Flexitarian Versus Vegan Diet: What Does the Research Say?

Disclaimer: plant-based diets such as vegan diets require vitamin B12 supplementation. Never change your diet without discussing such changes with your medical provider if you are being treated for a medical condition such as diabetes or you are taking medications such as anticoagulants. This post is for information purposes only.

What diet is best for your health?  A flexitarian diet that includes meat, eggs, and dairy but in small amounts?  Or a vegan diet that excludes all animal products?  What does the research say at this time?

Flexitarian Diets as defined by the Cleveland Clinic

Flexitarian diets as defined by the Cleveland Clinic are “relaxed” vegetarian diets.  According to the Cleveland Clinic, adherents of the flexitarian diet pattern eat meat.  However, they eat less of it, around 9-28 oz per week.

Adherents are encouraged to eat lean cuts of meat, along with dairy, eggs, and whole-food plants (such as legumes, whole grains, nuts, fruits, and vegetables.  Eating entirely plant-based 3 to 4 days per week is encouraged.

Diet proponents advocate health outcomes “similar to vegetarians” while allowing for greater diet variety and simplicity.  What are these health outcomes, and how do they compare with vegan health outcomes?

How do health outcomes compare for flexitarian diets versus vegans?

Many research studies on veganism compare the vegan diet to standard western-style diets.  The research is very clear that vegans enjoy superior health outcomes versus those consuming omnivore diets (diets that eat animal foods without any restrictions).   

Analyses of flexitarian diets versus vegan diets are less abundant, though the research that has been completed comparing the plant-based dominance of various diets to date appears to suggest that indeed, vegan diets maintain superiority against even a “healthy” diet containing small to moderate amounts of animal protein. 

Research findings comparing various diets including flexitarian diet

In a cross-section study of 10,797 participants conducted over a period of 12 years in Switzerland, participants were separated and analyzed in the following groupings according to diet:

• Omnivore (anyone in the study that ate meat/ poultry more than once a week)
• Pescatarian (a vegetarian diet with dairy, eggs, and fish, but no red meat or poultry)
• Flexitarian (anyone in the study that ate meat no more than 1 time per week)
• Vegetarian (no meat, but consume eggs and dairy)

Vegans were NOT analyzed as a separate group.  Nonetheless, the strict definition of flexitarian (meat no more than one time a week) is useful in providing comparisons versus vegetarians who avoid all meat.  

Five cardiovascular health markers were analyzed, including BMI (a measure of weight proportionate to height), blood pressure, cholesterol (as measured by mmol/L with readings above 6.5 mmol/L diagnostic of high cholesterol levels) and fasting glucose as measured in mmol/L (with greater than 7 diagnostic of diabetes). 

Despite the very low frequency of meat consumption in the flexitarian diet, vegetarian cardiovascular health outcomes proved superior as follows:

• Lower BMI: As compared to omnivores, vegetarians had the lowest BMIs (2.22 points lower than omnivores), followed by pescatarians (1.7 points lower than omnivores), then flexitarians (only 0.66 points lower than omnivores).   
• Lower total cholesterol: Vegetarians had a 0.44 mmol/L reduction in total cholesterol levels versus omnivores, pescatarians had readings 0.34 lower than omnivores, while flexitarians had no advantage over omnivores in terms of cholesterol.  LDL reduction levels were similar between pescatarian and vegetarian participants (0.36 and 0.33 mmol/L lower than omnivores respectively).  
• Lower blood pressure:  Pescatarians had the largest reduction in blood pressure versus omnivores (systolic bp reduced by 4.9 points, diastolic by 3.31 points), followed by vegetarians (systolic bp reduced by 3.1 points, diastolic by 1.9 points), with flexitarians experiencing the least blood pressure reductions (systolic reduced by 0.97, diastolic by 0.7). 

Some of the results (lower blood pressure for flexitarians and vegetarians) appear to be attributable to the lower BMI of these groups.  The same is true for the lower cholesterol levels in pescatarians.  None of the groups experienced diabetes at a lower rate than the omnivores.    

