Analysis of myocardial response to significant weight loss in obese rats

We evaluated cardiac response to weight loss induced by a very-low-energy (VLE) diet similar to commercially available protein-sparing diets. Such diets have been implicated in sudden death, and whether organ and tissue responses to them are untoward is not known. Rapid weight loss was induced in rats with weights ranging from obese to normal, and cardiac mass and myocardial histomorphometry were assessed. Over 3 wk body weight dropped from 544 +/- 12 to 417 +/- 21 g (P < 0.001). Heart weight was less in the VLE group than in obese controls (1246 +/- 115 vs 1625 +/- 179 mg, P < 0.001), as were the weights of the left ventricle (805 +/- 81 vs 1061 +/- 134 mg, P < 0.001) and right ventricle (198 +/- 27 vs 265 +/- 40 mg, P < 0.002). Reduction in heart weight was commensurate with loss of body weight (r = 0.89). Myocyte cross-sectional area was reduced in the VLE group (452.6 +/- 108.6 to 331.8 +/- 41.5 microns 2, P < 0.05), with no structural abnormalities. We conclude that weight loss in the weight range studied is accompanied by proportional reduction in cardiac mass and myocyte size. Myocardial regression is not accompanied by myocyte dropout or edema, and likely represents a simple adaptation to reduced body size.