As in many things in life, there is not a simple 'Yes' or 'No' answer. But I'll try to give you a picture what supplements are and which are the ones do their jobs.
What is supplement?
According to Wiki explanation, "A dietary supplement is intended to provide nutrients that may otherwise not be consumed in sufficient quantities.Supplements as generally understood include vitamins, minerals, fiber, fatty acids, or amino acids, among other substances. U.S. authorities define dietary supplements as foods, while elsewhere they may be classified as drugs or other products".
Most people consume supplements of some form.
Do we need supplements?
Simply, if you have deficiency in certain nutrients, supplements are supposed to make up for these.
Do they work?
in recent years science has proven that some supplements can enhance performance and provide the additional nutrients your body needs, particularly if you not getting enough from your normal meals and are training hard. Whatever it is, never replace your meals with supplements. Supplements are meant to be used in addition to your normal diet, which should be already in good order.
Some of the purported uses of supplements:
provide protection against possible nutrients deficiency
enhance performances (i.e. building more muscles, burning fat, increasing sexual function, increasing endurance, becoming stronger)
Which ones are proven useful?
Many unscrupulous supplement companies are going overboard and make wild claims about their products. While some claims are substantiated, most are not. So how many of these are the useful ones? I'll apply the 80/20 (Pareto's principle, aka law of the vital few. Will talk more about this important principle in future) here:
80% of all proposed benefits come from 20% of all supplements
That is to say, only 2 out of 10 of the supplements on the market can truly yield the above benefits and the rest are pure garbage!
What are the some of the 20% useful ones and 80% useless ones?
Note that 80% of supplements are not just plain useless but some can be quite toxic and dangerous when consumed in excess. What should you do, as a consumer, to guard yourself from falling into the traps?
Just beware of all exaggerated claims. Those that sound too good to be true, usually are.
I have listed here some of the common supplements that are being marketed for various fitness/health use and have concluded, based on my experience and available research, their effectiveness.
Multi-Vitamin and Multi-Mineral
Claims: For all round insurance against any possible micronutrient deficiency.
Effectiveness: Not really. A multi-vitamin and multi-mineral supplement is supposed to provide the best protection from its complete spectrum of all the vitamins and minerals your body everyday. However, you could be paying for most of the nutrients you don't need at all and still not get those you truly need. As a general rule, using a multi-vitamin is not a great idea. Calcium and Magnesium are notoriously underdosed in most multivitamin supplements.
Specific Vitamins and Minerals
Claims: For specific protection against certain micronutrient deficiency as deficiency in a single nutrient can lead to severe consequences.
Effectiveness: Yes. Most of us tend to be lacking in:
Vitamin D (underexposed to sun),
Vitamin K (usually not enough from food),
Calcium (hard to get adequate amount from food)
Iron (for people with low/no meat intake and/or are anemic),
Magnesium (deficiencies are common in the western diet because grains are poor sources of magnesium), and
Zinc (for those who tend to sweat more).
Taking specific supplements will fill in the gaps.
Protein powders (including whey protein, soy protein and egg & milk protein).
Claims: For muscle growth and repair.
Effectiveness: Yes, particularly if you are not consuming enough protein in your normal diet to match your activity and goals.
Claims: For weight gain, recovery and muscle growth.
Effectiveness: Yes if used as a post-workout recovery drink or if one is severely underweight. If consumed in excess under normal circumstances, the weight gain would be mostly fat.
Claims: For muscle growth.
Effectiveness: No. You would be better off sticking with protein powders and cheaper too.
Fish oil/Omega 3s
Claims: For treatment of many health issues including: heart disease, ADHD, anxiety, depression, high cholesterol, inflammatory bowel disease, arthritis, alzheimer’s disease, eczema, diabetes, cancer, weakened immunity, autoimmune disease and macular degeneration.
Effectiveness: For most parts, yes. Fish oil is a great general health supplement, and is taken as a source of omega-3 fats. It has been proven to be effective in aiding many health related issues above. Most notably, in reducing triglycerides levels, ADHD, inflammation, cholesterol and depression.
It is not needed if one eats enough fatty fish.
Claims: For increasing strength and power, which translates to muscle building effect
Effectiveness: Yes. This is by far, one of the most researched and scrutinized sports supplement on the market today. And most supported its effectiveness. Many athletes are taking creatine to enhance performances. Possible side effects include diarrhea and water retention.
Claims: For increasing endurance and support fat-oxidation
Effectiveness: Yes. Just like creatine, caffeine has been proven to work by many researches. Taken in moderate amount, it could help increase performance and utilization of fat for endurance activity, especially for those well-trained. Possible side effects include stomach upset, irritability, nervousness, and diarrhea.
Claims: For boosting testosterone and building muscles
Effectiveness: Mixed results. There are many forms of testosterone booster out there. Most are useless stuffs. Proven ones are: Dhea (for those above 40), Vitamin D (if you have D deficiency) and Zinc (if you have Zinc deficiency).
Claims: For reducing fat mass.
Effectiveness: Mixed results. Proven ones are Green Tea (EGCG), Caffeine, L-Carnitine and Ephedrine (not recommended. It is banned in many countries because of its associated side-effects)
Which supplements, how and when should you take?
It's best to seek an expert's opinion to make the ideal recommendations. Quality, brands, optimising absorption, necessity, timing, possible drug interactions (if you are taking certain medications) etc are some of the areas you need to look at.