In Binary Logistic Regression, a positive interaction effect between A and B means that when A increases the effect (in this case log odds ratio) of B increases and a negative interaction effect means that when A increases the effect of B decreases.
For detail, please refer to http://www.ats.ucla.edu/stat/stata/seminars/interaction_sem/interaction_sem.htm

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胖子得高血压的概率随饭量递减