Why are slow and fast muscles differentially affected during prenatal undernutrition?

The biceps brachii and soleus muscles (fast and slow twitch muscles, respectively) were taken from guinea-pig fetuses, undernourished via maternal dietary intake to 60% of ad lib, and examined for differences from their controls. At birth, muscle fiber number was found to be reduced by 26% in the biceps brachii but was unaltered in the soleus muscle. An examination of the muscles at ages through gestation revealed that any reduction in fiber number was associated with a reduction in the secondary/primary fiber ratio. There was a stage in soleus myogenesis at which fiber number was significantly lower than the control. The deficit was corrected by the continuation of secondary fiber hyperplasia beyond the time of usual cessation. A similar delay was found in the biceps brachii but it did not enable recovery to a full complement of fibers. The difference between the 2 muscles was thought to be related to the relative proportions of primary fibers in the muscles, primary fiber development being apparently unaffected by undernutrition in utero.