Use the same healthy skepticism with online information that you use to evaluate other nutrition claims.
Does the information come from a reliable source? Usually, web addresses with the suffixes .gov, .edu and .org are the most reliable. Those ending in .com are commercial sites, and .net are networks, Internet service providers or organizations. Many .com's have responsible consumer information, but you need to be a savvy consumer and ask good questions
Are a site's nutrition and health claims backed up by specific scientific research? You want cold, hard facts, not opinions and personal testimonials.