In the 19th century protein was thought to be the main energy source for physical activity. (1) Today we know this to be false and that the carbohydrates and fats are the main energy sources, while protein plays only a minor role in total energy consumption.
Protein intake is the main subject of conversation for people in involved in strength sports, fitness, and bodybuilding. The reason for this is the abundance of scientific literature confirming the importance of protein intake for maintaining and increasing muscle mass. However, protein intake has an important place in endurance sports as well, where it's often overlooked because of the necessary prioritization of energy (carbohydrate) intake and maintaining glycogen levels in the body. Carbohydrate intake is and must be primary goal for anyone involved in endurance sports, but for optimal results, along with other dietary factors (read: healthy diet), we must meet our daily protein requirements.
Proteins have three basic roles in endurance sports. They're used for repairing and building muscle proteins and as an energy source.
Proteins and muscles
Physical activity of a high enough intensity and volume stimulates the muscle protein synthesis (anabolism), but also breaks down muscle tissues (catabolism). (2) The type of physical activity determines which type of protein will be built. Strength training (weights, resistant bands or your own body weight) increases the quantity of myofibrillar proteins, causing muscle hypertrophy and increase in strength, while endurance training results in increased oxidative capacity – the muscular resistance to fatigue, through increasing mitochondrial volume and density. (3) The balance between the process of synthesis and catabolism conditions the increase (hypertrophy) or decrease (atrophy) of muscle tissue.
Dietary intake, specifically protein intake, which is the intake of essential amino acids, can shift the balance towards positive. It seems that myofibrillar proteins are the most sensitive to protein intake (hyperaminoacidemia) and that the synthesis of mitochondrial proteins after endurance training isn't influenced by protein intake. (4) This represents the basic reason why protein needs are greater in strength sports. Lack of dietary intake will shift the balance between synthesis and catabolism towards negative, which will result in loss of muscle protein and suboptimal training adjustment. Carbohydrates intake doesn't have a positive effect on synthesis, but it inhibits the protein breakdown, also moving the balance towards positive.
Proteins are also used as energy source – they oxidize, either directly or after the transformation to glucose in the process called gluconeogenesis. Their share in total energy spent during physical activity is 1-10% (14) and it decreases with the activity durtion. We must emphasize that this is the reducing of a relative part in energy consumption. The absolute quantity of protein spent for energy actually increases, because energy consumption increases multiple times in activity as compared to period of rest. The protein part increases as energy and/or carbohydrates intake decreases. (5) In other words, adequate carbohydrate intake will stop the excess protein oxidation. BCAA (branched chain amino acids): isoleucine, leucine, and valine are primarily used as energy sources. (6) Regardless of this, BCAA supplementation isn't necessary because we find them in sufficient quantities in quality protein sources. Interestingly, protein oxidation in women is lower than in men. (7)
I'd like to emphasize another, at first glance counterintuitive, occurrence. It's intuitive to think of protein catabolism as damaging, negative. But this stance is being questioned more and more since catabolism is strongly connected to reactive remodeling – fixing the muscle tissue. So, catabolism could represent a signal for recovery. This would mean that nutritious attempts to acutely suppress catabolism may not be the best idea. (8) At the moment this does not have direct implications for recommendations but in my opinion it removes the unnecessary obsession with quantity and timing of protein intake.
Protein needs are expressed in grams per kilogram of body mass. RDA (Recommended Dietary Allowances) for general population is 0,8 g/kg BM, even though newer studies based on better methodology suggest raise of this recommendation to 1,0 g/kg BM. (9)
As I mentioned before, protein needs in endurance sports are greater than the general population. But we must distinguish the needs between recreational and active athletes. There's a big difference in dietary needs as well as protein needs with those who jog 3-4 times a week with 45% VO2max intensity and professionals who train at 60-85% VO2max intensity 8-40 hours per week.
Considering this differences, the protein intake recommendations for recreational athletes do not exceed the ones for general population and are 1,0 g/kg BM, while recommendations for top athletes go around 1,6 g/kg BM. (10) In cases where a person is trying to decrease amount of fat tissue while engaging in endurance sports, need for protein could be higher, up to 2,0 g/kg BM. (11)
Timing of protein intake is one of the most commonly misinterpreted factors. Regardless of the type of activity, but especially in strength sports. Maybe it's logical, considering the intuitive importance of speedy intake and the interests of supplement industry. Timing is an important factor, but if the daily intake is sufficient, timing is secondary and its importance is blown out of proportion. (12) But that doesn't mean it should be neglected. General recommendation is to not allow more than 5-6 hours without at least 20-30 g of protein. In practice this means at least three main meals, which shouldn't be a problem for an athlete. An exception from this rule is made during the night, because it's more important to get a good night's sleep than foollow the rule. In cases of energy restriction, reducing the amount of fat tissue, timing could play a more important role so I would suggest reducing the time without protein to 3-5 hours.
