Jack Lalanne (American fitness guru) stated "If man made it, don't eat it". I recall reading that and scratching my head at the sheer number of food products that encompassed. I thought it a rather presumptuous statement.
Five years later and after many books and articles on nutrition I now fully agree with him. When I hit the grocery store I skip the middle aisles and head straight for the fish and meat counter or the fresh fruit and veg section.
My only (dietary) vices are coffee and the odd glass of wine (both very much man made).
I think that the human body has evolved to run on a certain fuel and of course we adapt but it takes time. Grains and milk are relatively recent to our diet (past 10000 years) so you will see some people who can and some who can't digest them. Sugar however and all the other myriad man made products on store shelves are even more recent and apart from as occasional treats should really be avoided.
As an aside one incredibly beneficial thing people can do for their health is exercise. The lymphatic system which helps rid the body of waste substances is 'pumped' by respiration and physical activity. If you want to eat that cupcake or enjoy that cold glass of beer be sure to exercise it off. It really is incredible how effective exercise can be at covering a multitude of dietary sins!