Protein - 0.8g/kg Insufficient for Female Athletes, Too. Plus: Revised RDA is 1.71-2.2 g/kg/d Depending on Sport & Sex

With average daily protein intakes of only 90±24 g (Gillen 2017), many female athletes are probably missing the sweet spot of ~1.7g/kg/d.
I guess I am preaching to the choir when I write that the recommended daily allowance (the magical "RDA") for protein is laughable; the number of studies which show that an increased protein intake will have beneficial metabolic effects is increasing by the day; the subjects of these studies are men and women, kids, teens, tweens, adults, and grandparents, perfectly healthy or sick, lean or fat... and in all of these increasing the protein intake to ~2x the RDA of 0.8g/kg has been found to be associated with significant beneficial effects on body composition and/or health in the majority of the studies (learn more).
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Unfortunately, humans lack a measuring gauge that would allow us to decide when we've satisfied our daily protein requirements and/or consumed the optimal amount of protein. I guess this is also why we don't even know for sure how exercise will affect our protein need. Yeah, we do have plenty of evidence that extra protein will augment size and strength gains in resistance training and, with the publication of pertinent studies, even endurance trainees are now jumping aboard the "protein supplementation wagon" (learn more).

Stuart M. Philips and colleagues are among those researchers who have been dabbling with the idea of increased protein requirements in athletes and non-athletes, young and old, sick and diseased for years (Phillips 2007; Phillips 2017). As the Canadian scientists point out in one of their latest reviews (Phillips 2017), there are two main methods to measure human beings protein balance. There is...
  • measurement of an individual's nitrogen balance - This method has been used for more than 60 years, despite its various shortcomings (Young 1986); and it is also the methodology that underpins the "RDA", a term of which Phillips writes in his review in Front Nutr. in 2017 that it would "inherently imply that the protein intake is 'recommended,' and it is all that you are 'allowed' to eat" (Phillips 2017). Now, I don't have to quote an expert for you to understand that this is, "[o]f course, [...] strictly speaking [not] true" (ibid.). 
  • the more recent alternatives are the indicator amino acid oxidation (IAAO) method and the 24h-indicator amino acid oxidation and balance method (24h-IAAO/IAAB)- Having been explicitly developed as an alternative to the nitrogen balance (free reviews can be found in Elango et al. 2008 & 2012), they yield significantly higher values for optimal protein intakes in than nitrogen balance studies, especially in older individuals.
Even though, I believe that this is the first time some of you read about this, the above ain't 'news' in the strict sense of the word and wouldn't have made it into this article, if it wasn't necessary to know that the IAAO method has recently been applied by Wooding et al. (2017) to determine a dietary protein requirement in active females performing variable-intensity intermittent exercise using the indicator amino acid oxidation (IAAO) method.
If we go by the Institute of Medicine's AMDR concept, 0.8g/kg is the absolute minimum, not the recommended protein intake.
What is currently recommended? You already know that the RDA, i.e. the "recommended daily allowance" for protein is 0.8g/kg. Despite its name, it's yet not the RDA, but the AMDR, i.e. the Acceptable Macronutrient Distribution Range that defines the "range of intakes for a particular energy source that is associated with reduced risk of chronic diseases while providing adequate intakes of essential nutrients" (Institute of Medicine 2005). The AMDR says that 10–35% of one's total energy intake should come from protein. As Phillips calculates in the previously cited study, this means that a 55-year-old man who is 1.80 m, weighing 80 kg, would not have to consume the laughable 64 g protein/day the RDA would recommend, but rather 65 g/day as an absolute minimum (that's 10% of the total energy intake and thus the lower AMDR).

Optimal intakes for our exemplary 55-year-old man, on the other hand, would be somewhere within the AMDR of 65–228 g protein/day (assuming an energy requirement of 2,600 kcal/day or 10.9 MJ/day) - in other words, the "optimal" range, i.e. the range where the Institute of Medicine says that it takes "into account the trends related to decreased risk of disease identified in epidemiological and clinical studies" is, as Phillips highlights in his previously cited review from 2017 "well above the RDA (up to 2.8 g/kg/day) that are associated with good health".
The scientists from the University of Toronto started their study with the hypothesis that these requirements would be greater than current IAAO-derived estimates in non-active adult males. And indeed, even though the IAAO method has been around for a couple of years, there's still a paucity of evidence for various populations - including female athletes participating in team sports.

