The simple answer is that its not. There are so many certifying agencies that range from total joke to excellent. But there is nothing standing in your way of providing fitness and nutrition advice without any type of certification. That may change at some point as I am sure the government will want to make it seem like they are doing something by requiring dieticians and nutritionists to have some government approved license to practice. Until then though the only authority you need is the one given to you by your clients.