[4] As an assistant research professor of medicine, Gardner collaborated with John W. Farquhar to study the effectiveness of ginkgo biloba supplements to treat peripheral artery disease.

[7] He also oversaw the largest and longest-ever comparison of Atkins, Zone, LEARN or Ornish diets to see which led to the greatest weight loss and changes in cardiometabolic risk factors.

This landmark study tested whether there is a predisposition to success on one diet or the other based on either a potential genetic pattern or a metabolic condition known as insulin resistance.

[11] These interests led him to be invited to join the Scientific Advisory Board of the Menus of Change, a collaboration between The Culinary Institute of America and Harvard T.H.

[10] Gardner was appointed the Rehnborg Farquhar Professor of Medicine at Stanford which supports research in disease prevention in June 2017.

[18][19] In 2023, Gardner published research on cardiometabolic risk factors in pairs of identical twins randomized to follow either a vegan or omnivorous diet.

[2] The study found that a vegan diet was associated with statistically significant reductions in LDL cholesterol and fasting insulin levels.