Turning back the clock? New vegan diet study sparks controversy

30 Jul 2024 --- The findings of a recent twin study, which posited that following a vegan diet for eight weeks is associated with a decrease in biological age estimations based on DNA methylation levels, were met with several cautionary reactions from other experts in the field.

The study, published in BMC Medicine, evaluated the impact of a vegan diet versus a non-vegan diet in twins on DNA methylation levels through the analysis of blood samples collected at baseline, four weeks and again at eight weeks. DNA methylation levels, the chemical modification of DNA that alters gene expression, were used mainly to infer the biological ages of the participants’ organs.

The researchers observed a decrease in biological age estimates — known as epigenetic aging clocks — in participants who followed a vegan diet but not in those who followed the omnivorous diet. After eight weeks, the ages of the heart, liver, inflammatory and metabolic systems appeared younger in the participants who followed a vegan diet. The researchers say the extent to which the observed differences between the two groups can be attributed to dietary compositions is unclear.

Meanwhile, an eight-week trial study comparing vegan and omnivore diets in 22 pairs of twins found that the former improves cardiovascular health. A correlation to healthy aging was also established from the vegan diet.

Experiment of comparison
The researchers investigated the molecular effects of a short-term vegan diet by instructing one-half of each twin pair to eat an omnivorous diet for eight weeks — including between 170 and 225 g of meat, one egg and one and a half servings of dairy each day — and the other half to eat a vegan diet for the same length of time.

The findings are based on a small randomized controlled trial of 21 pairs of adult identical twins. The pairs were controlled for sex, age and ethnicity, primarily from the Stanford Twin Registry and other twin registries, including Netflix’s pre-recruited participants interested in a documentary on vegan diets. On average, they were 40 years old and had an average body mass index of 26 kg. 

The researchers note that the participants who followed a vegan diet lost around 2 kg more on average than those who followed an omnivorous diet, which was associated with the differences in caloric content of the meals provided in the first four weeks when specialists prepared their meals. 

The researchers suggest that the weight loss variations could have contributed to the differences in epigenetic age between the groups. Further research is needed to investigate the relationship between dietary composition, weight and aging and the long-term effects of vegan diets.

Questioning potency of outcomes
Researchers from the nutrition and dietetics department of King’s College London and a dietitian from the British Dietetic Association have scrutinized the findings. “This is a small study conducted in twins, one of whom was allocated to a vegan diet and the other to a mixed diet for two months,” says Tom Sanders, professor emeritus of nutrition and dietetics, King’s College London, UK.

“Measurement of some biological markers of aging such as telomere length and methylation of DNA were made. The report found some difference which might be favorable in terms of aging for vegans.” 

According to Sanders, the assertion does not consider that micronutrient deficiencies often take years to exert harmful effects. For example, unless a vegan diet is supplemented with vitamin B12, people develop vitamin B12 deficiency, which causes chronic and often insidious damage to the nervous system.The Stanford twin study drew the attention of several experts in the field. 

“Long-term observational studies of vegans also find adverse effects on bone density that are probably caused by very low calcium and marginally adequate protein intakes,” Sanders explains.

“Although observational studies indicate that vegan diets may have favorable effects on health in middle age, such as a lower risk of cardiovascular disease and type 2 diabetes, this is not the case in older vegans, who seem more likely to suffer from muscle loss, low bone density and neurological disorders, which have a significant impact on the quality of life. Life expectancy does not differ in vegans compared with those who select mixed diets.”

Continuation of previous limitations
Dr. Duane Mellor, dietitian and spokesperson for the British Dietetic Association, UK, describes the current study as a follow-up of one published in JAMA Open in 2023. This study exhibits the same limitations: The participants who followed the vegan diet ate fewer calories and lost weight, which could be the reason their DNA was methylated in a healthier way.

Mellor notes: “The authors found that changes in how the DNA was methylated, a normal way DNA is changed in response to the environment including diet on a vegan diet could possibly be linked to cells aging more slowly.” 

“However, there were no changes in how quickly cells divided or died, again a normal part of our biology, but if out of control, can be linked to diseases such as cardiovascular disease or cancer,” he details.

“Although the study compared a vegan diet to an omnivorous diet, these diets were not entirely matched with vegan participants on average consuming around 200 kcal fewer per day, resulting in an average 2 kg weight loss.” 

According to Mellor, this was caused by the vegans eating less protein and fat than the omnivores. A reduction in energy intake could have altered the participant’s DNA. Another important consideration was that the vegans ate at least double the number of vegetables, fruit, legumes, nuts and seeds compared to the omnivores.

“In part this was to replace meat, eggs and dairy foods, but this would mean fiber intake as well as the vitamin and mineral intakes are likely to be different between groups, which also could in part explain the differences reported,” Mellor explains. 

“Overall, the benefits in terms of altering DNA methylation are theoretical, and not directly linked to living longer and the changes due to the diet could be down to the vegan group eating more plants and not that a vegan diet is healthier than a mixed diet.” 

According to Mellor, the key to any diet — with or without animal products — is that it consists of a wide range of foods, including vegetables, fruits, nuts, seeds, beans, peas and lentils, with wholegrain and moderate amounts of meat and dairy-based on preference. 

“If you do not wish to consume animal products, then alternative sources of nutrients including iodine, iron, calcium along with vitamins B12 and D need to be included in your diet along with a source of omega-3 fatty acids,” Mellor concludes.

By Inga de Jong