High Fiber Diet Has Major Effect on Gut Microbiome

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Key Takeaways

  • Just two weeks on a higher-fiber diet can alter the gut microbiome enough to change nutrient intake, a study suggests.
  • Better gut health has been associated with improved immune function and lower risk of infection and cancer.
  • When increasing fiber intake, ramp up gradually, a dietitian suggests; that way, you’ll minimize problems like bloating.

 Even just two weeks of a high-fiber diet can have a significant effect on the gut microbiome and could lead to better nutrient absorption, according to a recent study published in the journal mSystems.

Dietary fiber consists of resistant carbohydrates found in vegetables, fruits, and whole grains. The gut bacteria metabolize this type of fiber into short-chain fatty acids, which are the main source of nutrition for the colon’s cells. These fatty acids have also been linked to lower risk for:

  • Inflammatory diseases
  • Obesity
  • Cardiovascular disease
  • Type 2 diabetes

For the study, researchers recruited 20 college students and increased their amount of fiber consumption to about 40 to 50 grams per day by giving them meals comprised mainly of commercially unprocessed, high-fiber foods.

The current recommendation from the American Heart Association is 25 to 30 grams of fiber per day, but most people in the U.S. average about 15 grams daily. The students were averaging about 25 grams per day before the study began.

Researchers took samples of gut microbes before, during, and after the study period and found considerable changes in composition and bacterial diversity. This type of robust, diverse microbiome is more effective at absorbing and using nutrients, the researchers noted.

Better Gut, Better Immunity

Consumption of dietary fiber has decreased dramatically in the last century, according to the study’s lead author, Katrine Whiteson, Ph.D., associate professor of molecular biology and biochemistry at The University of California Irvine. Currently, the average person in the U.S. consumes less than half of the recommended dietary fiber amount.

That is likely due to lower consumption of plant-based foods and increased intake of low-fiber processed foods, Whiteson states, and that can have a major impact on gut health.

Katrine Whiteson, PhD

The lack of fiber intake in the industrialized world is starving our gut microbes, with important health consequences.

— Katrine Whiteson, PhD

“The lack of fiber intake in the industrialized world is starving our gut microbes, with important health consequences,” she says. Poor gut health has been connected with increased colorectal cancer risk and prevalence of autoimmune diseases, and Whiteson adds that it could even decrease vaccine efficacy and response to cancer immunotherapy.

For example, a 2018 review article in Frontiers in Immunology noted that imbalances in gut health can make people more susceptible to infections, chronic inflammation, and cancer.

“At this time during a pandemic, when we need our immune health and healthy vaccine responses, we encourage everyone to think about the plant diversity of their diets,” says Whiteson.

Boosting Fiber

There’s no shortage of plant-based options for dietary fiber, and those choices can include:

  • Beans
  • Berries
  • Avocados
  • Leafy greens
  • Cruciferous vegetables
  • Apples
  • Flax seeds
  • Nuts
  • Sweet potatoes
  • Quinoa
  • Oats

The list goes on and on, with a focus on whole foods rather than highly processed products, which might have much of the fiber stripped out.

Slow and Steady Approach

Although the recent study found that ramping up considerably on fiber in a short timeframe was advantageous to gut microbes, anyone who wants to increase dietary fiber is advised to take a more gradual approach, suggests dietitian Tamara Duker Freuman, R.D., who focuses on the dietary management of digestive and metabolic diseases.

“When you add a lot of healthy options to your diet, especially with fiber, it can cause digestive issues if you try to do too much, too soon,” she says. That can lead to gas, bloating, discomfort, and sometimes even abdominal cramping and diarrhea. Your body will adjust over time, she adds, but it’s better to ramp up slowly to prevent these issues.

Tamara Duker Freuman, RD

When you add a lot of healthy options to your diet, especially with fiber, it can cause digestive issues if you try to do too much, too soon.

— Tamara Duker Freuman, RD

Also, Freuman adds that it’s helpful to understand where all of your fiber is coming from. Whole foods like fruits and vegetables are easy, but keep in mind that nut flours, bean pastas, and cauliflower pizza crusts can all be loaded with fiber as well. That’s good overall, but it should factor into your “slow and steady” approach to increasing fiber consumption.


What This Means For You

Incorporating more whole-food, plant-based options into your snacks and meals can help improve your gut health. In turn, that can bring larger health benefits like improved immune response.

2 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Oliver A., et al. High-Fiber, Whole-Food Dietary Intervention Alters the Human Gut Microbiome but Not Fecal Short-Chain Fatty Acids. mSystems Mar 2021, 6 (2) e00115-21; DOI: 10.1128/mSystems.00115-21

  2. Lazar V., et al. Aspects of Gut Microbiota and Immune System Interactions in Infectious Diseases, Immunopathology, and Cancer. Frontiers in Immunology 2018, 1830 DOI=10.3389/fimmu.2018.01830  

By Elizabeth Millard
Elizabeth Millard is a freelance journalist specializing in health, wellness, fitness, and nutrition.