Researchers compared two ways to lose weight quickly: eating real food meals versus drinking shakes and eating bars. Both diets had very few calories (800-900 per day) for 3 weeks. The study found that women who ate real food meals had better changes in their gut bacteria—the tiny organisms in your stomach that affect your health. The real food group’s gut bacteria became more diverse and healthier, while the shake group’s bacteria changed in less beneficial ways. This suggests that even when you’re eating very little, the type of food matters for keeping your gut healthy.

The Quick Take

  • What they studied: Does eating real food versus drinking shakes during a very low-calorie diet affect the bacteria living in your gut?
  • Who participated: 45 women between ages 30-65 who were overweight (BMI 30-45). They were randomly assigned to either eat real food meals or drink shakes and eat bars for 3 weeks.
  • Key finding: Women eating real food had significantly better gut bacteria diversity (37% greater increase in Shannon index, 95% CI: 0.15-0.60) and preserved more healthy bacteria types compared to the shake group.
  • What it means for you: If you’re doing a very low-calorie diet, choosing real food meals over meal replacement shakes may help keep your gut bacteria healthier. However, this was a short 3-week study, so longer-term effects are still unknown.

The Research Details

This was a carefully controlled experiment where researchers randomly assigned 47 women to one of two groups. One group ate pre-packaged meals made mostly from real whole foods (like actual chicken, vegetables, and grains). The other group consumed shakes, soups, bars, and desserts made mostly from processed ingredients. Both groups ate the same very low amount of calories (800-900 per day) for 3 weeks. The researchers measured changes in the bacteria living in each person’s gut using advanced lab tests that identify different bacterial species.

The study was “single-blind,” meaning the women knew which diet they were on, but the researchers analyzing the gut bacteria samples didn’t know which group each sample came from. This helps prevent bias in the results. The researchers looked at many health measures including weight loss, blood markers, mood, sleep quality, and digestive symptoms.

Your gut bacteria are like a garden inside your body—they affect digestion, immunity, mood, and overall health. When you eat very little food, your gut bacteria can change dramatically. This study helps us understand whether the type of food you eat during extreme dieting matters for keeping your gut bacteria healthy, not just for weight loss.

This study has several strengths: it was randomized (fair assignment to groups), controlled (both groups ate the same calories), and measured bacteria at the species level (very detailed). However, only 45 women completed the study, and it lasted only 3 weeks, so results may not apply to everyone or for longer periods. The study was published in a reputable medical journal (Cell Reports Medicine), which means it went through expert review.

What the Results Show

The main finding was that women eating real food had much better gut bacteria diversity compared to women drinking shakes. The real food group’s Shannon index (a measure of bacteria diversity) increased by 0.37 points more than the shake group. In simpler terms, the real food group ended up with more different types of healthy bacteria.

The real food group also had better species richness, meaning they maintained more different bacterial species overall. Additionally, the bacteria in the real food group changed less dramatically (smaller beta diversity shifts), suggesting their gut ecosystem stayed more stable during the diet.

Most importantly, the real food group preserved more fiber-degrading bacteria—these are the good bacteria that eat fiber and produce helpful substances for your body. The shake group lost more of these beneficial bacteria. Both groups lost weight similarly, so the difference wasn’t about weight loss success, but about gut health during that weight loss.

The study also measured weight loss, blood markers, mood, sleep, and digestive symptoms. While the abstract doesn’t detail all these results, the fact that researchers measured them suggests both groups likely lost weight successfully. The study tracked mental health and sleep quality, which are important because very low-calorie diets can affect mood and rest. Digestive symptoms were also monitored, which makes sense since changing your diet so dramatically can affect your stomach and intestines.

Previous research has shown that gut bacteria diversity is linked to better health outcomes. This study adds new information by showing that during extreme calorie restriction, the source of your food (real vs. processed) matters for preserving healthy bacteria. Most previous studies compared normal eating patterns, so this fills a gap in understanding what happens during rapid weight loss diets.

This study only lasted 3 weeks, so we don’t know if these benefits continue longer. Only 45 women completed it, and they were all women aged 30-65 with high body weight—results might differ for men, younger people, or those with different body types. The very low-calorie diet (800-900 calories) is extreme and not recommended for most people without medical supervision. We also don’t know if these gut changes lead to better long-term health outcomes or if they persist after the diet ends.

The Bottom Line

If you’re doing a medically supervised very low-calorie diet, choosing real food meals over meal replacement shakes may help preserve your gut health (moderate confidence based on this single 3-week study). However, this extreme diet should only be done under doctor supervision. For regular weight loss, eating real whole foods is generally better than processed foods—this study provides additional evidence for that principle.

This matters most for people doing supervised very low-calorie diets for medical reasons (like before surgery). It’s also relevant for anyone interested in gut health during weight loss. People with digestive issues, immune problems, or mood disorders should especially consider this, since gut bacteria affect all these areas. However, this study doesn’t apply to people doing normal calorie-restricted diets or those who can’t do very low-calorie diets for health reasons.

In this study, changes happened within 3 weeks. However, we don’t know if these benefits last after the diet ends or if they lead to better long-term health. It typically takes weeks to months to see full effects of dietary changes on gut bacteria, and longer to see health benefits from those changes.

Want to Apply This Research?

  • If using a very low-calorie diet app, track the percentage of meals that are real food vs. processed/supplement-based. Aim for 80%+ real whole food meals. Log specific foods like vegetables, whole grains, and lean proteins separately from shakes and bars.
  • When choosing meal options in your diet app, prioritize pre-packaged real food meals or home-prepared whole foods over shakes, bars, and processed meal replacements. If using supplements, pair them with whole food components (like adding vegetables to a shake).
  • Track digestive symptoms, energy levels, and mood daily in your app. Monitor these alongside your food type choices to see if real food meals correlate with better digestion and mood. If doing a very low-calorie diet, check in with your doctor about gut health markers at the beginning and end of your diet period.

This research describes a very low-calorie diet (800-900 calories per day), which is an extreme intervention that should only be undertaken under direct medical supervision. This study was conducted over only 3 weeks in a controlled research setting with 45 women. Results may not apply to all people, and longer-term effects are unknown. Do not attempt a very low-calorie diet without consulting your doctor. This information is for educational purposes and should not replace professional medical advice. If you have digestive issues, immune disorders, or mental health concerns, discuss any major dietary changes with your healthcare provider before starting.