A study from the University of Toronto Faculty of Kinesiology and Physical Education (KPE) has found that endurance athletes had increased protein requirements compared to non-active populations, and that these requirements are very similar between male and female endurance athletes.
The study, recently published in the Medicine and Science in Sports and Exercise flagship journal for the American College of Sports Medicine, was part of a doctoral thesis by Eric Williamson, who did his research in the Iovate/Muscletech Metabolism & Sports Science lab at KPE under the supervision of Associate Professor Daniel Moore, acting associate dean of research.
“Endurance athletes typically have very high training loads, which means increased stress on their bodies,” says Moore. “They use the protein in their bodies to adapt to this stress, but they also use it for energy, which means it needs to be replaced after exercise to help them recover and adapt to their training.
“As with many athletic populations, the requirement for this essential nutrient has been reported to be higher in endurance athletes compared to inactive populations. However, its important to note that almost all past research into daily protein requirements of endurance athletes has been conducted in male athletes.
“Our research tried to understand how much higher the protein requirements are for both males and female endurance athletes using noninvasive stable isotope methodology, which lets us track how the body is using the dietary amino acid building blocks of proteins to build new proteins in our body.”
Eight males and seven females in the midluteal phase (second half) of their menstrual cycle participated in the study. After two days of a controlled diet consisting of 1.4 grams of protein per kilogram of body mass per day and 10 and five km runs per day, respectively, participants completed a 20-km run before applying an at-home indicator amino acid oxidation (IAAO) test to determine their protein requirements.
A follow up analysis by the researchers showed no major differences between the sexes with the recommended intake estimated to be 1.81 and 1.89 grams per kilogram of body mass per day for males and females, respectively.
“Our findings indicate that endurance athletes consuming a daily protein intake toward the upper end of current consensus recommendations (1.85 grams per kilogram per day) will maximize whole-body protein synthesis (the process in which cells make proteins) during post-exercise recovery regardless of sex,” says Moore.
This is an important finding because women only represent about 30 per cent of the participants in exercise science research despite making up at least half, if not more, of the active population.
“We wanted to try and rectify this imbalance, but also to provide very relevant nutritional information on an essential macronutrient,” says Moore. ““This gets us a little bit closer to be able to say that it doesn't really matter if you're male or female – if you’re doing roughly the same training, the protein requirements are going to be roughly similar.”
But there is a caveat.
Initially, the researchers hypothesized that the female athletes would need less protein because high levels of estrogen produce what's known as the protein sparing effect. High estrogen means the body is better able to use fat as an energy source, so it burns less protein. And, when you’re burning less protein, you don't have to eat as much to replace it.
The researchers studied the females in the luteal phase of the menstrual cycle, which has a high estrogen concentration in the blood, but also a high progesterone concentration. This is the phase when female protein requirements would be most similar to males. In the mid follicular (first half) phase of the menstrual cycle, the level of estrogen is a little bit lower than in the luteal phase but progesterone is very low, which skews the ratio in favour of estrogen.
“So, if we had studied female athletes in the mid follicular phase, we might have seen that their protein requirements are lower than males because they would have had the protein sparing benefit of the estrogen,” says Moore. “But, if they ate according to the recommendations from our study, we hypothesize that the protein requirements would balance out across all of the phases.”
This points to the need to study protein requirements at different phases of the menstrual cycle when the ratios between estrogen and progesterone are different, but equally important, according to Moore, would be to look into the effect of oral contraceptives on protein metabolism.
“This study is the first step in many to get to the point where we're able to tailor protein requirements to the female athlete across a broad range of female biology - or be able to come to a point where we don't have to worry about sex differences because we can see that exercise nutrition can override them,” says Moore.
This is important because self-reported dietary intakes of female athletes seem to indicate that they are not meeting their protein requirements, which they need in order to support their training, recovery and performance goals.
“We need to better understand what is the true protein intake of these athletes and what are the consequences of not meeting this requirement – over the short term and long term. How does it affect their overall health or performance?”
In addition to Williamson and Moore, Hugo J W Fung and Carolyn Adams, both graduate students at the faculty, and Daniel W D West, who is cross-appointed to the faculty from Toronto Rehabilitation Institute, also contributed to the study.