Physiology Friday #217: Do You Need More Protein on a Rest Day Compared to a Training Day?
A 'contrarian' finding to what's often practiced in sports nutrition.
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Protein is the macronutrient du jour.
Everyone’s talking about how important protein is for healthspan and longevity. Children need it to grow, middle-aged adults need it to build muscle before they begin to age, and older adults need it to stave off sarcopenia and frailty.
I’m not sure why protein has become so popular, but I surmise it’s because most big-name health influencers have emphasized it. One of the most popular books in health of the last year — Peter Attia’s “Outlive” — has an entire section dedicated to protein in which he calls it a “performance-enhancing macronutrient.”
I don’t disagree — protein was demonized for too long and we’re now realizing that we can eat a lot of protein with several potential benefits and little repercussion (no, it doesn’t damage your kidneys). In fact “overeating” protein seems to be one of the smartest things to do if you’re trying to lose weight and build muscle.
But how much protein do you actually need?
This is a much-debated question, and there’s no one right answer. Rather, optimal protein needs exist within a broad range, with a majority of your needs depending on your body size and activity level.
Most research indicates that the lowest protein intake for a healthy, sedentary adult should be around 1.2 grams of protein per kilogram of body weight per day. (Side note, Examine.com has an incredibly neat protein intake calculator that you can use to calculate your daily protein requirements).
This is much higher than the RDA of 0.8 grams per kilogram per day (which is, in most expert opinions, highly inadequate).
It’s long been known that athletes have higher protein requirements than the general population. That’s because their overall energy requirements are higher and because they are using up and damaging more muscle proteins during exercise — dietary protein/amino acids are needed to kickstart the repair process and to help build muscle size and strength. Endurance athletes in particular tend to have very high protein requirements, even though runners and cyclist often don’t think much about protein.
For athletes, a protein intake of 1.4 to 2.0 grams of protein per kilogram of body weight is recommended, and for those looking to build lean muscle mass, an intake of 1.6 to 2.2 grams per kilogram per day may be optimal. Even if you’re not an “athlete”, this protein intake appears to be beneficial.
One interesting and applicable question for athletes is how to eat their protein — should you evenly distribute it throughout the week, or should you increase and decrease your protein intake to match your daily training or exercise load?
Intuitively, it makes sense that on a day when I don’t work out, I could probably eat less protein, and vice-versa for a day when I do work out. It also makes sense that I might need more protein on a day that I work out for 2 hours compared to a day when I only work out 1.
The results of a new study published in the journal Applied Physiology, Nutrition, and Metabolism question this “common sense” practice.1
The study aimed to compare estimated whole-body protein requirements on a recovery day, a low-volume exercise day, and a high-volume exercise day.
To do this, the researchers recruited a total of 7 healthy young distance runners. The group had an average age of 28 and an average VO2max of 67 — they were fit. All of the runners reported running at least 40 kilometers (or about 20 miles) per week and had a 5k personal best of 23 minutes or faster.
In a random order, the participants completed three separate metabolic test conditions in the lab:
In one condition, they arrived at the lab in a fasted state and didn’t perform any exercise — this was the rest/recovery day.
In another condition, the participants arrived at the lab, ate a high-carbohydrate breakfast, and then completed a 10-kilometer treadmill run, followed by another high-carb meal. This was the low-volume exercise day.
In the final condition, the participants arrived at the lab, ate a high-carb meal, and then completed a 20-kilometer run, followed by another high-carb meal. This was the high-volume exercise day.
In all conditions after the run (or rest/recovery), the participants consumed a liquid meal every hour for 8 hours that provided them with 7.5 grams of carbohydrates per kilogram of body weight and 0.93 grams of protein per kilogram of body weight.
On all 3 days, the participant's protein requirements were estimated using a method known as the indicator amino acid oxidation method (IAAO). Briefly, the IAAO method is based on the idea that when one amino acid is deficient for protein synthesis, all other amino acids (including the indicator amino acid or the amino acid being measured) will be oxidized. In other words, if a specific amino acid is lacking, the body prioritizes its use for protein synthesis, and the excess of other amino acids gets oxidized. The indicator amino acid is typically labeled with a stable isotope (in the case of this study, that was L-[1-13C]phenylalanine) to track its fate in the body.
