Protein is reemerging as an important nutrient in contemporary diet plans for improving weight management. Whether the directives of current approaches are as extreme as the Atkins diet or a more subtle approach such as the Zone or South Beach plans, protein has regained equal footing with carbohydrates. For many health/fitness professionals, the reemergence of this essential nutrient is a welcome change. Indeed, the vital role of protein in the maintenance, repair, and synthesis of skeletal muscle and other body tissues in physically active men and women has long been known.
Protein as a Macronutrient
Proteins serve a number of structural (i.e., skeletal muscle) and functional (i.e., enzymes) roles in the body. As a macronutrient, protein is unique, given that it contains essential amino acids that cannot be produced by the body and must be consumed as part of the diet (Table 1). Additionally, protein can be distinguished from carbohydrate or fat, the other macronutrients, because proteins are made from amino acids that contain nitrogen. Whereas all three of the macronutrients can be used for fuel by the body, this fate is the least preferable for protein. Rather, amino acids, in general, and the essential amino acids, in particular, should be used in the synthesis (growth), repair, or maintenance of body proteins. These processes are often referred to as protein turnover.
The Concept of Protein Turnover
Protein turnover refers to the dynamic exchange between free amino acid pools and proteins in the body in the ongoing cycle of protein synthesis and breakdown. Figure 1 illustrates the integration of various aspects of protein utilization by the body and the role of dietary protein in providing amino acids to the free amino acid pools. In the fasted state, the breakdown of the body's own protein stores is the source of amino acids to the free amino acid pools. Therefore, the amount of protein being consumed is important for maintaining rates of protein turnover, particularly in maximizing muscle synthesis and growth.
Recommended Intakes of Protein
In 2005, the Institute of Medicine published Dietary Reference Intakes (DRIs) for specific nutrients, including protein (1). The DRIs are a set of reference values that include the commonly recognized recommended dietary allowance (RDA; 0.8 g of protein/kg body weight). The RDA is defined as the average daily nutrient intake level sufficient to meet the nutrient requirement of nearly all (97% to 98%) healthy individuals (1). The DRIs are based on the concept that there is a range of protein intakes for optimal protein utilization when energy (calorie) intake is sufficient to meet energy needs.
Importance of Protein Source
Protein source is an important consideration in meeting protein needs and obtaining the essential amino acids from the diet. Foods such as meat, eggs, fish, and dairy products contain all of the essential amino acids and are considered sources of complete or high-quality proteins. These foods also are nutrient dense because they provide a number of other essential nutrients relative to their calorie content. The nutrient density of foods with specific regard to their protein or nitrogen content can be referred to as the energy (E) or calorie to nitrogen (N) ratio (i.e., E:N). The lower the E:N ratio, the more protein dense the food source (Table 2). For active individuals interested in weight maintenance, gains in lean body mass, or even weight loss, high quality proteins are found in nutrient-dense foods that are easily incorporated into daily menu plans.
Importance of Adequate Energy Intake to Optimal Protein Utilization
Because protein turnover (i.e., synthesis and breakdown) and amino acid metabolism are energy dependent, energy balance plays a central role in the best use of protein by the body (2). Studies using nitrogen balance methodology (NitrogenIn vs. NitrogenOut) provide an overview of protein utilization because nitrogen is present in every amino acid. When nitrogen balance is negative (NitrogenIn < NitrogenOut), the body is considered to be in a catabolic state, where amino acids are being used for energy production rather than for synthetic processes. A positive nitrogen balance (NitrogenIn > NitrogenOut) reflects an anabolic situation associated with incorporation of amino acids into body proteins (i.e., growth). Nitrogen balance improves with increasing energy intake for any given amount of protein consumed. However, increasing protein intake while energy intake is adequate does not improve nitrogen balance or protein utilization (3); rather, consumption of protein in excess of what is needed for maintenance, synthesis, or repair of proteins leads to an increase in the oxidation of protein as a fuel source (4). As a result, energy balance is more important to protein metabolism when protein intakes are at the lower range of the DRIs so that amino acids are spared for protein synthesis and not oxidized to assist in meeting energy needs. Because exercise training contributes to energy expenditure, participation in routine exercise programs challenges this relationship and suggests that training-specific nutritional strategies are needed for optimal protein utilization.
The Impact of Resistance Training on Protein Metabolism
For muscle to increase in size, two conditions must be met: the synthesis of new muscle proteins must increase and muscle protein synthesis must be greater than breakdown so that protein balance is positive. Numerous studies have shown that a single session of resistance training stimulates an increase in both muscle protein synthesis and breakdown in the period after the exercise. The increase in synthesis seems to be more pronounced in untrained subjects, where rates of synthesis remain elevated for 48 hours after exercise (5), whereas routine training leads to an attenuated response (6). As mentioned previously, in the fasted state, net muscle protein balance is negative (i.e., breakdown > synthesis; Figure 2). Although resistance exercise increases the rate of protein synthesis, it cannot match the rate of protein breakdown caused by fasting, and net protein balance remains negative. Only when adequate amounts of amino acids are provided does muscle protein balance become positive (7). In this way, resistance exercise and amino acids work together to stimulate muscle growth. Therefore, for muscles to experience growth, adequate amounts of amino acids should be available at the same time the stimulus for increased synthesis is present.
