Unlike breads made from wheat flour, rye bread has a low glycaemic index. Therefore, pumpernickel and rye breads are good substitutes for modern white breads.
Be careful though. Many pumpernickel and rye breads have more wheat flour than rye and thus have a greater fat-forming potential. The easiest way to tell which flour predominates is to examine the list of ingredients. If wheat flour is listed before rye flour, it means that there is more wheat flour in the bread.
Until this century, pumpernickel formed the basis of many European diets. Perhaps this is the reason why obesity is a modem phenomenon in those countries.