The chemistry of sports drinks

Carbohydrate is arguably the most important source of energy for athletes. No matter what sport you play, carbohydrates provide the energy that fuels muscle contractions. Both energy drinks and sports drinks are a good source of carbohydrate that is quickly absorbed into the body. Many energy drinks also contain caffeine which behaves in a totally different way. 

The energy required for muscle contraction comes from a molecule called adenosine triphosphate (ATP).

In ATP, an organic (carbon-based) group called adenosine is attached to three phosphate groups. The phosphate groups are involved in the energy storage. The loss of one of the phosphate groups produces adenosine diphosphate (ADP) and gives out 30 kJ mol⁻¹ of energy. It is an exothermic reaction. This reaction supplies the energy needed to make our muscles move.

At any one time, we only have a small amount of ATP in our muscles and so ATP must be regenerated in our bodies. In fact, the above reaction is reversible and ADP is reattached to a phosphate to make ATP. This requires an input energy of 30 kJ mol⁻¹ ie it is an endothermic reaction. This energy comes from the breakdown of food molecules such as carbohydrates, fats and proteins. The primary source is carbohydrates such as glucose, C6H12O6. In fact 180 g of glucose (1 mole) can release about 3000 kJ when reacted with oxygen. Our bodies release this energy gradually via the ATP/ADP cycle. 

Many sports drinks contain carbohydrates such as glucose which are quickly absorbed into the bloodstream. If the glucose is not needed straight away it is stored in the muscles and liver as a carbohydrate called glycogen, which consists of many glucose molecules joined together. Glycogen is the source of energy most often used for exercise. It is needed for any short, intense bouts of exercise from sprinting to weight lifting because it is immediately accessible. Glycogen also supplies energy during the first few minutes of any sport. During long, slow duration exercise, fat can help fuel activity, but glycogen is still needed to help breakdown the fat into something the muscle can use.

Our bodies release the chemical energy stored in glucose through the process of respiration. There are two main types of respiration; aerobic and anaerobic. During aerobic respiration, glucose reacts with oxygen to produce carbon dioxide and water.