Because resistance training stimulates an increase in protein turnover and results in muscle damage, it is widely recommended that individuals who resistance train consume protein at intakes above those recommended for healthy, nonexercising individuals. Although determining precise requirements for protein is difficult, analysis of several nitrogen balance studies indicate that a protein intake of approximately 1.3 g/kg protein/day is sufficient for strength-trained individuals to achieve nitrogen balance when energy balance exists (8). For individuals just beginning a strength training program, protein needs may be higher initially, given the heightened effect of resistance exercise on increasing skeletal muscle protein turnover. However, given that typical protein intakes for those who strength train is usually in excess of current recommendations, consuming adequate protein is likely not a concern unless they are restricting calorie intake to lose weight.
Much research has been done on the timing, composition, and quantity of amino acids needed in relation to resistance exercise to optimize muscle protein synthesis. Studies have found no difference in protein synthesis when amino acids are consumed before versus 1 hour after exercise, and no difference when amino acids are consumed at 1 hour versus 3 hours after exercise, providing individuals with some flexibility with timing of protein intake (9). Interestingly, when amino acids are consumed at 1 hour and again at 3 hours after exercise (10), there is an increase in synthesis after consumption of amino acids at both time points, suggesting an added benefit to two small feedings after exercise. The amount of amino acids needed to elicit this anabolic effect is surprisingly small, with as little as 6 g of essential amino acids (in combination with 35 g of sucrose) eliciting an increase in synthesis and resulting in a positive protein balance in muscle (9). Quality, and not quantity, of protein is an important concept, with studies showing that nonessential amino acids are not necessary to increase protein synthesis (11); rather, provision of adequate amounts of essential amino acids is important to maximize protein utilization in response to routine resistance exercise.
The Role of Protein in Endurance Exercise
The impact that endurance exercise has on protein turnover is far less studied than resistance exercise. Protein has typically not been considered to play a major role in the diets of endurance athletes because carbohydrates and fat are the primary fuel sources during endurance exercise and because muscle hypertrophy is not typically observed with endurance training. However, protein metabolism is affected by endurance exercise, especially when either energy intake or carbohydrate intake is inadequate (12, 13). During exercise, it is generally thought that protein synthesis is decreased and breakdown is either increased or not different from rest. After exercise, rates of protein synthesis are restored.
At rest, the use of amino acids for energy is very low. During aerobic exercise, however, amino acid oxidation for energy to support muscular work increases, especially with increasing intensity, duration, and low glycogen stores. Whether this increases the protein requirements for individuals who routinely participate in endurance exercise has been debated for several years. It is likely that significant increases in protein needs (up to 1.6 g/[kg · day]) apply only to "elite" endurance athletes exercising at high intensities for several hours a day (14). For people who engage in routine endurance exercise and have adequate energy intake, the recommendations for protein intake (1.0 g/[kg · day]) are not much greater than for nonexercisers (0.8 g/[kg · day]) (14). Although evidence exists that suggests people beginning an endurance exercise program may have increased protein needs during the initial few weeks (15), consumption of calories sufficient to maintain energy balance during this time will improve protein utilization in response to aerobic training (12, 16).
Considerations for Protein in Weight Management
Habitual reduction in energy intake for weight loss has been shown to have a negative impact on protein utilization because of the simultaneous slowing down of metabolic processes (17). Typically, weight loss is associated with a reduction in both body fat and lean body (i.e., muscle) mass, although the intent of diet interventions remains the loss of fat and maintenance of muscle mass. Recent reports by Donald K. Layman, Ph.D., et al (18, 19) have demonstrated that consumption of reduced calorie diets for which protein provides a greater percentage of calorie intake than conventional weight loss diet plans (i.e., 30% vs. ~15%, respectively) is associated with better maintenance of muscle mass during weight loss. When exercise is added to a weight loss program that uses a higher-protein diet intervention, reductions in body fat and sparing of muscle mass is even greater (19). These studies encourage integration of exercise programs with nutrition plans that provide a higher amount of dietary energy in the form of protein for weight management.
Summary and Recommendations
Protein is essential to the diets of healthy, physically active men and women. The essential amino acids provided by high-quality proteins are important to maximize rates of protein synthesis and to optimize protein utilization in response to habitual participation in resistance training and endurance exercise. The role of energy balance should not be overlooked. Clearly, anabolic processes (i.e., tissue building) require energy. Therefore, attention should be given to sufficient consumption of calories for either weight maintenance or increases in muscle mass if amino acids are to be directed to synthetic (i.e., anabolic) rather than catabolic processes. When negative energy balance is intended for the purpose of weight loss, a small amount of additional protein in the diet is likely beneficial to maintenance of lean body mass.
Condensed Version and Bottom Line
Dietary protein remains a critical component of the diet for healthy, fit men and women. Attention should be given to adequate energy intake for optimal use of dietary protein by the body and by the muscle of active individuals.
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