In terms of formal education, becoming a registered dietitian (4 year university degree + internship) and then becoming a board certified sports dietitian is probably a worthwhile pathway to consider if you are young and want to be a sports nutritionist as your main career. Of course, relevant internship and work experience is recommended in order to develop into a qualified professional. Other credentials that are generally recognized as being reputable include:
International Olympic Committee Diploma in Sports Nutrition- Bachelor's degree in a related field required (Nutrition/Dietetics, Exercise Science, Biology, etc). Comprehensive 2 year part-time program in sports nutrition that can mostly be completed online.
Certified Sports Nutritionist- International Society of Sports Nutrition (CISSN)- Any bachelor's degree required. Reputable sports nutrition certification that can be completed with self study and passing an exam. If a comparison were to be made with personal training certifications, this is analogous to the NSCA-CSCS but for nutrition.
Sports Nutrition Specialist- International Society of Sports Nutrition (SNS)- Basic sports nutrition certification, analogous to the standard personal training certifications offered by ACSM, NSCA, NASM, etc but for nutrition.
Precision Nutrition Level 1 Certification