Separate research analyzing specifically vegan diets, however, has shown diabetics placed on a vegan diet experienced 28% reductions in blood sugar versus a control group on a low-fat diet which only experienced a 12% reduction in blood sugar level.   

What do 25 studies say about flexitarian diets?

The above study is interesting but insufficient.  So what does a review of 25 flexitarian diet studies, including 4 randomized controlled trials tell us about flexitarian diets and health outcomes versus other diets?

Flexitarian diets in the studies that follow were frequently referred to as “semi-vegetarian,” and definitions varied with some restricting red meat, others restricting fish.  

The studies below involving the Adventists had a more clear definition of flexitarians/ semi-vegetarians, defining such as persons who ate dairy and poultry more frequently than once per month, but less frequently than once per week. 

Weight management and flexitarian diets

• Flexitarian diets are inferior to vegan diets for weight management.  Vegan diets significantly outperformed 4 other diets (including omnivores, flexitarian, pescatarian, and vegetarian diets) in helping participants lose weight during a six-month randomized-controlled trial.
• Cross-sectional data analysis on 71,751 participants in the Adventist study similarly found that BMI was lowest in strict vegetarians, while only slightly lower in flexitarians, versus omnivores.
• In a similar Adventist study data analysis of 22,434 men and 38,469 women from 2002 to 2006, vegan diets outperformed all other diet variants in terms of weight management.  BMI average findings were as follows:  vegans (23.6 kg/m2); lacto-ovo vegetarians (25.7 kg/m2), pesco-vegetarians (26.3 kg/m2), flexitarians / semi-vegetarians (27.3 kg/m2), and omnivores  (28.8 kg/m2).
• Flexitarian diets outperformed omnivore diets in terms of weight management in 9,113 Australian women

Similar to the study cited in the previous section, research on tens of thousands of individuals shows consistently that weight creeps up with the addition of animal foods.  Weight starts escalating compared to vegans, first as eggs and dairy are added into the diet, then increasing further with fish, then higher still in those that just occasionally eat meat (flexitarians), until reaching the highest levels in omnivores. 

Cancer prevention and flexitarian diets versus vegan diets

Flexitarian style diets did not appear to protect against breast cancer in an analysis of 96,001 adults conducted from 2002 to 2007.  In trending breast cancer incidents, the addition of animal foods corresponded with increased rates of breast cancer when compared with vegans. 

Vegans enjoyed the highest reduction in rates of breast cancer versus cancer rates in vegetarians, pescatarians, semi-vegetarians/ flexitarians, and non-vegetarians, though chance could not be ruled out (the results trended lower than the other groups, even when controlled for BMI, but did not achieve statistical significance).  

When BMI was factored into the findings, it appeared at least some of the possible protection against breast cancer was due to lower average weights in vegans. 

Flexitarian diets also failed to protect against prostate cancer in an analysis analyzing dietary patterns of 26,346 males.  Only vegans experienced significant reductions in prostate cancer rates versus omnivores when compared to males consuming other vegetarian, pescatarian, and flexitarian/ semi-vegetarian diets.

Similar to the findings in breast cancer reduction, the lower BMI appeared to play some role in the cancer rate reductions for vegans–though not necessarily explaining the difference in rates in totality.

Diabetes and flexitarian diets versus vegan diets

Flexitarian diets do appear to reduce the risk of diabetes as noted in the National Family Health Survey 2005–2006 that compared health outcomes and dietary patterns for 156,317 participants.  

Findings in that study did not note a protective effect of vegan diets.  However, the researchers note that in India, self-reported “vegans” likely consume ghee and butter (normally not consumed in vegan diets elsewhere), as well as likely routinely consume white rice. The authors note that in India, people do not necessarily go vegan for health considerations.

