Eating and teeth

Just got a question about laryngeals, and you guys are pretty
knowledgeable about this stuff. It's said that the words eat and
tooth are etymologically related. Eat comes from *ed and tooth from
*dont. This makes sense, until laryngeals come along. According to
theory these words are *h1ed and *h3dont. The h3 is needed to explain
the prothetic vowel in Greek. What is the reason for the variation
between h1 and h3 since h3d- should be the zero grade of h1ed?