Researchers wanted to know if eating beef as part of a Mediterranean diet (which includes lots of vegetables, healthy oils, and whole grains) affects your gut health differently than eating it in a typical American diet. They had 30 healthy adults eat different amounts of lean beef mixed into Mediterranean-style meals for 4 weeks at a time. The good news: when people ate beef as part of a Mediterranean diet, their gut bacteria became more diverse and healthier, and they had lower levels of a compound called TMAO that might be bad for heart health. This suggests that how you eat beef matters just as much as how much you eat.

The Quick Take

  • What they studied: Does eating different amounts of lean beef as part of a Mediterranean diet affect your gut bacteria and a heart-health marker called TMAO differently than eating beef in an American diet?
  • Who participated: 30 generally healthy adults who completed the study, eating four different diet patterns for 4 weeks each with breaks in between
  • Key finding: People eating beef in a Mediterranean diet had healthier, more diverse gut bacteria and 78-115% lower TMAO levels compared to those eating beef in an American diet, even when eating the same amount of beef
  • What it means for you: If you eat beef, combining it with Mediterranean diet foods (vegetables, olive oil, whole grains, legumes) may be better for your gut health and heart health than eating it with typical American diet foods. However, this is one study in healthy people, so talk to your doctor about what’s best for you.

The Research Details

This was a carefully controlled experiment where the same 30 people ate four different diets, one at a time, for 4 weeks each. Between each diet period, they took a break of at least one week to let their bodies return to normal. The diets were: a Mediterranean diet with very little beef (about 0.5 ounces per day), a Mediterranean diet with a moderate amount of beef (2.5 ounces), a Mediterranean diet with more beef (5.5 ounces), and a typical American diet with beef (2.5 ounces). The researchers collected blood samples, urine samples, and stool samples at the beginning and end of each diet period to measure different compounds and analyze the gut bacteria.

By having the same people eat all four diets, researchers could see exactly how each diet affected their bodies without other differences between people getting in the way. This type of study is very strong because it controls for individual differences. The researchers measured actual biological markers (TMAO and gut bacteria diversity) rather than just asking people what they ate, which makes the results more reliable.

This study is well-designed with several strengths: it’s a controlled feeding trial where researchers provided all the food (so people ate exactly what was planned), it used objective measurements of gut bacteria and metabolites, and it had people serve as their own controls by eating all diets. The main limitation is that it only included 30 generally healthy people, so results might differ in people with health conditions or different backgrounds. The study lasted 4 weeks per diet, which is long enough to see changes in gut bacteria but might not show long-term effects.

What the Results Show

The three Mediterranean diets all increased the diversity of gut bacteria compared to the American diet. This is important because more diverse gut bacteria are generally associated with better health. When people ate the American diet, their blood TMAO levels were about 78-104% higher than when they ate Mediterranean diets with small or moderate amounts of beef. Similarly, their urine TMAO levels were 76-115% higher on the American diet compared to all three Mediterranean diets. Interestingly, even the Mediterranean diet with the most beef (5.5 ounces per day) still had lower TMAO than the American diet with less beef (2.5 ounces per day). This suggests that the overall diet pattern matters more than the amount of beef.

The study measured many other compounds in the blood, urine, and stool. The Mediterranean diets appeared to affect various metabolites differently than the American diet, though the abstract doesn’t detail all of these. The fact that gut bacteria diversity increased on all Mediterranean diets, regardless of beef amount, suggests that the other components of the Mediterranean diet (vegetables, whole grains, olive oil) are driving these benefits.

Previous research showed that beef contains carnitine, which can be converted to TMAO—a compound linked to heart disease risk. This study confirms that concern but adds an important finding: eating beef as part of a healthy Mediterranean diet appears to prevent TMAO buildup. This suggests that the overall dietary pattern can modify how your body processes beef, which is more nuanced than simply saying ‘beef is bad.’

The study only included 30 generally healthy adults, so results might not apply to people with heart disease, diabetes, or other conditions. All participants were in a controlled setting eating provided meals, which is different from real life where people make their own food choices. The study lasted 4 weeks per diet, which is enough to see changes in gut bacteria but might not show what happens over months or years. The study doesn’t tell us whether these improvements in TMAO and gut bacteria actually reduce heart disease risk in real life.

The Bottom Line

If you eat beef, consider eating it as part of a Mediterranean-style diet that includes plenty of vegetables, whole grains, legumes, and olive oil rather than as part of a typical American diet high in processed foods and refined carbohydrates. You don’t need to avoid beef entirely—even 2.5 ounces per day as part of a Mediterranean diet showed benefits. Confidence level: Moderate. This is one well-designed study, but more research in different populations is needed.

This research is most relevant to people who eat beef and want to optimize their heart health and gut health. It’s particularly interesting for people following or considering a Mediterranean diet. People with existing heart disease, high cholesterol, or digestive issues should discuss these findings with their doctor. The study was done in generally healthy people, so results might differ for those with health conditions.

Changes in gut bacteria diversity can begin within weeks (this study saw changes in 4 weeks), but the full health benefits of improved gut bacteria and lower TMAO might take longer to appear. You probably won’t feel immediate changes, but over months, you might notice improvements in digestion or energy if you switch to a Mediterranean diet pattern.

Want to Apply This Research?

  • Track daily servings of lean beef and Mediterranean diet components (vegetables, whole grains, legumes, olive oil) separately. Set a goal of 2-3 ounces of lean beef per day combined with at least 5 servings of vegetables and 2-3 servings of whole grains.
  • If currently eating beef in an American diet pattern, gradually shift to a Mediterranean pattern by: replacing refined grains with whole grains, adding 2-3 extra vegetable servings daily, using olive oil instead of butter, and keeping beef portions to 2-3 ounces per meal. Make one change per week rather than all at once.
  • Track adherence to Mediterranean diet components weekly rather than daily to avoid overwhelm. Monitor how you feel (energy, digestion) monthly. Consider getting blood work done annually to check cholesterol and other heart health markers if you have risk factors, since this study suggests dietary changes may improve these markers.

This research suggests that eating lean beef as part of a Mediterranean diet may be associated with better gut health and lower TMAO levels compared to eating beef in an American diet pattern. However, this is one study in 30 generally healthy adults, and individual results may vary. TMAO levels are one of many factors affecting heart health. This information is not medical advice and should not replace consultation with your healthcare provider, especially if you have heart disease, high cholesterol, digestive issues, or other health conditions. Before making significant dietary changes, discuss them with your doctor or a registered dietitian to ensure they’re appropriate for your individual health situation.