Protein and leucine

The performance of athletes and the physically active in training and recovery is integral to success and can become compromised without appropriate nutrition. It has generally been accepted that the amount of dietary protein intake required to support metabolic adaptation, muscle repair and muscle protein synthesis (MPS) is higher in athletes than the general population. The recent Joint Position Statement by the Academy of Nutrition and Dietetics, Dietitians of Canada and American College of Sports Medicine on Nutrition and Athletic Performance supports this notion as well as placing an emphasis on the distribution, type and amount of protein across the day.

High quality protein

Protein is critical for a variety of functions in the body, however it is the contractile proteins found in skeletal muscles that often receive the most attention. Skeletal muscle allows the body to perform a wide range of physical movements from running a marathon through to performing a deadlift in the gym. With carbohydrates and fats responsible for providing a majority of the energy required during exercise, the primary role of protein lies in muscle repair and muscle growth.

Proteins are made of long chains of amino acid molecules linked together. When we digest food, proteins are broken down to individual amino acids that can then be relinked in different sequences to form proteins such as hormones, enzymes, haemoglobin and skeletal muscle.

We consume a variety of protein in our diet but not all proteins contain every amino acid required by the body. There are twenty different amino acids in total, nine of which are ‘essential’ and must be obtained from the diet. The other eleven amino acids are ‘non-essential’ and can be produced by the body. Foods containing all the essential amino acids are termed high-quality protein sources. Dairy, meat, eggs, and soy are examples of high-quality protein sources. Meat and dairy are considered high biological value (HBV) proteins because they contain all the essential amino acids and are more readily digestible than plant-based protein.

Resistance training is the major driver of MPS and research indicates that there is an upregulation of MPS for up to 24 hours following a single bout of exercise. The primary benefit of including protein post-exercise is to not only limit exercise-induced muscle protein breakdown (MPB), but also maximise MPS. This means we end up with a net gain in muscle growth.

When determining an individual’s protein needs, a person’s weight as well as the volume and type of exercise must be taken into consideration. Daily requirements are expressed as grams of protein per kilogram of body weight per day (g/kg/d). Depending on energy intake and body composition goals, for most athletes this commonly ranges from 1.2 to 2.0 g/kg/d. For a 70kg person, this equates to 84-140g per day.

Including a regular intake of HBV protein spread across the day and following strenuous exercise supports optimal muscle repair and growth in most situations. For a 70kg person, this can be achieved with 20-30g of protein at three main meals and an additional dose following exercise. Including dairy foods in the diet makes this easily achievable.

BCAA and leucine

Branched chain amino acids (BCAAs) are three essential amino acids; leucine, isoleucine and valine and are particularly important in MPS. Dairy protein contains two types of protein: casein (80%) and whey (20%). Whey protein has a high content of leucine, which is known to assist with the upregulation of MPS. Whey protein is also more rapidly digested than casein, resulting in a rapid increase in plasma leucine and essential amino acid concentrations after ingestion.

Protein is required to build muscle, but it is leucine that is the primary trigger of MPS following exercise. To use an analogy, MPS is akin to turning on a light - leucine flicks the switch while protein is the electricity that powers the light. Current research shows that 2-3g leucine after a workout is optimal to trigger MPS. The amount of leucine varies widely between foods with animal protein providing the highest concentration. Milk, cheese and yoghurt also have a high leucine content, which makes them ideal foods to consume at snacks or main meals.

While adequate amounts of protein can be acquired through a well-balanced and nutritious diet, protein supplements offer an effective method for increasing protein consumption without significantly increasing energy intake. Whey protein isolate (WPI) is made by separating the whey protein from milk and offers a convenient concentrated dose of protein and BCAAs. 20g of WPI with 250ml milk, provides approximately 30g of protein and 3g of leucine, making it an ideal way to meet post-exercise targets. Numerous studies have highlighted the effectiveness of whey protein at increasing MPS following resistance exercise.

More recent research suggests including a moderate dose of casein protein before bed is able to prevent some of the MPB that occurs overnight. The slower digestion of casein helps to maintain plasma protein levels overnight when a majority of MPB occurs.

In conjunction with regular strength or endurance training, consuming moderate amounts of HBV protein at each main meal and post-exercise as well as a moderate dose of casein before bed is the most effective way to optimise MPS and support training.

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