In the research on Adventists in North America, vegan diets were superior and significant at protecting against diabetes, even when the study controlled for BMI, activity levels, income, age, television watching, and other factors.  Findings noted (with controls as described above in place):

• Vegans were 49% less likely to experience diabetes than omnivores
• Lacto-ovo vegetarians were 46% less likely to experience diabetes than omnivores 
• Pesco-vegetarians were 30% less likely to experience diabetes than omnivores
• Flexitarians were 26% less likely to experience diabetes than omnivores

Wrap-up

While flexitarian diets appear to be somewhat useful in reducing cardiac risk, risk of obesity, and risk of diabetes based on current research findings, most of these studies suggest it is inferior to vegetarian and vegan diets in terms of reducing risks for these conditions.  The strict definition of flexitarian is also interesting… suggesting that even small amounts of meat may undermine an otherwise heavy plant-based meal in terms of reducing disease risk.  

Nonetheless, while potentially inferior to vegan and vegetarian diets, flexitarian diets are still better in terms of health outcomes/ disease prevention compared to standard omnivore diets (where there are no restrictions on meat consumption).  More robust research is needed, such as large, randomized-controlled trials analyzing different diets as interventions for specific diseases and comparing results over time.  

However, based on research available at this time, flexitarian diets are perhaps good starting points for people who want to go more “plant-based.” Those seeking to maximize health outcomes may choose to adopt a more strict plant-based diet such as veganism.

References

Cleveland Clinic. (2021). What is the flexitarian diet? Retrieved https://health.clevelandclinic.org/what-is-the-flexitarian-diet/

Derbyshire E. J. (2017). Flexitarian Diets and Health: A Review of the Evidence-Based Literature. Frontiers in nutrition, 3, 55. https://doi.org/10.3389/fnut.2016.00055

Marrone, G., Guerriero, C., Palazzetti, D., Lido, P., Marolla, A., Di Daniele, F., & Noce, A. (2021). Vegan Diet Health Benefits in Metabolic Syndrome. Nutrients, 13(3), 817. https://doi.org/10.3390/nu13030817

Penniecook-Sawyers, J. A., Jaceldo-Siegl, K., Fan, J., Beeson, L., Knutsen, S., Herring, P., & Fraser, G. E. (2016). Vegetarian dietary patterns and the risk of breast cancer in a low-risk population. The British journal of nutrition, 115(10), 1790–1797. https://doi.org/10.1017/S0007114516000751

Tantamango-Bartley, Y., Knutsen, S. F., Knutsen, R., Jacobsen, B. K., Fan, J., Beeson, W. L., Sabate, J., Hadley, D., Jaceldo-Siegl, K., Penniecook, J., Herring, P., Butler, T., Bennett, H., & Fraser, G. (2016). Are strict vegetarians protected against prostate cancer?. The American journal of clinical nutrition, 103(1), 153–160. https://doi.org/10.3945/ajcn.114.106450

Tonstad, S., Butler, T., Yan, R., & Fraser, G. E. (2009). Type of vegetarian diet, body weight, and prevalence of type 2 diabetes. Diabetes care, 32(5), 791–796. https://doi.org/10.2337/dc08-1886

Turner-McGrievy, G. M., Wirth, M. D., Shivappa, N., Wingard, E. E., Fayad, R., Wilcox, S., Frongillo, E. A., & Hébert, J. R. (2015). Randomization to plant-based dietary approaches leads to larger short-term improvements in Dietary Inflammatory Index scores and macronutrient intake compared with diets that contain meat. Nutrition research (New York, N.Y.), 35(2), 97–106. https://doi.org/10.1016/j.nutres.2014.11.007

U.S. News & World Report. (n.d.). The flexitarian diet. Retrieved from https://health.usnews.com/best-diet/flexitarian-diet

Wozniak, H., Larpin, C., de Mestral, C., Guessous, I., Reny, J. L., & Stringhini, S. (2020). Vegetarian, pescatarian and flexitarian diets: sociodemographic determinants and association with cardiovascular risk factors in a Swiss urban population. The British journal of nutrition, 124(8), 844–852. https://doi.org/10.1017/S0007114520001762