This is a very controversial issue... So many people hear about this supplement and that supplement... Truth is, if you eat a well-balanced diet (and are not at a nutritional/disease risk), and you are taking vitamin/mineral supplements because you heard it was "good for you" then you are just going to have expensive urine! A well-balanced diet is called a "well-balanced diet" for a reason... it provides your body with ALL the nutrients it NEEDS! (This is why having variety in your diet is important) The human body can only metabolize so much at a time and whatever it can't store or doesn't need is excreted. There are countless supplements on the market and it's hard to know which ones are good and not so good. Before taking supplements, it's important to know if you really need them. So talk to your doctor/dietitian before taking them.
The thing about taking supplements is that our bodies do not metabolize nutrients the same way when in supplement form. It metabolizes and utilizes them BEST when they come from our FOOD! Lets say you take a calcium supplement that has 1000mg of calcium; your body will NOT metabolize all 1000mg. It depends on how much calcium your body needs at that time, and if there is room to store any extra, if only 500mg are needed, then 500mg are going to be excreted.
Some people, however, do need supplements. Not everyone eats a well-balanced diet. These people may be at nutritional risks and should see a dietitian to determine exactly what they need. Not just guess or try a new fad product seen on the drug store shelf!
I was asked about supplementation for triathletes. Even if you are eating a well-balanced diet, you may need supplementation depending on how hard you are training...However, this is all individualized. One answer does NOT fit all! But if you are following a well-balanced diet that is meeting YOUR nutritional needs, then you probably do not need supplements. The only supplementation you may need is electrolytes, due to a high loss while sweating. Electrolytes are essential to replace to maintain normal body functions, so many people turn to sports drinks for this. Sports drinks can be high in sugar, so unless you need those carbs, try a lower-carb option. It's important to maintain your energy during training/racing, but if you aren't pushing yourself as hard one day or your training for a particular day is lighter than others, then definitely go with the lower-cal options (at least for that day). You can even make your own electrolyte water to avoid calories/carbs if you need to (it may not taste too great, but there are ways to fix that!) Again, a lot of this is based on the individual. It's important to know what YOUR body needs. Set a meal plan that will allow you to meet those needs, and if it is difficult for that person to meet his/her needs, then supplementation may be needed.