What Happens When One Twin Goes Vegan and the Other One Keeps Eating Meat?

In November 2023 an interesting study from Stanford University selected pairs of twins to test the health effects of different types of food.  This was a randomized controlled trial encompassing 22 pairs of healthy identical twins 18 years of age or older (44 participants) who had signed up in the Stanford Twin Registry database in order to participate in research. (1)

One twin from each pair was assigned to eat a vegan diet and the other to eat an omnivorous diet for a period of eight weeks.  Both diets were created to be as healthy as possible, containing lots of fruits, vegetables including beans and whole grains and avoiding sugar and refined carbohydrates.  The omnivorous diet also allowed fish and chicken, dairy, cheese and eggs.  The vegan diet was entirely plant-based and included no meat or animal products such as eggs and dairy products.  (1)

The first four weeks of meals (breakfasts, lunches and suppers) were provided at no cost to the twins through the services of a meal-delivery facility.  The last four weeks of meals were prepared by the study participants themselves. Participants also purchased their own snacks following guidance from health educators supplied by the study.  It was expected that, after the first 4 weeks of healthy food counselling, participants would understand the amounts and types of foods they should prepare in order to adhere to their assigned diet for the last 4 weeks.  A registered dietitian was on call throughout the 8-week span of this research to answer questions and offer suggestions to the participants about their diets. (1)

The researchers recorded the weight of each participant and took blood samples three times during the trial; at the start of the study, at four weeks into the study, and at eight weeks (the end of the study).  The participants were interviewed about their dietary intake via unannounced 24-hour dietary recall interviews with participants which were conducted three times during week zero (baseline), three times during week four, and three times during week eight. Participants also logged their daily food consumption in detail on “Cronometer”, an app that tracks diet and health data. (1)

Here are the results (1).

  • Both the vegan and the omnivorous groups saw improvements in their cholesterol levels and modest reductions in weight over the eight weeks, however the benefits were greater among the twins who followed the vegan diet.
  • At four weeks, the participants who were eating vegan meals showed significantly higher gains in their cardiometabolic health compared to the participants who were eating omnivorous meals. These improvements included lowered LDL-cholesterol, lowered insulin levels and lowered body weight.
  • At eight weeks, the twins who were eating vegan meals showed a 14% greater reduction in their LDL-cholesterol levels and a 20% greater reduction in their fasting insulin levels compared with the twins consuming the omnivorous diet. In addition, the vegan eaters had lost an average of 4.2 pounds more than the omnivorous eaters.

The researchers noted several reasons why additional differences between the study groups were not observed in this trial. (1)

  • Both the twins eating a vegan diet and the twins eating an omnivorous diet were eating a healthier diet during the trial than they were before the study began. All participants increased their consumption of vegetables and whole grains and decreased their intake of added sugars and refined grains.
  • The vegan group primarily substituted animal products with meat alternatives (vegan meats). This translates into a relatively high intake of ultra-processed food for them in spite of their instructions to choose minimally processed foods.
  • All the twins were relatively healthy at the start of the study. For example, their average starting LDL-cholesterol levels were 2.9 mmol/L.  This is very close to the healthy range for LDL-cholesterol leaving minimal room for improvement.
  • The length of the study was only 8 weeks, a short amount of time to see substantial health changes.
  • Food eaten during the second half of the study was planned and prepared by the participants. Some of them reported difficulties during this process and commented that choosing appropriate foods, learning new methods of preparation and cooking and ordering suitably healthily at restaurants were new concepts and sometimes difficult to carry through.
  • In spite of these challenges, significant improvements were seen after only 4 weeks in the three health outcomes of LDC-cholesterol levels, insulin levels and weight, and the benefits continued through the full 8 weeks of the study.

The investigators also stated that the use of identical twins as participants was a great strength of this study.  This eliminated the confounding influences of age, sex and genetic factors due to their nearly identical genes and similar experiences during their upbringing so that observed differences in health outcomes could be largely attributed to the diet itself. (1)

Previous science has also shown positive links between the consumption of a plant-based diet and health risks such as heart disease, type-2 diabetes and obesity.  Many of these earlier studies were observational in nature and therefore couldn’t answer the question of whether their results showed that healthier people are more likely to follow a plant-based diet and not that the result of eating a more plant-based diet is better health.  It takes a randomized clinical trial, the gold standard for scientific research, to identify cause and effect.  And indeed there were some early randomized trials that discovered similar healthy outcomes to those found in this latest study in participants eating vegetarian, vegan or plant-based diets. (2,3,4,5,6,7,8,9,10,11)

To sum up, the results of this recent randomized clinical trial are a convincing indicator that ingesting more plants can significantly improve health and they add robust strength to the conclusion that eating more plant-sourced foods directly results in substantial health benefits.



