"Isocaloric diets" refers to meal plans that have the same number of calories. It means that you consume the same amount of energy, even if the composition of the foods may differ. Full definition
Rats fed isocaloric diets in which the fat source was varied among three groups — a beef tallow group (primarily saturated fat, 3 % PUFA), an olive oil group (primarily unsaturated, 10 - 15 % PUFA), and a safflower oil group (78 % PUFA)-- had highly variable weight gains. (paleodietnews.com)
In contrast, the same subjects receiving isocaloric diets containing at least 50 g of carbohydrate showed no significant changes in either T3 or rT3 concentration. (perfecthealthdiet.com)
In a landmark study, researchers gave participants isocaloric diets (meaning they both had the same amount of calories) with two different ratios of carbohydrates to protein. (sciencedrivennutrition.com)