Researchers studied 22 pairs of identical twins to compare plant-based (vegan) and meat-eating diets over 8 weeks. Both groups ate healthier versions of their chosen diet style—the vegan group ate more beans and fiber, while the meat-eating group got more B-12 and cholesterol. The interesting part? Both groups significantly improved their overall diet quality during the study, even though their diets were quite different. This shows that you can eat healthy on either diet type, as long as you focus on nutritious foods.

The Quick Take

  • What they studied: Whether people eating plant-based diets and people eating meat-based diets could both improve their diet quality while staying true to their chosen eating style.
  • Who participated: 44 people (22 pairs of identical twins) who were randomly assigned to either eat vegan or eat meat. They participated for 8 weeks, with the first 4 weeks using meal delivery and the second 4 weeks preparing their own meals.
  • Key finding: Both groups improved their diet quality scores significantly. Vegans improved by 14.2 points in the first 4 weeks and maintained a 12.0-point improvement by week 8. Meat-eaters improved by 9.0 points at 4 weeks and 7.9 points at 8 weeks. Both groups ate more vegetables and less added sugar.
  • What it means for you: You don’t have to choose between being healthy and following your preferred diet style. Whether you eat plants or meat, focusing on whole foods and reducing added sugars can improve your diet quality. However, this was a small, short-term study, so more research is needed before making major dietary changes.

The Research Details

This study looked at data from an 8-week diet experiment with 22 pairs of identical twins. The researchers randomly assigned each pair to either a vegan diet (no animal products) or an omnivorous diet (including meat and animal products). For the first 4 weeks, participants received pre-made meals from a delivery service to ensure they were following their assigned diet correctly. For the second 4 weeks, they prepared their own meals and snacks while following the same diet guidelines.

The researchers collected detailed food records at the start, after 4 weeks, and after 8 weeks. They used a scoring system called the Healthy Eating Index-2015 to measure how nutritious each person’s diet was. This score looks at things like how many vegetables people eat, how much added sugar they consume, and whether they’re getting enough whole grains and other healthy foods.

Using identical twins was clever because twins share the same genes, so any differences between the vegan and meat-eating groups would be due to diet, not genetics.

This approach matters because it shows that researchers can fairly compare different diet styles. In the past, studies comparing vegan and meat-eating diets sometimes had one group eating much healthier than the other, making it hard to know if differences were from the diet type itself or just from eating better overall. This study made sure both groups were eating high-quality versions of their chosen diet.

This is a secondary analysis, meaning researchers looked at data from an existing study rather than conducting a brand-new experiment. The sample size is relatively small (44 people), which limits how much we can generalize the findings to everyone. The study was short (only 8 weeks), so we don’t know if these improvements would last longer. However, the use of identical twins and careful food tracking makes the data fairly reliable for what it does measure.

What the Results Show

Both the vegan and meat-eating groups showed meaningful improvements in their diet quality scores during the study. The vegan group’s diet quality improved more dramatically at first (14.2 points after 4 weeks) compared to the meat-eating group (9.0 points), though both groups maintained most of their improvements by week 8.

When researchers looked at specific foods and nutrients, they found that both groups made similar healthy changes: they ate more vegetables and reduced their added sugar intake. These are two of the most important changes for improving overall health.

The groups also showed expected differences based on their diet type. The vegan group ate significantly more legumes (beans, lentils, peas) and got more fiber from plant sources. The meat-eating group, as expected, consumed more cholesterol and vitamin B-12, which comes naturally from animal products.

These results suggest that both diet approaches can be healthy when people focus on whole, nutritious foods rather than processed options.

The study also showed that the meal delivery phase (weeks 0-4) seemed to help both groups make bigger improvements in diet quality compared to the self-preparation phase (weeks 4-8). This suggests that having structured, healthy meals available makes it easier to eat well. When people had to plan and prepare their own meals, improvements were still present but slightly smaller, which is realistic since meal planning takes more effort and knowledge.

Previous research has shown that both well-planned vegan diets and omnivorous diets can support good health. This study adds to that evidence by showing that when both diet types emphasize whole foods and minimize processed foods, they can both achieve high diet quality scores. It also supports the idea that diet quality (what you eat) may matter more than diet type (whether you eat meat) for overall health.

The study only included 44 people, which is a small number for drawing broad conclusions. The 8-week timeframe is relatively short—we don’t know if people could maintain these improvements for months or years. The study used identical twins, which is good for controlling genetics but may not represent the general population. Additionally, people in the study knew they were being monitored, which might have motivated them to eat better than they normally would. Finally, this was a secondary analysis, meaning the original study was designed for different purposes, so some details about the diet quality improvements might not have been the main focus.

The Bottom Line

If you’re considering changing your diet, focus on eating more whole foods like vegetables, fruits, whole grains, and legumes, while reducing added sugars—regardless of whether you choose a plant-based or meat-inclusive diet. Both approaches can be healthy when done thoughtfully. (Confidence level: Moderate—this is based on a small, short-term study, so larger and longer studies would strengthen this recommendation.)

This research is relevant for anyone considering a vegan or vegetarian diet who worries about whether they can eat healthily without meat. It’s also useful for meat-eaters who want to improve their diet quality. People with specific health conditions (like vitamin B-12 deficiency or iron deficiency) should consult with a healthcare provider or registered dietitian before making major dietary changes, as this study doesn’t address individual medical needs.

Based on this study, you might see improvements in your diet quality scores within 4 weeks if you focus on eating more whole foods and less added sugar. However, health benefits like improved energy, better digestion, or weight changes may take longer—typically several weeks to months. Individual results vary based on your starting diet and overall lifestyle.

Want to Apply This Research?

  • Track your daily vegetable servings and added sugar intake. Aim to increase vegetables to at least 2-3 servings per day and identify and reduce sources of added sugar (like sugary drinks, desserts, and processed snacks). Use the app to log these two metrics weekly and watch for improvements over 4-8 weeks.
  • If you’re following a vegan diet, ensure you’re including legumes (beans, lentils, chickpeas) at least 3-4 times per week and track your fiber intake. If you eat meat, focus on getting enough vegetables alongside your protein sources. In either case, use the app’s meal planning feature to pre-plan healthy meals for the week, similar to how the meal delivery service helped participants in this study.
  • Use the app to calculate your Healthy Eating Index score (or a similar diet quality score) at baseline, after 4 weeks, and after 8 weeks. Track food groups like vegetables, whole grains, legumes, and added sugars. Set reminders to log meals consistently, as detailed food tracking was key to the improvements seen in this study. Review your progress monthly to stay motivated.

This research is informational and should not replace professional medical advice. Before making significant dietary changes, especially if you have existing health conditions, take medications, or have nutrient deficiencies, consult with your healthcare provider or a registered dietitian. This study was small and short-term; individual results may vary. The findings suggest both diet types can be healthy but don’t prove one is superior to the other for all people.