I have non-alcoholic fatty liver, but my gi doc doesn't know why because i have zero risk factors. do i need a liver biopsy? should it be investigated

Loose weight: Only proven way to help fatty liver is to loose weight. You don't need biopsy

Need more info: It depends on how the diagnosis was arrived at: do you have elevated liver enzymes? What imaging studies have been performed?

Genes play a role: One major gene was just reported in the New England Journal of Medicine. Don't ask 'why', but focus on your best treatment -- aerobic fitness. This will greatly improve your life both now and in the decades to come.