It really depends where the food is from. Lithium isn’t used to amend soil in large amounts as far as I know so it would mostly be from natural sources (there are studies about lithium battery dumps releasing lithium into the soil and water too). You could have a potato with undetectable amounts of lithium or a potato that was grown near a lithium mine and watered with the river water contaminated by the mine as well, maybe a battery dump close by too, you would have high amounts.