Researchers studied 558 middle-aged men at risk for type 2 diabetes over three years to see how eating more fiber affected their diabetes risk. They discovered that people who ate more fiber had a much lower chance of developing diabetes. Interestingly, the benefit was even stronger for men who carried a specific gene variant that increases diabetes risk. The study also found that a lifestyle program successfully helped people increase their fiber intake from about 45% to 64% of participants meeting the recommended amount.

The Quick Take

  • What they studied: Whether eating more fiber could help prevent type 2 diabetes in men who already had early warning signs of the disease, and whether a person’s genes affected how much fiber helped them.
  • Who participated: 558 Finnish men aged 50-75 years who had impaired fasting glucose (higher-than-normal blood sugar levels but not yet diabetic). All participants were part of a 3-year lifestyle program that encouraged healthier eating and exercise.
  • Key finding: Men who ate more fiber (at least 3 grams per megajoule of food) had about 54% lower risk of developing type 2 diabetes compared to those eating less fiber. For men carrying a specific diabetes-risk gene, the protection was even stronger—about 94% lower risk.
  • What it means for you: If you’re at risk for diabetes, eating more fiber-rich foods may significantly reduce your chances of developing the disease. The benefit may be especially strong if you have a family history of diabetes. However, this study was only in men, so results may differ for women. Talk to your doctor about your personal diabetes risk.

The Research Details

This was a long-term study that followed the same group of men for three years. Researchers measured how much fiber each man ate using detailed food diaries (4 days of records) and also used a blood test to double-check their fiber intake measurements. They tracked whether men developed diabetes and measured their blood sugar levels using a standard glucose tolerance test. The researchers also identified which men carried a specific gene variant (TCF7L2) that increases diabetes risk, then compared how fiber intake affected men with and without this gene variant.

The men were divided into two groups based on their fiber intake: those eating less than 3 grams per megajoule (a measure of food energy) and those eating 3 grams or more. Researchers then looked at who developed diabetes in each group and how their blood sugar levels changed over time.

This approach is important because it shows how genes and diet work together. Some people may benefit more from dietary changes than others based on their genetics. By studying the same people over three years, researchers could see real-world effects rather than just short-term changes. The study also used multiple ways to measure fiber intake, making the results more reliable.

The study’s strengths include a reasonably large sample size (558 men), a long follow-up period (3 years), and multiple methods to measure fiber intake. The researchers adjusted their analysis for many other factors that affect diabetes risk (exercise, smoking, alcohol, different types of fat intake). However, the study only included Finnish men aged 50-75, so results may not apply equally to women, younger people, or other ethnic groups. The study was observational in nature, meaning researchers tracked what people naturally did rather than randomly assigning them to eat different amounts of fiber.

What the Results Show

The lifestyle intervention program successfully increased fiber intake. At the start, 45% of men were eating enough fiber (3 grams per megajoule or more), but by the end of the three years, 64% were eating this amount—a significant improvement.

Men who achieved the higher fiber intake had substantially lower diabetes risk. Overall, high fiber intake was associated with 54% lower risk of developing type 2 diabetes. This means that for every 100 men eating low fiber who would develop diabetes, only about 46 men eating high fiber would develop it.

The most striking finding was that men carrying the TCF7L2 risk gene benefited even more dramatically. These men had 94% lower diabetes risk when eating high fiber compared to those eating low fiber. This suggests that people with this genetic risk factor may be especially responsive to dietary fiber.

Beyond diabetes development, men with the risk gene who ate more fiber also had better blood sugar control. Their fasting blood sugar didn’t rise as much, their overall blood sugar response to food was better, and their pancreas function (measured by something called the disposition index) was preserved better.

The study found that the protective effect of fiber was particularly strong in men carrying the T allele (risk version) of the TCF7L2 gene. Men without this risk gene still benefited from higher fiber intake, but the benefit was less dramatic. This suggests that genetic testing might eventually help doctors personalize dietary recommendations, though this is not yet standard practice. The study also showed that a group-based lifestyle intervention program was effective at helping people increase their fiber intake, which is encouraging for public health efforts.

Previous research has shown that fiber helps prevent type 2 diabetes in general populations. This study builds on that knowledge by showing that the benefit may be even stronger in people with specific genetic risk factors. The finding that genes and diet interact to affect diabetes risk aligns with growing evidence in nutrition science that one-size-fits-all dietary recommendations may not work equally well for everyone. However, more research is needed to confirm these gene-diet interactions in other populations.

The study only included men aged 50-75 from Finland, so the results may not apply equally to women, younger people, or people from other ethnic backgrounds. All participants were in a structured lifestyle intervention program, so results may not reflect what happens in everyday life without such support. The study was observational, meaning researchers couldn’t prove that fiber directly caused the lower diabetes risk—other unmeasured factors could be involved. Additionally, the study measured fiber intake using food diaries, which depend on people accurately remembering what they ate. Finally, the sample size for the genetic analysis was relatively small, so the very strong protective effect seen in risk gene carriers should be confirmed in larger studies.

The Bottom Line

Based on this research, men at risk for type 2 diabetes should aim to eat at least 3 grams of fiber per megajoule of food consumed (roughly equivalent to 25-30 grams of fiber daily for most people). This can be achieved by eating whole grains, beans, lentils, vegetables, fruits, and nuts. Combining increased fiber intake with regular exercise and maintaining a healthy weight appears most effective. Confidence level: Moderate to High for men; Lower for women and other groups since this study only included men.

This research is most relevant for men aged 50-75 who have been told they have impaired fasting glucose or prediabetes. It’s also important for anyone with a family history of type 2 diabetes. While the study only included men, the benefits of fiber for diabetes prevention likely apply to women as well, though this needs confirmation. People without diabetes risk factors can still benefit from eating adequate fiber for overall health. This research is less directly applicable to people already diagnosed with type 2 diabetes, though they should still discuss fiber intake with their healthcare provider.

The study followed people for three years, and improvements in blood sugar control appeared within this timeframe. However, most people should expect to see meaningful changes in blood sugar levels within 3-6 months of increasing fiber intake, especially if combined with exercise and weight management. The full protective effect against developing diabetes may take longer to become apparent.

Want to Apply This Research?

  • Track daily fiber intake in grams, aiming for 25-30 grams per day. Log all foods eaten and note their fiber content. Set a daily goal and monitor progress weekly. Users can also track blood sugar readings if they have a glucose monitor, noting correlations between high-fiber days and blood sugar levels.
  • Start by identifying high-fiber foods you enjoy (whole grain bread, beans, berries, vegetables, nuts). Add one new high-fiber food to each meal. Use the app to set reminders to eat fiber-rich snacks. Create a shopping list of fiber-rich foods and check them off when purchased. Track which fiber sources work best for your digestion and preferences.
  • Weekly review of average fiber intake with a goal of reaching 25-30 grams daily. Monthly tracking of how you feel (energy levels, digestion, hunger). If available, quarterly blood sugar measurements from your doctor to see if fiber intake correlates with improvements. Long-term: annual check-ins with your healthcare provider to assess diabetes risk reduction.

This research suggests associations between fiber intake and diabetes risk but does not constitute medical advice. The study was conducted in men aged 50-75 and may not apply equally to other populations. If you have been diagnosed with prediabetes, impaired fasting glucose, or type 2 diabetes, consult your healthcare provider before making significant dietary changes. Genetic testing for TCF7L2 is not yet standard clinical practice and should only be done under medical supervision if recommended by your doctor. This information is for educational purposes and should not replace professional medical evaluation and personalized treatment recommendations.