Your clients do not need dietary supplements unless they have a documented vitamin deficiency or they do not eat a balanced diet. Using supplements as an alternative to a sound diet can lead to serious deficits in the consumption of other nutrients (Benardot et al. 2001). It is always healthier to acquire vitamins and minerals from food than to obtain them from a pill. However, serious vitamin deficiencies do occur in a small proportion of the population (Benardot et al. 2001), and supplements are useful for making sudden improvements in vitamin status.
Supplements for losing fat or building muscle are rapidly becoming popular. Claims that “fat-burning” supplements will decrease body fat by increasing either mobilization or oxidation of free fatty acids (FFAs) are faulty at best. Untrained individuals have a greater ability to mobilize FFAs than they do to oxidize them. Therefore, supplements that increase FFA mobilization are not of any value for untrained people. For supplements to directly enhance FFA oxidation, the insulin response to the carbohydrates in those supplements would need to be eliminated (since insulin inhibits fat oxidation), and this is unlikely to happen (Coyle 1995). Exercise alone increases the muscles’ capacity to oxidize FFAs.
For those who eat a balanced diet, there is no evidence that muscle-building supplements, including protein powders and amino acids, build muscle mass (Clarkson 1998; Eichner et al. 1999). The few supplements whose muscle-building potential is supported by research (e.g., creatine) are effective mostly in elite athletes who have undergone many years of training (Eichner et al. 1999). Source: http://www.ideafit.com/fitness-library/top-10-most-frequently-asked-questions-in-a-fitness-center-and-their-answers