Stanford Medicine Releases Confirmation Bias Study; Media Takes the Bait

By David Lightsey MS — Dec 13, 2023
This past month, Stanford Medicine posted the following press release headline: “A Stanford Medicine-led trial of identical twins comparing vegan and omnivore diets found that a vegan diet improves overall cardiovascular health.”
Image by 1035352 from Pixabay

In 2020, I published the college secondary text The Myths About Nutrition Science, outlining the seven most prominent reasons for unreliable health news. One of the principal reasons is that most journalists rely on institutional press releases, which often overstate the study’s findings instead of examining the actual published study itself. A 2015 study published in PLOS/One identified 312 worldwide news reports related to the same press release. Of those 312 reports, “85.6% of all the stories were derived wholly or largely from a secondary source.” In other words, 85.6% of the reporters either failed to evaluate the original data, resulting in a failure to identify the shortcomings of the “study.” Eight years later, this problem persists. 

The latest example is a study touted by Stanford’s media machine. 

“a groundbreaking way to assert that a vegan diet is healthier than the conventional omnivore diet.” -Stanford Medicine News Center [emphasis added]

What followed was uncritical widespread media coverage of it. Simply Google “Stanford identical twin vegan study,” and you will notice just about every news service provides optimistic, gullible headlines.

According to Christopher Gardner, Ph.D., professor at the Stanford Prevention Research Center, their “study suggests that anyone who chooses a vegan diet can improve their long-term health in two months, with the most change seen in the first month.” 

Is this fact or fiction?

Consider a few relevant points in the actual published study, which pours cold water on this hot news story and Stanford’s claim.

The participants, ages 26-52, were all determined to be without cardiovascular disease and then split into paired groups, one vegan and one omnivore. None of the participants reportedly had been following a vegan diet before the study but consumed conventional diets and were determined to be free of cardiovascular disease. Because of the length of the study, 8 weeks, there were no actual health improvements, only “better” biomarkers. 

“The authors found the most improvement over the first four weeks of the diet change. The participants with a vegan diet had significantly lower low-density lipoprotein cholesterol (LDL-C) levels, insulin, and body weight — all of which are associated  with improved cardiovascular health — than the omnivore participants.” [emphasis added]

Of course, we all know that association is not causation.

  • The LDL-C for the vegan group dropped from a baseline average of 110.7 mg/dl to 95.5 mg/dl by the end of the study. For the omnivore group, the LDL-C dropped from 118.5 mg/dl at baseline to 116.1mg/dl by the end of the study. This is not some epiphany to support veganism; it is simply common sense. LDL is a natural and essential molecule involved in the distribution of cholesterol. Vegan diets contain no cholesterol, so it is well known that a vegan diet can dramatically reduce LDLs. However, all of the LDLs, both at baseline and at the completion of the study, fall within the near-optimal levels of 100 to 129 mg/dl. When more cholesterol is present, more LDL-C will be present. In this instance, lower does not necessarily mean better; it's just lower with no discernable health benefits. 
  • “The vegan participants also showed about a 20% drop in fasting insulin — higher insulin level is a risk factor for developing diabetes.” This is data spinning. The vegan group had an insulin level of 11.4 uIU/mL vs. 14.7 uIU/mL for the omnivore group. Both were well within the normal range and certainly no indication of pre-diabetes. The 20% sounds beneficial, but in reality, it is a meaningless comparison between two perfectly normal values. 
  • Dietary B12 availability to the vegan group dropped by 63% during the first four weeks and 65% in the latter four weeks. The average B12 availability was only 1.24 mcg in the first four weeks and 1.19 mcg in the second four weeks. This is significantly below what the NIH recommends in every age group from 9 years old and upward. A true vegan diet can result in other nutritional deficiencies if not supplemented with vitamin D, iodine, selenium, calcium, and iron. That is why some nutritionists favor a plant-forward rather than a purely plant-based diet.    
  • Both diets “were healthy, replete with vegetables, legumes, fruits and whole grains and void of sugars and refined starches. The entirely plant-based vegan diet included no meat or animal products such as eggs or milk. The omnivore diet included chicken, fish, eggs, cheese, dairy, and other animal-sourced foods.” There is absolutely nothing in the literature to suggest that individuals who regularly consume diets “replete” with plant-based foods as well as animal products, aka a healthy omnivore diet, are going to shorten their lifespan or be more susceptible to any disease—quite the contrary. Here, I illustrated, using a 24-year longitudinal study, not the eight weeks of the Stanford study, that the average life expectancy of population groups who adhere to a healthy omnivore diet and good lifestyle habits lived a few years longer on average. 

Dr. Christopher Gardner commented that “our study used a generalizable diet that is accessible to anyone, because 21 out of the 22 vegans followed through with the diet.” That assertion bears some consideration. During the first four weeks of the study, all participants had pre-prepared meals delivered to them. They were provided nutritional education on three separate occasions to reinforce adherence to the program and instruction on preparing healthy meals for themselves during the second four weeks. How realistic is this to the average uneducated consumer?

Closing comment:

The Stanford study does not support what it states in the Key Points section under Meaning

“The findings from this trial suggest that a healthy plant-based diet offers a significant protective cardiometabolic advantage compared with a healthy omnivorous diet.” 

The fact that there was a significant deficiency of Vitamin B12 illustrates how problematic a vegan diet is, countering the author’s assertion that a vegan diet is superior to a healthy omnivore diet. Additionally, in addition to the common nutrient shortcomings already mentioned, it would be naive to assume that the average child, adolescent, or even adult would be cognizant enough daily to mix and match the necessary plant-based foods to acquire the essential amino acid combinations to ensure their complete protein needs were being met. 


Source: Cardiometabolic Effects of Omnivorous vs Vegan Diets in Identical Twins A Randomized Clinical Trial JAMA Network Open DOI: 10.1001/jamanetworkopen.2023.44457