, but most studies didn’t compare different types of fiber directly. This study is unique because it tests two fibers with different fermentation properties side-by-side. The finding that resistant starch increases conjugated bile acids aligns with some previous research on resistant starch and gut health. However, the specific connection to Akkermansia bacteria is a newer finding that adds to our understanding of how resistant starch works in the body.
The study only lasted 50 days, so we don’t know if these effects continue if people take these supplements for months or years. All participants were healthy adults, so these results may not apply to people with digestive diseases, obesity, or other health conditions. The study measured bile acids at only one time point after the intervention, so we don’t know how quickly these changes happen. Additionally, the study didn’t measure clinical outcomes like cholesterol levels or weight changes, only the biological markers of bile acid metabolism.
The Bottom Line
Based on this research, there is moderate evidence that resistant starch and polydextrose affect your body differently. If you’re interested in adding fiber to your diet, this study suggests that the type of fiber may matter. However, this research alone is not strong enough to recommend one fiber over another for general health. Talk to a healthcare provider before making significant dietary changes, especially if you have digestive issues. Current dietary guidelines recommend 25-38 grams of total fiber daily from various sources.
This research is most relevant to people interested in optimizing their digestive health and understanding how different foods affect their body. It may be particularly interesting to people with metabolic concerns like high cholesterol or blood sugar issues, since bile acids play a role in fat and cholesterol metabolism. People with irritable bowel syndrome or other digestive conditions should consult a doctor before trying these supplements, as individual responses may differ. Healthy people looking to improve their diet can use this as one piece of information among many.
The changes in bile acids and gut bacteria happened within 50 days in this study, suggesting these effects develop relatively quickly. However, the long-term effects beyond 50 days are unknown. Most people would need to take these supplements consistently to maintain the changes. If you try adding these fibers to your diet, you might notice digestive changes (like increased gas or bloating) within days, but the deeper metabolic changes measured in this study take weeks to develop.
Frequently Asked Questions
What’s the difference between resistant starch and polydextrose?
Resistant starch and polydextrose are both dietary fibers, but they’re made from different sources and ferment differently in your gut. Resistant starch comes from starchy foods and increases certain bile acids in your blood while boosting beneficial bacteria. Polydextrose is a synthetic fiber that changes how your body processes bile acids in stool without increasing blood levels.
Do I need to take fiber supplements if I eat a healthy diet?
Most people can get enough fiber from whole foods like vegetables, fruits, beans, and whole grains. However, if you’re not meeting the recommended 25-38 grams daily, supplements may help. This study suggests different fiber types have different effects, so talk to a doctor about which might be best for your individual needs.
How long does it take for fiber supplements to work?
This study found changes in bile acids and gut bacteria within 50 days, suggesting metabolic effects develop over weeks rather than days. You might notice digestive changes like increased gas within days, but the deeper health effects measured in research take 4-6 weeks to develop.
Can resistant starch help with weight loss or cholesterol?
This study measured bile acid changes, which are connected to fat and cholesterol metabolism, but didn’t directly measure weight or cholesterol levels. While bile acid changes suggest potential metabolic benefits, more research is needed to confirm whether resistant starch actually improves cholesterol or weight loss in real people.
Is it safe to take both resistant starch and polydextrose together?
This study included a group that took both supplements together for 50 days without reporting safety issues. However, combining fibers may increase digestive side effects like bloating or gas. Start with one fiber type, then add another gradually while monitoring how your body responds. Consult a healthcare provider if you have digestive conditions.
Want to Apply This Research?
- Track daily fiber intake by type (resistant starch vs. other sources) and log digestive symptoms (bloating, gas, bowel regularity) to see if you notice patterns with different fiber types over 4-6 weeks.
- Start by adding one type of fiber supplement consistently for 4 weeks while tracking symptoms, then try switching to another type to compare how your body responds. Keep a simple log of energy levels, digestion comfort, and any changes you notice.
- Use the app to set a daily reminder to take your fiber supplement at the same time each day. Track weekly averages of digestive comfort and energy levels. After 6 weeks, review your data to see which fiber type seemed to work best for your individual body.
This research describes how two specific fiber supplements affected bile acid metabolism and gut bacteria in healthy adults over 50 days. These findings do not constitute medical advice and should not replace consultation with a healthcare provider. Individual responses to fiber supplements vary significantly based on age, health status, medications, and existing digestive conditions. People with irritable bowel syndrome, inflammatory bowel disease, or other gastrointestinal disorders should consult a doctor before starting fiber supplements. Pregnant or nursing women should seek medical guidance before supplementation. This study measured biological markers only; it did not assess clinical health outcomes. Long-term safety and efficacy beyond 50 days are unknown.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.