By labeling the amino acid phenylalanine and then requiring the participants to consume drinks containing the labeled amino acid, the researchers could track the fate of the amino acid in their breath (F13CO2), how much of the amino acid was being oxidized, and whole-body amino acid (phenylalanine) balance. Phenylalanine excretion (F13CO2) is an inverse proxy for whole-body protein synthesis.
For 2 days before each condition, the participants followed a standardized diet containing 8 grams of carbohydrates per kilogram of body weight per day and 1.4 grams of protein per kilogram of body weight per day. On consecutive days, they performed a 10k run and a 5k training run.
Results
On the recovery day, amino acid excretion (F13CO2) was greater when compared to the 10k and the 20k training day, indicating that whole-body protein requirements are elevated on a rest compared to a training day.
Interestingly, there was no difference in amino acid excretion (F13CO2) between the 10k and the 20k training day. In other words, training volume does not seem to impact whole-body protein requirements.
Phenylalanine oxidation — a measure of the body’s breakdown/utilization of this amino acid — was also greater on the rest day compared to both training days but wasn’t different between the 10k and 20k training days.
Finally, phenylalanine net balance was greater on the 10k training day and the 20k training day compared to the rest day. A higher net protein balance is indicative of a positive (beneficial) protein synthesis. In other words, the dietary protein intake during the study days was more sufficient to meet protein requirements on the training days compared to the rest day.
Like me, you’re probably wondering why protein requirements would be higher on a non-training day. It’s contrary to common sense and common practice.
The answer to this isn’t quite clear, but one explanation could be that a greater protein intake is needed on a recovery day to remodel muscle protein affected by the previous day’s training. The physiological requirement for protein might increase on a non-training day due to a real need for amino acids to build and repair tissues. It could also be due to a suboptimal protein intake on training days.
The latter explanation aligns with my personal experience. Sometimes on days that I train hard, my appetite is low and I might not eat enough to match the day’s training demands. The day after, I tend to be a bit more ravenous than usual — perhaps this is a sign from my body that I’m in need of a bit more fuel to make up the protein deficit incurred the day prior.
Also, the participants performed both of the training runs in a carbohydrate-replete state. So long as you perform exercise with adequate carbohydrate stores to start and don’t exceed your body’s carbohydrate storage capacity during exercise, you probably won’t use that much protein during exercise (more on this later).
The other possibly confusing result from this study is the lack of a difference in protein requirements between the 10k training day and the 20k training day. Despite running double the distance, the runner’s protein requirements were the same on these days. Wouldn’t more exercise mean more protein breakdown?
Not really.
When we look at the actual contribution of amino acids to energy production during endurance exercise, it’s minimal (at least relative to carbohydrates and fat). Only about 2–5% of the energy provided during exercise comes from amino acid oxidation. For a 20k training run that burns about 1,500 calories (for a 75-kilogram person), that means only 7–20 grams of protein (at most) will be oxidized. Take half of that for a 10k run. The difference in protein oxidation between a 10k run and a 20k run is thus a meager 3–10 grams of protein, or about one whole egg.
Of course, none of this is to say that you don’t need to prioritize protein every day, especially if you’re exercising hard.
I don’t think that it makes much sense — unless you’re so obliged — to oscillate your daily protein intakes based on how much you exercise. Finding a protein intake that works for you within that 1.4–2.2 grams per kilogram per day range and eating it consistently each day is likely a fine strategy.
However, based on the results of this study, I might start to slightly increase my protein intake on a day when I’m not exercising to make sure my body fully recovers from the previous several week’s cumulative training load and is ready to start the next cycle.
Finally, although these are interesting findings, remember that the study comprised 7 young healthy male endurance runners. Using these results to make sweeping generalizations about the protein needs of everyone else would be premature, but I think that they can still be used to inform us about dietary strategies for optimal health and performance.
Thanks for reading. See you next Friday.
~Brady~
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Do you know how the calorie content of the high carb breakfast was in comparison to the energy expenditure from the runs? I would be interested in the overall caloric balances in the different conditions (and have no access to the publication).
Useful post. Thanks! Could you extrapolate and say that the day after a very hard training day (say, a 20 mile trail run), protein requirements are even higher on the subsequent rest day? Also, are these results concordant with other studies showing that timing of protein intake post-exercise is less important?