Twin Nutrition Study (TwiNS): Vegan VS. Omnivore

Study Aim

A vegan diet has been promoted as having a much lower environmental impact, but there is some controversy about the health effects of following a vegan diet. Some believe that this plant-based diet can help prevent many of the chronic diseases that affect us, such as heart disease, cancer, and Alzheimer’s disease and would also help in managing body weight. Others claim that the vegan diet does not provide all the required nutrients, such as adequate amounts of protein, vitamin B12 and the minerals iron and calcium, and thus it would not promote optimal health. This study is designed to investigate the health impact of a vegan diet compared to an omnivorous diet. We plan to study these diets in twins, where one twin follows a vegan diet and the other twin follows an omnivorous diet, thus we control for genetic differences that might impact the effect of the diet.

Eligible Participants

  • 18 years or older

Study Design

Twin pairs will be randomized to follow either a vegan or an omnivorous diet for 8 weeks. They will receive pre-made meals delivered to their home for the first 4 weeks. They will be responsible for their own meals the last 4 weeks. Participants will have 3 in-person clinic visits, as well as provide stool and micro-samples of blood from home. Changes in food patterns and behaviors and dietary intake assessment will be addressed using multiple tools employed successfully over dozens of studies by the Nutrition Studies Group.


The findings indicate that a vegan diet improves cardiovascular health. Vegan eaters’ low-density lipoprotein - which is the bad cholesterol - dropped on average by 15.2 milligrams over eight weeks. The omnivore participants' low-density lipoprotein fell by 2.4 milligrams over eight weeks. Vegan participants shed an average of around 4 more pounds than omnivores, and their insulin dropped by 20 percent more. A full recap of results can be found in Stanford Medicine’s News Center.

The study results were published in the JAMA Network Open in November 2023.