If you tell him that you haven't been drinking (he's already smelled the odor of an alcoholic beverage), he knows you're lying. You can give him the standard answer of a couple of beers (regardless of how much you drank) which he will note as an admission of drinking. Finally, if you give him an honest answer the DA and the cop will focus on the amount you admitted to drinking -- not over how long a period or what you had to eat during that time.
The only correct answer is to refuse to answer any questions until you're able to speak to an attorney.
If you find yourself charged with DWI, call my office so we can start working to save your driver's license.