The energy cost of adjuvant-induced arthritis in rats

To test the hypothesis that rats with adjuvant-induced arthritis (AIA) need more metabolic energy to maintain body weight than healthy control rats or rats with AIA treated with methotrexate (MTX).

Rat AIA was induced by Mycobacterium butyricum injection at the base of the tail. The MTX-treated group was injected with MTX (1.0 mg/kg/week) in phosphate buffered saline. Negative controls (i.e., disease-free) and the MTX-treated group were pair-fed with positive controls (i.e., untreated AIA rats) to ensure equal mean body weights.

An additional 0.85 gm of food per day per rat was needed by the positive control group and 0.54 gm per day by the MTX-treated group to maintain a body weight comparable with that of the negative control group during days 5-15 post-adjuvant injection. During days 15-34 post-adjuvant injection an additional 1.4 gm of food per day per rat was needed by the positive controls and 0.62 gm by the MTX-treated group.

The results of this study indicate that adjuvant arthritis has a metabolic cost, which increases substantially as the disease becomes clinically apparent (days 15-34). MTX treatment does not completely eliminate the caloric cost of the disease. During days 5-15 post-adjuvant injection, an average of 6% of the total calories eaten by the positive controls was metabolized to support subclinical inflammation and other physiologic processes of this disease. During the active phase of the disease (i.e., clinical inflammation), this value increased to 18% of total calories.