Well if they take the typical government approach. We will all have to keep logs of everything we eat to document that we aren't on the Atkin's diet. Then we will have to save our Grocery Receipts. Our refrigerators and meal-times will be subject to surprise audits.
There, of course will be a hotline where anyone with a grudge against you can start an investigation of you where the burden of proof is on you, by anonymously reporting that they saw you ate at Western Sizzlin.
That won't be sufficient though. Eventually you will have to choose a diet plan. People will object because not everyone needs to diet. Two years later they will make us chooose a "meal plan" to avoid the stigma of the word "diet".
Then they will find a couple of examples where people chose inappropriate diets. So our choice of meal plans will now have to be approved by either a Doctor or a registered dietician.
If you have a blood test and the results are inconsistent with the meal plan that you are on, you will be subject to further investigation.
Of course they will go after the distributors. Grocery stores will be required to report purchases, so that the Feds can zero in on communities that aren't following the meal plans.
The low fat (high carb) diets will cause more obesity, diabeties, heart disease and vitamin deficiency. So the government will react by vigorously prosecuting those whose weight gain and symptons don't match what the government perceives the results of low fat diets should be.
Maybe we need to put the government on a diet before it puts us on one.