What Were the Results of the Stanford Twin Diet Study?

December 6, 2025 •

3 min read

In a groundbreaking 2023 study published in JAMA Network Open, researchers at Stanford University used identical twins to compare a healthy vegan diet against a healthy omnivore diet. The results of the Stanford twin diet study showed that the vegan diet offered significant improvements in cardiovascular health metrics over an eight-week period.

Quick Summary

Stanford's twin study demonstrated superior health outcomes for the vegan group, including lower LDL cholesterol, reduced fasting insulin, and greater weight loss, compared to the omnivore group after eight weeks. Identical twins were used to control for genetic variables.

Key Points

Vegan Diet Lowers LDL Cholesterol: Identical twins on a vegan diet saw a significantly greater drop in 'bad' LDL cholesterol compared to their omnivore counterparts.
Improved Insulin Sensitivity: The vegan group experienced a notable decrease in fasting insulin levels, indicating better insulin sensitivity and reduced risk for diabetes.
Greater Weight Loss: The study showed that the twins eating a plant-based diet lost more body weight than those on the omnivore diet over the eight-week period.
Protective Effect on Telomeres: Participants following the vegan diet exhibited slower telomere shortening, which suggests potential anti-aging benefits at the cellular level.
Controlled for Genetics: By using identical twins, the study provided strong evidence that the observed health differences were due to diet, not genetic or lifestyle variations.
Healthy Diets Compared: Both the vegan and omnivore diets were designed to be healthy, underscoring that even a 'healthy' omnivore diet was outperformed by the plant-based approach.

Study Design and Methodology

To conduct the Cardiometabolic Effects of Omnivorous vs Vegan Diets in Identical Twins trial (TwiNS), researchers led by Dr. Christopher Gardner recruited 22 pairs of healthy adult identical twins from the Stanford Twin Registry. This unique methodology was critical as it effectively controlled for genetic factors, lifestyle, and upbringing, allowing for a more accurate comparison of dietary impacts.

For the eight-week intervention, one twin in each pair was assigned a healthy vegan diet, while the other followed a healthy omnivorous diet. Both diets were designed to be equally nutritious, emphasizing vegetables, fruits, whole grains, and legumes, while limiting refined starches and sugar. For the first four weeks, all meals were provided via a meal-delivery service to ensure strict adherence. For the final four weeks, participants prepared their own meals with guidance from the research team.

Key Findings on Cardiometabolic Health

At the conclusion of the eight-week study, the vegan participants showed significant and favorable changes in several key health markers compared to their omnivore counterparts. The most pronounced differences were observed in cardiovascular health metrics. Not only were these results statistically significant, but some improvements were evident as early as the first four weeks of the study.

Comparison Table: Vegan vs. Omnivore Diet Results


Health Metric	Vegan Diet Group	Omnivore Diet Group	Key Takeaway
LDL Cholesterol	Average drop of 15.2 mg/dL.	Average drop of 2.4 mg/dL.	Vegan diet led to a far greater reduction in "bad" cholesterol, despite most participants starting with healthy levels.
Fasting Insulin Levels	20% greater drop than omnivores.	Modest change observed.	This result indicates a significant improvement in insulin sensitivity, reducing a risk factor for diabetes.
Body Weight	Lost an average of 4.2 more pounds.	Lost less weight overall.	Greater weight loss in the vegan group occurred without calorie counting, highlighting the impact of diet composition.
Telomere Length	Slower loss of telomeres, a marker of cellular aging.	Faster telomere shortening observed.	The vegan diet showed a protective effect on telomere length, suggesting slower aging at a cellular level.

Factors Contributing to the Results

Several factors likely contributed to the more positive health outcomes seen in the vegan group. The plant-based diet inherently contained less saturated fat, more dietary fiber, and a different micronutrient profile compared to the omnivore diet. The vegan diet eliminated all animal products, a major source of saturated fat, and led to a higher intake of fiber-rich whole foods. The researchers noted that these changes—reduced saturated fat, increased fiber, and weight loss—are the key drivers for improving cardiovascular health.

While both diets were structured to be healthy, the absence of animal products proved to be a decisive factor. The study's design, using identical twins, provided robust evidence that these health changes were primarily due to diet rather than genetics or other confounding variables. For those interested in a deeper look, a Netflix docu-series, You Are What You Eat: A Twin Experiment, documents the study participants' experiences.

The Takeaway for a General Audience

The study's lead author, Dr. Christopher Gardner, summarized the findings by stating that a more plant-based diet offers a significant cardiometabolic advantage. While a full vegan diet showed the most dramatic results, the implication is that any move toward a more plant-based eating pattern is likely beneficial for long-term health and longevity. The study demonstrates that individuals can see measurable health improvements in a relatively short timeframe, even without being underweight or having severe health issues at the start. It provides compelling, controlled evidence for the health benefits associated with reducing or eliminating animal products from one's diet.

Conclusion: The Cardiometabolic Advantage of a Vegan Diet

In conclusion, the Stanford twin diet study provided compelling evidence that a healthy vegan diet leads to significant improvements in cardiometabolic health compared to a healthy omnivorous diet. The findings revealed notable reductions in LDL cholesterol, fasting insulin, and body weight for the vegan participants. By utilizing identical twins, the study effectively isolated the dietary factors influencing these outcomes, reinforcing the powerful impact of plant-based eating on health. This research adds substantial weight to the body of evidence suggesting that shifting towards a more plant-based diet can yield long-term health benefits. The changes were rapid, with improvements visible in just two months, showing that dietary adjustments can quickly and effectively improve health markers. For those looking to improve their heart health, the study's results offer a clear path forward.

Here is the full text of the original study from JAMA Network Open.

Frequently Asked Questions

The study was led by Dr. Christopher Gardner and his research team at Stanford Medicine, with the results published in JAMA Network Open in 2023.

The trial involved an eight-week dietary intervention for the participants, with health metrics measured at both the four-week and eight-week marks.

Identical twins were used to control for genetic variations and shared environmental influences. This design allowed researchers to isolate the effects of the diet itself on the participants' health.

The primary outcome was that the healthy vegan diet provided significant cardiometabolic health benefits, including reduced LDL cholesterol, lower fasting insulin levels, and greater weight loss, compared to the healthy omnivorous diet.

For the first four weeks, participants received all meals from a delivery service to ensure strict adherence. For the final four weeks, they prepared their own meals with guidance.

LDL cholesterol is often called 'bad' cholesterol. Its reduction is important because high levels are a major risk factor for cardiovascular disease, including heart attack and stroke.

Yes, the results are relevant for the general population. The use of twins simply provided a powerful tool to control for variables, making the findings on diet's effect more robust and generalizable.