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The authors of a study published in the Journal of Nutrition found out that rye bread was more effective than commonly used laxatives in relieving mild constipation and it also improved gut metabolism. As a part of the study rye bread was also tested against white wheat bread and the results showed that rye bread was much more effectual.
Holma et al., 2010. Constipation Is Relieved More by Rye Bread Than Wheat Bread or Laxatives without Increased Adverse Gastrointestinal Effects. The Journal of Nutrition. 140(3) 534-541.