First of all, what are dietary supplements?

"Dietary supplements consist of single or multiple concentrated minerals, vitamins or other substances and have nutritional or physiological effects." [2]

It is very important to know that supplements can be added to a balanced diet but should never replace it! The most popular supplements used in the fitness industry are proteins/amino acids and creatine. They are mostly consumed in the form of protein shakes, protein bars, compounds such as l-carnitine and branched chain amino acids (BCAA) [1]. 

Numerous studies have shown that amino acids have a positive effect on recovery after exercise [3]. Likewise, consuming protein concentrates directly before exercise can delay the time of exhaustion, as they prolong the decrease of serotonin levels in the brain, meaning that during long training sessions fatigue sets in later [3].

Creatine is a substance produced naturally in the body and is responsible for rapidly providing energy in the muscles. It has been shown that supplementing creatine can lead to faster recovery after exercise and increased performance during strength training [5].

Increased protein intake can lead to increased muscle protein turnover. However, higher protein turnover does not equal muscle gain. [4] ATTENTION: Too much protein can also be harmful! If you consume too much protein, you can damage your kidneys and liver, therefore you should not consume more than 2 grams of protein per kg body weight per day [3].

If you don't want to resort to supplements you can also improve your athletic performance with the help of a coffee, because the caffeine contained in coffee promotes perseverance during long-lasting endurance efforts [4]. 

Conclusion: In order to increase physical fitness dietary supplements are not necessarily needed. Those who follow a balanced diet can also increase their physical fitness without adding supplements. However, supplements can be useful when following a vegetarian/vegan diet and when the training workload is quite high [3].