The Marine Body Reviews. Is Jason Klein Course Credible? Jason Klein is a former Marine and Navy fitness instructor as I mentioned earlier and beside training the best of the best he also trained Army Special Forces. The Marine Body author spent years training hundreds of Marines and soldiers in the fastest time possible so he knows exactly how to get you maximum results in the least amount of time possible. He also has a masters in exercise science so if you’re looking to burn fat, lose weight and get into some great shape Jason is a good guy to get help from in my opinion.