1  Landry, M.J., Ward, C.P., Dunanan, K.M., et al.  Cardiometabolic Effects of Omnivorous vs Vegan Diets in Identical Twins: A Randomized Clinical Trial.  JAMA Netw Open. 2023; 6(11): e2344457. Doi:10.1001/jamanetworkopen.2023.44457.

2  Wang, T., Kroeger, C.M., Cassidy, S., Mitra, S., Ribeiro, R.V., Jose, S., Masedunskas, A., Senior, A.M., Fontana, L. Vegetarian Dietary Patterns and Cardiometabolic Risk in People With or at High Risk of Cardiovascular Disease: A Systematic Review and Meta-analysis. JAMA Netw Open. 2023 Jul 3; 6(7): e2325658. Doi:10.1001/jamanetworkopen.2023.25658. PMID: 37490288; PMCID: PMC10369207.

3 Remde, A., DeTurk, S.N., Almardini, A., Steiner, L., Wojda, T. Plant-predominant eating patterns—how effective are they for treating obesity and related cardiometabolic health outcomes? a systematic review. Nutr Rev. 2022; 80(5): 1094-1104. Doi: 10.1093/nutrit/nuab060.

4  Termannsen, A.D., Clemmensen, K.K.B, Thomsen, J.M., Nørgaard, O., Díaz, L.J., Torekov, S.S., Quist, J.S., Faerch, K. Effects of vegan diets on cardiometabolic health: A systematic review and meta-analysis of randomized controlled trials. Obes Rev. 2022 Sep; 23(9): e13462. Doi: 10.1111/obr.13462. Epub 2022 Jun 7. PMID: 35672940; PMCID: PMC9540559.

5 Gibbs, J., Gaskin, E., Ji, C., Miller, M.A, Cappuccio, F.P. The effect of plant-based dietary patterns on blood pressure: a systematic review and meta-analysis of controlled intervention trials. J Hypertens. 2021; 39(1): 23-37. Doi: 10.1097/HJH.0000000000002604.

6 Lee, K.W., Loh, H.C., Ching, S.M., Devaraj, N.K., Hoo, F.K. Effects of vegetarian diets on blood pressure lowering: a systematic review with meta-analysis and trial sequential analysis. Nutrients. 2020; 12(6): 1604. Doi: 10.3390/nu12061604.

7 Joshi, S., Ettinger, L., Liebman, S.E. Plant-based diets and hypertension. Am J Lifestyle Med. 2019; 14(4): 397-405. Doi: 10.1177/1559827619875411.

8 Viguiliouk, E., Kendall, C.W., Kahleová, H., et al.. Effect of vegetarian dietary patterns on cardiometabolic risk factors in diabetes: a systematic review and meta-analysis of randomized controlled trials. Clin Nutr. 2019; 38(3): 1133-1145. Doi: 10.1016/j.clnu.2018.05.032.

9 Li, S.S., Blanco Mejia, S., Lytvyn, L., et al. Effect of plant protein on blood lipids: a systematic review and meta-analysis of randomized controlled trials. J Am Heart Assoc. 2017; 6(12): e006659. Doi: 10.1161/JAHA.117.006659.

10 Huang, R.Y., Huang, C.C., Hu, F.B., Chavarro, J.E. Vegetarian diets and weight reduction: a meta-analysis of randomized controlled trials. J Gen Intern Med. 2016; 31(1): 109-116. Doi: 10.1007/s11606-015-3390-7.

11 Barnard, N.D., Levin, S.M., Yokoyama, Y. A systematic review and meta-analysis of changes in body weight in clinical trials of vegetarian diets. J Acad Nutr Diet. 2015; 115(6): 954-969. Doi: 10.1016/j.jand.2014.11.016.


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My name is Debra Harley (BScPhm) and I welcome you to my retirement project, this website. Over the course of a life many lessons are learned, altering deeply-rooted ideas and creating new passions.

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