Diagnostics is first and often the most important step in the process of introducing changes to our lifestyle. Of course there are no miracle diets, which will work for everyone. Each one of us is different and each one of us have different needs when it comes to macro and microelements. We can even have two very similar persons, with the same height, weight, muscle and fat mass level, but with a very different needs and tolerances for various nutrients. A high in fat diet can be very harmful for one person, whilst the other person may suffer if on a high protein diet. Fortunately, nowadays an offer of diagnostic tests is huge. We can therefore be very precise in defining dangers of any diet for any person. We can also do tests, which will tell us if using a specific diet by a particular person can be effective or maybe that it is pointless, because it would not allow them to achieve the desired effects. That also applies to training plans. The most important role of any dietitian is to select a diet that would meet the individual preferences, training needs, as well as proneness of a particular organism to metabolism. It is therefore worth taking diagnostic tests, which can help avoiding unnecessary experimentation and waste of time and health using a trial and error method!