In preclinical safety studies in which the administration of a test co
mpound causes reductions in food consumption, body weights, and organ
weights, it may be difficult to differentiate direct compound-induced
effects on organ weights from those simply due to reduced nutrition. T
o address this problem in reference to the heart, hearts were obtained
from rats that were known to have had reductions in body weights and
absolute heart weights as a result of feed restriction. Rats (40/sex)
were divided into 4 groups (10/sex) and given quantities of ad libitum
diet for 2 wk as follows: Group 1, 100%; Group 2, 75%; Group 3, 50%;
and Group 4, 25%. Routine histologic evaluation was performed on longi
tudinal sections of paraffin-embedded hearts stained with hematoxylin
and eosin. Computer-assisted image analysis was conducted on left vent
ricular free-wall of picrosirius red-stained sections for histomorphom
etric evaluation of ratio of cross-sectional area occupied by myofiber
s versus interstitium and for quantification of myofiber width. No dif
ferences were detected histologically among groups, and no difference
in the mean myofiber: interstitium ratio was detected between Groups 1
(9.1) and 4 (9.5). Mean values for myofiber width ranged from 24.6 mu
m for Group 1 to 17.3 mu m for Group 4. Two-way ANOVA revealed a stro
ng effect of dietary restriction on reduction of myofiber width but no
consistent gender effect. The significant dietary effects occurred in
Groups 3 and 4 compared to corresponding controls. The present author
s speculate that, if reductions in feed intake were <50% in short-term
preclinical studies, any reductions in myofiber width could imply a p
rimary test article effect. Conversely, if reductions in feed intake w
ere greater than or equal to 50% in such studies, reductions in myofib
er width could be caused either solely by inadequate nutrition or by c
ombined effects of nutrition and test article.