 His best-known research centres on what he termed the "isodynamic law" of calories (demonstrated in 1873, and published a decade later), according to which the form of human calorie intake is irrelevant to its effect on energy balance, often paraphrased as "a calorie is a calorie".In 1902, Rubner expressed his belief that this was over-simplistic, stating "the effect of specific nutritional substances upon the glands" may modify the effect of specific foods on energy balance, a view that is now increasingly accepted.With Otto Heubner (1843–1926), he performed important studies involving energy metabolism in infancy.