BCAAs, also known as branched chain amino acids, include valine, leucine and isoleucine. These are 3 essential amino acids that make up ⅓ of muscle proteins.

What do they do?

BCAAs may help prevent the breakdown of muscle tissue during intense exercise. They are converted into two other amino acids called glutamine and alanine. These are released in large quantities during intense aerobic exercise like running, cycling, swimming. They can be used as fuel by the muscles especially when muscle glycogen levels are low.

Some evidence and studies suggest taking 4g of BCAA supplements during or after exercise may reduce muscle breakdown. It may also help preserve muscle in athletes who consume a low carbohydrate diet.
These benefits are very similar to an athlete consuming a carbohydrate drink so it is not clear whether chronic (long term) use of BCAA supplements benefits performance. BCAAs may not offer advantage over carbohydrate drinks taken during exercise.

Do I need them? Are they necessary?

If an athlete is consuming enough calories, protein and carbohydrates then there is little benefit in taking a BCAA supplement.
However, if an athlete is in a calorie deficit (consuming fewer calories than the body expends) or consuming very little carbohydrate and protein food sources then doses of 6-15g of BCAA supplements may help improve recovery during hard training sessions by reducing muscle protein breakdown.

Are there any side effects?

BCAA supplements are relatively safe to consume because branched chain amino acids are naturally found in protein food sources in the diet. It is important to note that excessive intake may inhibit/reduce the absorption of other amino acids. BCAA might cause stomach problems like nausea, vomiting, diarrhea and stomach bloating.