Protein shakes are a popular muscle-building supplement.
Protein is one of the body's main building blocks for muscle, bone, skin, and other tissues.
Protein shakes come in combinations of protein, carbohydrates and fats. They can range from 100% protein to mostly carbohydrates with added protein and fat.
Athletes involved in higher strength events have a need for more protein than most people.
Having extra protein from shakes and other sources doesn't build muscle on its own - exercise is still needed to give the muscles a workout.
Protein shakes come in a variety of flavours and may be sold ready-made or as powder needing to be mixed up.
Protein shakes are used by athletes and sports people before, during and after training to help enhance performance and to aid recovery.
The shakes may also be taken at mealtimes, or between meals as a high-protein snack.
Choosing a protein shake
Although protein shakes can be convenient, the British Dietetic Association says most people could get similar benefits from eating more high-protein foods in their meals and snacks.
Protein shakes are not always suitable as meal replacements.
Although protein is important for good health, too much protein can be bad for us, increasing the risk of osteoporosis and worsening any existing kidney problems.
The Department of Health advice is not to eat or drink more than twice the recommended daily intake of protein, which is 55.5g for men and 45g for women.
Protein supplements are not suitable for children.
If shopping for protein shakes online, remember that products from outside the UK or Europe may not pass the same safety standards at those sold at home.
Seek advice from a registered dietitian if you think you need extra protein for exercise.