The invention is based on a correlation observed between serotonin level and intestinal flora. The invention relates to a relative proportion of bifidobacteria in the intestine and in particular to a product increasing the ratio of bifidobacteria to Clostridia, to be used for raising the serotonin level. The invention also relates to a method for estimating a health risk associated with serotonin depletion by determining a relative proportion between bifidobacteria or Clostridia in the intestine or their ratio to one another. The invention further relates to a method for selecting a form of therapy for raising the serotonin level by determining a relative proportion between bifidobacteria or Clostridia in the intestine or their ratio to one another.