Dr. Michael Roizen, an anesthesiologist and the chief wellness officer at Cleveland Clinic, is 78 years old, however, his “biological age” is around 57, meaning that his body appears to be two decades younger.

One’s biological age is determined through the study of levels of methylation, which are chemical changes in our DNA.

Here are the diet principles Roizen follows.

Eat a Mediterranean diet

The Mediterranean diet has linked it to better heart health, weight loss, and less cognitive decline. The South African food-based dietary guidelines and the Healthy Eating Food Guide compiled for South Africans has many similarities to the Mediterranean diet.

The Mediterranean diet primarily consists of whole foods such as fruits and vegetables, legumes, low-fat protein and dairy, and limits red meat, processed foods, and alcohol.

Trout and salmon, which are packed with vitamin D and omega-3 fatty acids, are the main source sources of animal protein.

Eat a big meal at lunch

The biggest meal we consume should be at lunch. Rozen says that he and eats “very little” at dinner — usually just a salad.

He added that he doesn’t sleep well after a heavy meal and feels “much worse the next day,” he said.

A study released this year by researchers at the University of Alagoas in Brazil found that eating most of your calories at lunch could help to prevent and treat obesity, regardless of the quality of the diet.

The study suggested that eating this way may align better with the body’s natural rhythms.

The Longevity diet

Rozien said he’s been following the Longevity diet, which involves restricting calories five days a month to mimic the effects of fasting with a mostly plant-based diet and fish.

The Longevity Diet was developed by Valter Longo, a professor of gerontology (the scientific study of the ageing process), and the director of the University of Southern California Longevity Institute.

A study released this year by Longo’s team found that participants on the fasting-mimicking diet had lower biological ages by an average of 2 ½ years after three months on the diet.

Participants consumed about 1,100 calories on the first day of the diet and then about 700 calories on days two to five.