Constipation is defined as fewer than three stools per week. Common causes of constipation include a lack of fiber in your diet, dehydration, not moving your bowels when you have the urge and a lack of physical activity. Other causes of constipation are related to specific conditions; for example, pregnancy can cause constipation. Heavy resistance training is not a direct cause of constipation and most likely not the primary culprit if a weight trainer is constipated.
Link Between Weight Training and Constipation
There are certain circumstances where heavy resistance training could be associated with, but not cause, constipation. Dehydration is a big factor for constipation, as the colon holds on to fecal matter to get water from the stool. Heavy resistance training with no fluid replenishment may increase dehydration in those with low pretraining hydration levels. This dehydration may lead to constipation, but the resistance training itself is not the cause.