What certifications or degrees could help someone who has a passion for fitness break into the personal training/exercise science field?

I'm unsure about the US but in Australia you would require a Cert 4 in Fitness from a good TAFE (technical college) or private institution to get the basic level of knowledge and qualififcation. From there find a gym or PT who has values and goals similar to your own and work on getting them as a mentor and/or working within their business. Learn from them and what made them successful. A common thing i see working with fitness professionals is that its not always how brilliant you are at training its the whole package of presentation, your involvement with the client and your attention. You can be brilliant and know exactly what would help every single patient but still have a poor business. On the other hand of seen some rather scary personal trainers that run amazing businesses. I'd suggest a bit of both :-). Look at what you love, who you want to work with, what this audience requires, where they hang out, what are their needs. Address all these and put it into a package that stands out from everyone else. Clients in general have no idea who is 'better' they are much better at discerning 'different' or 'unique'. Probably a large reason why many of the trends like cross fitt, TRx training etc burst on to the scene and achieve rapid success.

I have been a personal trainer for 16 years and have a business degree from a very good college. The one thing that helps me to stand out from the pack is my experience. I have had all the certifications there are and all it did was cost me a lot of money. No book out there is going to tell you how to help each individual person hit their goals, this only comes from experience. Getting it done and seeing common mistakes, pitfalls, syndromes will make you better and able to help more people. Do whatever it takes to get clients and to learn from each experience you have. I use to find the best looking person at a gym and offer to train them for free, people will associate you with that fit person and want to train with you. After that you just have to produce.