We do already know that endurance athletes need more, much more ...

As Wooding et al. point out, their "group previously used the minimally-invasive indicator amino acid oxidation (IAAO) method to demonstrate that protein requirements in endurance athletes are ~50% greater than sedentary individuals" (Wooding 2017). That, however, is still a pretty conservative estimate, as it marks the protein requirements, not the optimal intakes that will "maximises whole body protein synthesis" and thus enhance recovery from, and potentially adaptation to, an exercise stimulus" (ibid.) - a value the scientists sought to determine in their latest study which involved six healthy, active young adult females (see Table 1).
Figure 1: Modified LIST in which participants followed an audio prompt to complete the variable-intensity intermittent exercise pattern 10 times per block. There were four blocks separated by 5-min breaks, totaling 75-min of exercise plus a 5-min warm-up and cool-down at a self-selected pace (Wooding 2017).
Four individuals were varsity athletes (three rowing, one ice hockey), one was a national level volleyball player, and one was a highly active recreational athlete.
"Participants were required tobe healthy (PAR-Q+), have habitual activity levels of more than 45 min/d on 5 d/wk of moderate vigorous physical activity (I-PAQ for adults aged 15-69), a predicted VO2max ≥44.6 mL O2/kg/min (Leger Multistage Fitness Test), no current use of hormonal contraceptives, and a predictable menstrual cycle (25-33d) during the previous year as determined by interviews and the participants’ records of the 2-3 months prior to study enrollment" (Wooding 2017).
Each participant completed 5-7 metabolic trials (n = 36) during the predicted luteal phase. The women consumed a 2-d adaptation diet containing 1.2 g/kg/d of protein prior to each trial. Wooding et al. chose this intake "in order to minimize metabolic variability during the trial day" and to "provide a level that was previously determined to be sufficient for non-exercising males by IAAO" (ibid.) (15). These adaptation diets contained sufficient energy to meet the subjects' individual habitual daily caloric expenditure of ~2700kcal/d.
Figure from an exemplary study by Kriengsinyos et al. (2002),
How does this strange NaH13CO2 + tracer amino acid test work? Let's take a look at the exemplary data from Kriengsinyos et al. (2002) that has been reprinted in Elango's previously cited 2008 review. Kriengsinyos et al. investigated the effect of lysine intake on the production of 13CO2 from the oxidation of L-[1-13C] phenylalanine (F13CO2) when tracer was infused either i.v. or orally in healthy adult humans (note: one of the important results of the study at hand is that the estimated protein/amino acid requirements don't differ for i.v. vs. oral administration).

