A 7-year follow-up showed that binge-eating,not purging , was predicted by frequently dieting.   其中分词not purging 是什么用法？修饰全句吗？

purging在此不是现在分词，而是动名词。可以理解为插入语。

binge-eating: 暴食、吃得过多。purging: 用泻药、通便。predict在本句为预示之意。

本句实际是将二个that从句合并为一个，第二个that从句与第一个从句的结构完全相同，不同的只是主语，以及第二个是否定句。比较本句与下一句：

A 7-year follow-up showed that binge-eating was predicted by frequently dieting but that purging was not predicted by frequently dieting. 

参考译文：一项7年的随访研究表明，频繁节食预示以后会暴饮暴食，而不是使用泻药。