Scientists reviewed 17 studies involving over 690 lab animals to understand how high-carbohydrate diets affect gut bacteria. They found that different types of carb-heavy diets change the balance of bacteria living in the intestines in various ways. High-carb, high-fat diets consistently shifted bacteria toward patterns linked to obesity in humans. However, the specific type of carbohydrate - whether fructose, sugar, or starch - made a big difference in how gut bacteria responded and how this affected overall health, including blood pressure and immune function.

The Quick Take

  • What they studied: How different high-carbohydrate diets affect the balance of bacteria living in animal intestines
  • Who participated: More than 690 laboratory rodents across 17 different research studies
  • Key finding: High-carb, high-fat diets consistently increased a bacteria ratio linked to obesity, while low-fat carb diets had mixed effects depending on the type of carbohydrate
  • What it means for you: The type of carbohydrates you eat may influence your gut bacteria balance, but more human research is needed before making specific dietary changes

The Research Details

This was a systematic review, meaning researchers gathered and analyzed existing studies rather than conducting new experiments. They searched five major scientific databases through September 2024 to find all relevant animal studies examining how carbohydrate-heavy diets affect gut bacteria. The researchers used a specialized tool called SYRCLE to evaluate the quality and potential bias of each study they included. This approach allows scientists to see patterns across multiple experiments and draw broader conclusions than any single study could provide.

Systematic reviews are valuable because they combine results from many studies to identify consistent patterns. By looking at over 690 animals across 17 studies, researchers could see which effects happened repeatedly versus which might have been coincidental in individual experiments.

The researchers used established scientific methods to search for studies and evaluate their quality. However, this review only included animal studies, so the findings may not directly apply to humans. The large number of animals studied increases confidence in the patterns observed.

What the Results Show

The most consistent finding was that high-carbohydrate, high-fat diets increased the ratio of Firmicutes to Bacteroidetes bacteria in animal intestines. This ratio, often called the F/B ratio, has been linked to obesity in human studies. When animals ate diets high in both carbs and fat, this bacterial shift happened almost every time across different studies. However, the story became more complex when researchers looked at low-fat, high-carbohydrate diets. These diets didn’t consistently increase the F/B ratio, suggesting that fat content plays an important role in how carbohydrates affect gut bacteria.

The specific type of carbohydrate made a significant difference in outcomes. Diets high in fructose, table sugar, or starch each affected gut bacteria differently and led to different health effects in the animals. These changes went beyond just bacterial balance - they also affected blood pressure, the integrity of intestinal walls, immune system function, and metabolism. Importantly, when researchers added interventions like probiotics or other treatments during or after the high-carb feeding, they could often reverse or modify these bacterial changes.

These findings align with human studies showing that people with obesity often have higher F/B ratios in their gut bacteria. The research also supports previous work suggesting that diet composition, not just total calories, influences gut bacterial communities. However, this review highlighted that the relationship is more nuanced than previously understood, with the combination of macronutrients being crucial.

The biggest limitation is that all studies were conducted in laboratory animals, primarily rodents, so the results may not directly translate to humans. The studies also used different types of carbohydrates, feeding durations, and measurement methods, making direct comparisons challenging. Additionally, the review couldn’t determine optimal carbohydrate types or amounts for maintaining healthy gut bacteria.

The Bottom Line

Based on this research, it appears that combining high carbohydrates with high fat may negatively affect gut bacteria balance. However, since this was animal research, people shouldn’t make dramatic dietary changes based solely on these findings. If you’re concerned about gut health, consider focusing on the quality and variety of carbohydrates rather than eliminating them entirely.

People interested in gut health, those managing weight, and individuals with metabolic concerns may find this research relevant. However, anyone considering significant dietary changes should consult with healthcare providers, especially those with existing health conditions or taking medications.

The animal studies showed bacterial changes within weeks to months of dietary changes, but human responses may differ significantly. More research is needed to understand realistic timelines for gut bacteria improvements in people.

Want to Apply This Research?

Use the Gram app to:

  • Track your daily intake of different carbohydrate types (fruits, vegetables, grains, added sugars) and fat content to identify patterns in your diet
  • Experiment with reducing high-carb, high-fat food combinations while monitoring how you feel and any digestive changes
  • Log energy levels, digestive comfort, and overall well-being weekly while making gradual changes to carbohydrate quality and fat intake

This research was conducted in laboratory animals and may not apply to humans. This information is for educational purposes only and should not replace professional medical advice. Consult your healthcare provider before making significant dietary changes, especially if you have existing health conditions.