As Elango et al. explain in their 2012 paper, "the IAAO technique is based on the concept that when one indispensable amino acid (IDAA) is deficient for protein synthesis, then all other amino acids including the indicator amino acid (another IDAA, usually L-[1-13C]phenylalanine) are in excess and are therefore oxidized" (Elango 2012). That's logical, after all, dietary protein can only either be stored or oxidized. If a rate limiting amino acid is missing, storage is not an option and oxidation will take place. As the intake of said limiting amino acid increases, the oxidation of the indicator amino acid will decrease -  a decrease that reflects the increasing incorporation of amino acids into protein. Yet, once the requirement is met for the limiting amino acid, there will be no further change in the oxidation of the indicator amino acid with increasing intake of the test amino acid (see breaking point in the figure). The inflection point where the oxidation of the indicator amino acid stops decreasing and reaches a plateau is referred to as the ‘breakpoint’. The breakpoint, identified with the use of two-phase linear regression analysis, will then "indicate the estimated average requirement (EAR) of the limiting (test) amino acid" (Elango 2012) - in the example from Kriengsinyos et al. (2002), the breakpoint for lysine is 36mg/kg/d.
On test days the participants reported fasting at the lab, ingested a CHO sports drink and performed a modified version of the Loughborough Intermittent Shuttle Test (LIST; cf. Figure 1), an exercise protocol that simulates the activity pattern of a soccer match:
"Briefly, the test involved four 15-min blocks in which a 17-m variable-intensity shuttle pattern was repeated 10 times with 5-min rests between each block. The entire test was 75 min in length, which also included a 5-min warm-up and cool-down at a self-selected pace. The running pace in our study was based on the percentage of average maximum speed obtained by the entire group of participants during the aerobic assessment (3.45m/s) instead of the percentage of individual VO2max. In this way, the same audio prompt was used for all participants with the following 17-m shuttle paces: 1.7m/s (walk), 2.1m/s (jog) 3.1m/s (run), and an all-out sprint. The average energy expenditure during the LIST was 8.7 ± 0.2 kcal/kg (range 8.5-9.1kcal/kg, determined from accelerometer)" (Wooding 2017). 
Following exercise, participants consumed their first of eight hourly mixed meals containing the test protein intake (0.2-2.66 g/kg/d) and sufficient energy and CHO content in the form of test beverages containing crystalline amino acids, protein-free powder, fruit flavoring powder, grapeseed oil, and maltodextrin as well as protein-free cookies. The actual measurement was done by including a priming dose of NaH13CO2 (0.176 mg/kg) and L-[1-13C]phenylalanine (1.86 mg/kg) in every hourly meal beginning at meal #5. The turnover of this tracer was determined from urinary [13C]phenylalanine enrichment.
Figure 2: Relationship between protein intake and F13CO2. Six participants completed 5-7 metabolic trials each (n=36). F13CO2 breakpoint was used to estimate protein requirements and recommended daily allowance (Wooding 2017).
The breaking point (compare explanation in the blue box above) was determined by analyzing the subjects' breath for F13CO2. As you can see in Figure 2, the F13CO2 curve displayed the expected robust bi-phase response (R² = 0.66) and points, as the author's point out, "to an estimated average requirement (EAR) of 1.41 g/kg/d and an RDA of 1.71 g/kg/d" (Wooding 2017).
Overview of new, IAAO based protein intake minimums and recommendations in different groups of athletes; the figures across the bar denote the values in units of the current RDA of 0.8  g protein /kg/d (Wooding 2017 women in team sports; Kato 2016 male endurance athletes; Bandegan 2017  bodybuilding men)
Bottom line: The study at hands provides further evidence that the well-known and repeatedly cited population guidelines which are based on net protein breakdown analyses are inadequate when it comes to the protein requirements of the athletic population, for whom contemporary sports science consensus statements suggest a broad range of protein intakes from 1.2-2.0 g/kg/d should be consumed by athletic populations (Thomas 2016).

What the study adds to the existing evidence are definite numbers for the minimal and recommended protein intake of female team-sports athletes, a subject group that is often overlooked in clinical research.

More specifically, Wooding's data reveals that an acute bout of variable-intensity exercise results in an increase of young women's protein requirements to 1.41 g/kg/d and an approximate RDA, i.e. an intake that can be expected to be sufficient for 95% of the population, of 1.71 g/kg/d. As previously hinted at, this result is consistent with the scientists' previous observations in endurance-trained males on a day in which they trained (Kato 2016) and in bodybuilders on a rested day (Bandegan 2017) - all numbers you know from the SuppVersity Facebook or Twitter News | Comment!
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  • Elango, Rajavel, Ronald O. Ball, and Paul B. Pencharz. "Recent advances in determining protein and amino acid requirements in humans." British Journal of Nutrition 108.S2 (2012): S22-S30.
  • Gillen, Jenna B., et al. "Dietary protein intake and distribution patterns of well-trained dutch athletes." International journal of sport nutrition and exercise metabolism 27.2 (2017): 105-114.
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  • Phillips, Stuart M., Daniel R. Moore, and Jason E. Tang. "A critical examination of dietary protein requirements, benefits, and excesses in athletes." International journal of sport nutrition and exercise metabolism 17.s1 (2007): S58-S76.
  • Phillips, Stuart M. "Current Concepts and Unresolved Questions in Dietary Protein Requirements and Supplements in Adults." Frontiers in Nutrition 4 (2017).
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  • Wooding, Denise J., et al. "Increased Protein Requirements in Female Athletes after Variable-Intensity Exercise." Medicine and science in sports and exercise (2017).
  • Young, Vernon R. "Nutritional balance studies: indicators of human requirements or of adaptive mechanisms?." The Journal of nutrition 116.4 (1986): 700-703.
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