The study found after eight weeks of a plant-based diet, there were also decreases in the ages of the heart, hormone, liver, and inflammatory and metabolic systems.

However, these changes were not seen in those whose diet included meat, eggs, and dairy.

Those who ate a vegan diet also lost two kilos more on average, due to them consuming 200 fewer calories through the meals provided during the initial four weeks of the study.

According to the experts, the weight loss could have contributed to the observed differences in biological age between both groups.

Some experts suggest that, long-term, a vegan diet may result in nutritional deficiencies and may not suit all ages, while others say the differences could have been down to the weight loss.

The findings are based on a small study of 21 pairs of identical twins aged 39, half of whom ate a vegan diet, while the other sibling ate an omnivorous diet.

By the end of the study, the researchers observed decreases in biological age estimations based on levels of DNA methylation – a type of chemical modification of DNA that is used to estimate biological age.

Biological aging refers to the decline in functioning of tissues and cells in the body, as opposed to chronological age.

Previous research has reported that increased DNA methylation levels are associated with aging.

However, Varun Dwaraka, of epigenetic testing company TruDiagnostic Inc, and Christopher Gardner of Stanford University, California, and colleagues, said it is unclear how much the differences between the pairs can be attributed to the differences in diet.

Further research is needed to investigate the relationship between diet, weight and aging, in addition to the long-term effects of vegan diets, they add.

The findings are published in the BMC Medicine journal.