I think you'd be better off with Robb Wolf's or Mark Sisson's book if you want to explain paleo laws. But here's my quick primer:
Fructose is not bad per se, but excess fructose (the kind that comes from added sugar in processed foods and drinks) is generally bad because too much can overwhelm the liver's ability to metabolize it, leading to a whole host of problems.
Some PUFAs are critical (EFAs, the essential fatty acids), but excess PUFAs (especially those in industrially produced veggie oils) are harmful because they are unstable fats that can oxidize (not a good thing) and do bad things like contribute to inflammation.
Gluten is a protein in wheat that causes problems for sensitive folks (and modern wheat is particularly bad). Sensitivity to gluten has been linked to all sorts of problems especially gut and/or auto-immune related.
Excess fructose overloads the liver and excess PUFA increases inflammation. The quantities of fructose and PUFA consumed on SAD have only be possible in the last 100 years or so, and we are not that well adapted to them.
In lieu of a biochemical explanation, just present an evolutionary scenario that explains all our weird human physical traits (so you know it's gotta be right) as consequences of carnivorous behaviors.
This has convinced a couple people in my circle to at least hold their tongue instead of criticizing my diet. One of them might go paleo rather soon, a full-fledged convert, if I don't drop dead from carb deficiency.