Vegan Diets and Cardiometabolic Benefits: A Closer Look

Within the past 3 years, a trend toward eating plant-based or vegan diets has emerged and become growingly popular. According to Google search data, interest in veganism has shown a recent increase in 8 countries, including the U.S. More specifically, online searches involving veganism have doubled in America since 2015. As of late, more plant-based options have also become available in restaurants and grocery stores, perhaps to accommodate the growing interest in vegan products. The vegan diet has clearly become a hot topic in the U.S. and around the world, which has sparked research into its health benefits. 

Overview of the Stanford Twin Study

One of the most prominent recent studies on plant-based diet was conducted by Stanford researchers and published in JAMA in 2023. This clinical trial used 22 pairs of identical twins as subjects. One twin from each pair was randomly assigned to a vegan diet, while the other twin was assigned to an omnivorous diet for a total time period of 8 weeks. 

The dietary intervention was divided into two 4-week periods. During the first 4 weeks, meals were delivered to the participants at no cost in order to expose them to the parameters of their assigned diet, along with nutrition counseling. Then, in weeks 5-8, participants were expected to adhere to the vegan or omnivorous diet while making their own meals and choosing their own snacks. Thus, the second portion of the experiment was largely self-guided. Guiding principles were given to the participants regarding what types of foods to consume. These were:

• Choose minimally processed foods.
• Build a balanced plate with vegetables, starch, proteins, and healthy fats.
• Choose variety within each food group.
• Individualize these guidelines to meet preferences and needs. 

Data on food intake was collected mainly through unannounced evaluations of a participant’s food intake in the past 24 hours. These assessments were conducted by a registered dietician on a phone interview. Subjects were also encouraged, though not required, to log their food intake on an online platform. Weight loss was not the goal of the study, so participants were told to eat according to their hunger and fullness cues. To study metabolic effects, blood draws and stool samples were conducted at baseline, 4 weeks, and 8 weeks. Blood draws were done mainly to measure LDL cholesterol levels. Stool samples were intended to show changes in gut microbiome composition and assess for inflammatory markers. 

Interestingly, caloric intake was lower than baseline for both the vegans and the omnivores at the end of the study period. This could possibly be an effect of monitoring and tracking food intake, which can cause people to eat less or make healthier choices than they would if they were not tracking. The drop in caloric intake could also be a product of eating more vegetables and minimally processed foods, as stated in the dietary guidelines of the study.

The main finding of this study was that there was a significant decrease in LDL cholesterol levels in vegan participants compared to omnivores. This difference in LDL levels was present at both 4 weeks and at the end of the 8-week study period. Secondary findings included a decrease in fasting insulin levels in the vegan group compared to the omnivorous group. Another key secondary finding to note was that both groups experienced weight loss, although the participants on a vegan diet experienced significantly more weight loss than the participants on an omnivorous diet. This weight loss could, at least in part, explain the decrease in LDL levels and fasting insulin levels observed in the participants. A final secondary finding was that those on the vegan diet had lower levels of dietary satisfaction than those on the omnivorous diet.

TMAO Levels

Another important finding of this study involves TMAO levels. For some background, TMAO is a metabolite produced by digestion and metabolism of red meats. Recent literature has linked this metabolite to cardiovascular disease. This could further explain the connection between excess red meat and increased CVD risk, which was previously thought to be only a result of saturated fat content. Individuals with increased levels of TMAO in their blood are at higher risk of stroke, heart attack, and other serious cardiovascular problems. 

In this study, there was no significant difference in TMAO levels between the vegan and omnivorous diet groups. This is contrary to what would be expected, given the difference in meat consumed. However, due to tracking of food and the guidelines provided, the omnivorous group may not have ate enough red meat to cause an increase in TMAO. This suggests that eating meat alone does not cause an inherent increase in cardiovascular disease risk. It is likely that quantities of red meat consumed and other lifestyle factors also play a role in modulating cardiovascular health. 

Strengths

A major strength of this study when compared to other nutritional analyses is that it eliminates the component of genetic variation by using identical twin pairs. When measuring cholesterol, genetic factors may be a major confounding variable overriding diet and lifestyle. By eliminating this factor, the effects of diet on health measures can be better observed.

In addition, the health measures used in this study, including LDL cholesterol, fasting insulin, and TMAO levels are well-studied and have a significant impact on CVD risk. Many of these values are also used in clinical settings to evaluate and diagnose patients. Therefore, using such values as markers for cardiometabolic health is accurate and evidence-based. 

A third strength is that meals were provided to the participants in the first 4 weeks of the study. This eliminates variation that can be seen with self-chosen foods and controls for caloric intake as well. It also greatly improves adherence to the guiding principles listed in the study. As a result, one can be more sure that health measures are results of the vegan versus omnivorous diet category rather than individual dietary variation.

Limitations

However, there are some major limitations in this study that are important to consider when evaluating its results, many of which are noted by expert critics. These limitations begin in the study design itself. The sample size of n=44 is low, and the exclusion criteria state that individuals who weighed under 100 lbs, had a BMI greater than 40, an LDL of 190 mg/dL or higher, a systolic blood pressure of >160 mmHg, or a diastolic blood pressure > 90 mmHg were not included in the study. This is potentially problematic because it impacts the ability of the results to be generalized on a population scale. 

In addition, there was relatively loose dietary control, particularly in the second, self-guided portion of the study. This leads to potentially significant variability in individual diet choices and calorie consumption, which can play a role in health outcomes. There was also no control of other lifestyle factors, with examples being physical activity and smoking. Lifestyle choices outside of diet can have a large impact on health outcomes measured in this study.

Another important issue is that categories of vegan versus omnivorous do not necessarily encompass health value of a diet. For example, those on a vegan diet can choose to eat primarily fruit, vegetables, and whole grains. On the other hand, there is also an opportunity to eat increasingly popular, highly processed vegan junk food. The same is true with an omnivorous diet. Although this study attempted to make both diets rich in fruits and vegetables and minimally processed foods, this is not a realistic representation of dietary variability in vegans and omnivores. 

The effects of tracking and monitoring diet should also be considered. Although this was not intended to be a weight loss study, both groups did lose weight by the end of the 8-week treatment period, possibly as a result of tracking dietary habits. This weight loss was more pronounced in the vegan group, which could be a major confounding factor that may explain the reduction in LDL and fasting insulin levels in the vegan group compared to the omnivorous group. 

Summary/Conclusion

Overall, this study concludes that a vegan diet has a positive impact on cardiometabolic health through decreasing LDL cholesterol and, secondarily, decreasing fasting insulin levels. Although these findings were statistically significant, there are several key limitations that must be considered along with the study’s results. 

Lack of a representative sample does not allow widespread application and generalization of these results. In light of so many uncontrolled confounding variables, a conclusive statement cannot definitively be made about vegan versus omnivorous diets and their impact on measures of cardiometabolic health. Further research is needed to clarify the cardiovascular and metabolic impact of vegan and omnivorous diets.