The answer to this question, in short, is: “It depends,” but before we dive deeper into that, let’s first review why we need protein and what the recommendations are.
Dietary protein has many functions in the body with regulation of skeletal muscle mass being a primary role. Skeletal muscle is constantly evolving with a balance between breakdown (muscle protein breakdown [MPB]) and regeneration (muscle protein synthesis [MPS]). Maintaining adequate protein intake is paramount to ensure that the rate of MPS is at least equal to the rate of MPB, and muscle mass is maintained. With the breakdown effects of training, this can add to the complexity regarding timing, amounts, and day to day intake. The current recommendation for baseline intake of sedentary men is 0.8 g/kg/day and for women is ~1.0g/kg/day. But note, this is the minimum amount needed in sedentary individuals, and exercise, sex, and age can all influence protein needs.
Endurance exercise increases protein oxidation (as fuel during exercise) and therefore total daily requirements, which is also influenced by the training load of the day. A Canadian study in 2019 looked at the protein requirements from daily activity and training in male triathletes and determined the baseline intake to maintain protein metabolism and exercise performance is closer to 2.0g/kg/day. Although less is known in female athletes, we do know that the elevation of progesterone in the luteal phase increases protein oxidation during exercise and at rest, increasing the daily needs at baseline to 1.6g/kg/day in the follicular (low hormone) phase and 1.8-2.0g/kg/day in the luteal (high hormone) phase.
Protein intake with calorie deficit with the goal of losing body fat while building or maintaining lean mass is another consideration. A recent review paper reported that high levels of dietary protein in women supports the maintenance of muscle mass and whole-body protein metabolism during caloric deficit, although these benefits are diminished with increased deficit (e.g. it does not help if a woman is in low energy availability). In men who follow a low carbohydrate diet, protein needs increase due to a substrate switch during exercise: the body becomes more reliant on fatty acids and amino acids (greater protein oxidation) therefore there’s a reduced availability of amino acids for muscle protein synthesis post-exercise.
So, returning to the original question of how much protein does a triathlete need? This will vary based on whether they are male or female, as well as their age and how much training they are doing. Other important factors include: total calorie intake across the day as well as energy availability (are you in a low energy availability state or eating adequately? Is there a purposeful calorie deficit taking place?). With all of these factors in mind, men who eat a well balanced, nutrient dense diet should look to eat approximately 2g/kg/day, while for women the range goes from 1.6 to 2g/kg/day, depending on the menstrual cycle phase they are in.