However, a direct cause-and-effect relationship between energy intake and visceral tissue mass, or O2 consumption, is difficult to ascertain from data sets that have compared level of alimentation or dietary energy density because of confounding factors such as diet chemical composition and DMI. Case-in-point, Reynolds et al. (1991) reported that although PDV O2 consumption increased with increasing dietary MEI, O2 consumption was less in heifers fed a 75% concentrate vs. a 75% forage diet at equal MEI. This observed difference in O2 use between diets reflects the cumulative impact of factors that affect visceral mass, potentially including site of energy substrate assimilation in the gastrointestinal tract.