Protein intake during activity
Protein intake during endurance training/competition is still not researched enough. In theory, it has a triple role, serves as energy source, a signal to boost synthesis and inhibit protein break down. (13)
If the carbohydrate intake before and during the activity is adequate, protein intake won't represent an additional energy source. (14) In other words, it is energetically unnecessary. On the other hand, a high protein intake during an activity could cause gastrointestinal problems so you should be careful.
There are certain indications that in activities longer than 4-5 hours protein intake could have a beneficial effect on recovery and building of muscle tissue, (15), but this data is far from conclusive. Considering everything, for safety sake, I would recommend the intake of 30 g of easily degradable protein (for instance whey) during the activities longer than 3 hours.
Protein quality depends on their amino acidic composition and biological value. Generally speaking, quality of protein from animal sources is greater than the one from plants. Recommendations for intake are based on average intake quality of the population, including animal sources. This means if you're vegetarian or vegan, your needs will be 10-20% higher due to compensation for lower quality. If you eat everything and don't significantly reduce foods from animal sources, protein sources are not something to be bothered with.
1,0 – 1,6 g/kg BM in energetic balance, 1,4 – 2,0 g/kg BM during restriction diet
20-30 g of protein at least every 5-6 hours in balance, every 3-5 hours during restriction diet
30 g of protein during activities longer than 3 hours
(1) von Liebig, Justus Freiherr, and William Gregory. Animal chemistry: or, Organic chemistry in its application to physiology and pathology. John Owen, 1842.
(2) Wolfe, Robert R. "Protein supplements and exercise." The American journal of clinical nutrition 72.2 (2000): 551s-557s.
(3) Jones, D. A., and O. M. Rutherford. "Human muscle strength training: the effects of three different regimens and the nature of the resultant changes." The Journal of physiology 391.1 (1987): 1-11.
(4) Breen, Leigh, et al. "The influence of carbohydrate–protein co‐ingestion following endurance exercise on myofibrillar and mitochondrial protein synthesis." The Journal of physiology 589.16 (2011): 4011-4025.
(5) Tarnopolsky, Mark A., J. Duncan MacDougall, and Stephanie A. Atkinson. "Influence of protein intake and training status on nitrogen balance and lean body mass." Journal of Applied Physiology 64.1 (1988): 187-193.
(6) Smith, Kenneth, and Michael J. Rennie. "The measurement of tissue protein turnover." Baillière's clinical endocrinology and metabolism 10.4 (1996): 469-495.
(7) Phillips, Stuart M., et al. "Gender differences in leucine kinetics and nitrogen balance in endurance athletes." Journal of Applied Physiology 75.5 (1993): 2134-2141.
(8) Tipton, Kevin D. "Protein for adaptations to exercise training." European journal of sport science 8.2 (2008): 107-118.
(9) Humayun, Mohammad A., et al. "Reevaluation of the protein requirement in young men with the indicator amino acid oxidation technique." The American journal of clinical nutrition 86.4 (2007): 995-1002.
(10) Tarnopolsky, Mark. "Protein requirements for endurance athletes." Nutrition 20.7 (2004): 662-668.
(11) Tipton, Kevin D., and Oliver C. Witard. "Protein requirements and recommendations for athletes: relevance of ivory tower arguments for practical recommendations." Clinics in sports medicine 26.1 (2007): 17-36.
(12) Aragon, Alan Albert, and Brad Jon Schoenfeld. "Nutrient timing revisited: is there a post-exercise anabolic window." J Int Soc Sports Nutr 10.1 (2013): 5.
(13) Coyle, Edward F. "Fluid and fuel intake during exercise." Journal of sports sciences 22.1 (2004): 39-55.
(14) Jentjens, Roy LPG, et al. "Addition of protein and amino acids to carbohydrates does not enhance postexercise muscle glycogen synthesis." Journal of Applied Physiology 91.2 (2001): 839-846.
(15) Koopman, René, et al. "Combined ingestion of protein and carbohydrate improves protein balance during ultra-endurance exercise." American Journal of Physiology-Endocrinology and Metabolism 287.4 (2004): E712-E720.