There are 18 words that start with 'diet'.
| diet | diets | dieted | 
| dieter | dietary | dieters | 
| diether | dieting | dietetic | 
| diethers | dietaries | dietarily | 
| dietetics | dietician | dietitian | 
| dieticians | dietitians | dietetically | 
Note: These 18 words that start with 'diet' are found from the USA & Canada version of the tournament Scrabble word list.