Researchers looked at 14 different studies to understand how very-low-calorie ketogenic diets (a strict, high-fat, low-carb eating plan) affect the trillions of bacteria living in your digestive system. They found that this type of diet can increase the variety of bacteria in your gut and boost certain helpful bacteria, especially in people over 40 with obesity. However, the diet also reduced some other beneficial bacteria. While these changes look promising for weight management, scientists say we need more research to understand if these changes are safe and helpful in the long run.

The Quick Take

  • What they studied: How does eating a very-low-calorie ketogenic diet (a strict diet that’s very low in carbs and very high in fat) change the bacteria living in your stomach and intestines?
  • Who participated: The researchers combined results from 14 different studies that looked at people with obesity who tried this type of diet. The studies included people of different ages and weights.
  • Key finding: The diet increased the variety and diversity of gut bacteria in most people. It also boosted a helpful bacteria called Akkermansia, especially in people over 40. However, it reduced another helpful bacteria called Bifidobacterium.
  • What it means for you: This diet may help improve your gut bacteria in ways that could support weight loss and health. However, because it also reduces some helpful bacteria, it’s not a one-size-fits-all solution. Talk to your doctor before starting this diet, especially if you plan to follow it long-term.

The Research Details

This is a meta-analysis, which means researchers looked at 14 different published studies about the same topic and combined all their results together. They searched four major medical databases (PubMed, EBSCOhost, Cochrane Library, and Web of Science) to find all studies published up to June 2025 that tested how very-low-calorie ketogenic diets affect gut bacteria.

The researchers looked at studies that measured different aspects of gut bacteria health, including how many different types of bacteria were present (diversity) and how much of each type was present. They also looked at whether results were different depending on people’s starting weight, age, and how long they followed the diet.

This approach is stronger than looking at just one study because it combines information from many different research projects, giving a clearer picture of what’s really happening.

By combining results from multiple studies, researchers can see patterns that might not be obvious in a single study. This helps us understand whether the effects of this diet on gut bacteria are real and consistent across different groups of people. It also helps identify which people might benefit most from this approach.

This meta-analysis is a strong type of research because it combined 14 different studies. However, readers should know that the individual studies may have had different methods and quality levels. The researchers did look at whether results changed based on people’s age and weight, which shows they were careful about understanding different groups. Some results had small sample sizes, which means we should be a bit cautious about those specific findings.

What the Results Show

When people followed a very-low-calorie ketogenic diet, the variety of bacteria in their gut increased. Scientists measure this using two different methods called the Shannon index and Faith’s Phylogenetic Diversity index. Both methods showed that bacterial diversity improved, meaning people had more different types of bacteria living in their gut.

The diet also increased a specific helpful bacteria called Akkermansia. This bacteria is interesting because it’s been linked to better metabolism and weight management in other research. The increase was especially strong in people over 40 years old and in people with a BMI (body mass index) between 30-35.

Another change was an increase in the ratio of two major types of bacteria called Firmicutes and Bacteroidetes. However, the diet decreased another helpful bacteria called Bifidobacterium, which is normally considered beneficial for digestive health. This decrease was most noticeable in people over 40 and in those who followed the diet for 12 weeks or less.

The researchers found that the effects of the diet were not the same for everyone. People with different starting weights and ages experienced different changes in their bacteria. For example, Akkermansia increased more in people aged over 40 and those with a BMI between 30-35 kg/m². The timing also mattered—some bacteria changes happened more quickly (within 6 weeks) while others took longer to develop. These findings suggest that this diet might work differently depending on your age, starting weight, and how long you follow it.

Previous research has suggested that gut bacteria diversity is important for overall health and weight management. This study supports that idea by showing that this diet can increase diversity. However, earlier studies have also shown that Bifidobacterium is usually helpful for digestive health, so the decrease in this bacteria is a new concern that wasn’t fully understood before. The increase in Akkermansia aligns with other research showing this bacteria may help with metabolism, which is a positive finding.

The researchers combined 14 studies, but they didn’t tell us exactly how many people were in all those studies combined. Some of the studies may have been small or used different methods to measure bacteria, which could affect the overall results. Most importantly, these studies only looked at short-term changes (up to a few months). We don’t know what happens to gut bacteria if someone follows this diet for years. Also, the studies didn’t always measure whether these bacteria changes actually led to better health outcomes—they just measured that the bacteria changed. Finally, most of the research was done in specific countries or populations, so results might be different for other groups of people.

The Bottom Line

If you’re considering a very-low-calorie ketogenic diet, talk to your doctor first, especially if you have digestive issues or take medications. The research suggests this diet may help improve gut bacteria diversity and increase helpful bacteria like Akkermansia, which could support weight loss efforts. However, because the diet also reduces Bifidobacterium (another helpful bacteria), it’s not clearly better than other diets for long-term gut health. This diet might be worth trying for short-term weight loss (a few weeks to a few months), but we need more research on whether it’s safe and beneficial long-term. Consider working with a registered dietitian who can personalize the approach for your specific needs.

This research is most relevant for adults with obesity who are looking for weight loss strategies and are interested in how diet affects their digestive health. People over 40 may see more pronounced changes in their bacteria. However, this diet is NOT recommended for pregnant women, children, people with a history of eating disorders, or those with certain medical conditions like kidney disease. If you have digestive problems, diabetes, or take medications, talk to your doctor before starting this diet.

Based on the studies reviewed, changes in gut bacteria can start happening within 2-6 weeks of starting the diet. However, some changes take longer to develop. The full effects on your health and weight loss may take several weeks to months to become noticeable. Remember that changing your gut bacteria is just one part of weight loss and health—other factors like overall calories, exercise, and sleep also matter.

Want to Apply This Research?

  • Track your daily adherence to the diet (percentage of meals that follow the plan) and monitor digestive symptoms weekly (bloating, energy levels, digestion comfort) using a simple 1-10 scale. This helps you see if the diet is working for you personally.
  • Start by replacing one meal per day with a ketogenic option (like eggs with avocado instead of toast with jam) rather than switching everything at once. This gradual approach helps your gut bacteria adjust more slowly and may reduce digestive discomfort.
  • Use the app to log your meals and track your weight weekly rather than daily. Also monitor how you feel—energy levels, digestion, mood, and hunger—because these are often better indicators of whether the diet is working for you than bacteria changes alone. Set a check-in point at 4 weeks and 8 weeks to decide if you want to continue.

This research summary is for educational purposes only and should not replace professional medical advice. Very-low-calorie ketogenic diets are restrictive and may not be appropriate for everyone. Before starting this or any diet, especially if you have existing health conditions, take medications, are pregnant or breastfeeding, or have a history of eating disorders, consult with your doctor or a registered dietitian. The studies reviewed measured changes in gut bacteria but did not always confirm that these changes led to better health outcomes. Long-term safety and effectiveness of this diet remain unclear. Individual results vary significantly based on age, weight, genetics, and other factors.