> That means 10-20% excess per person. If everyone started eating the right amount overnight, grocery spend would drop 10-20%

I doubt that $ spend on the top end of caloric intake scales linearly with # of calories because of high caloric density foods.

If I spend $500 a month on groceries, lets say I need $400 to keep me alive and hygienic and the last $100 are going to be the candy and sodas that make me fat. So to give an example, reducing the caloric intake by 50% could be achieved by reducing spend by only 5% if there's very caloric foods making me fat.

Also need to account for restaurant spending.