The internet will have you downing a shake before you reach the car park...
...you can shower first, but there is one timing rule that does hold up.
For about twenty years, gym culture swore by a rule that turned out to be wrong. You finished your session, you had 30 minutes to guzzle your protein shake or stuff you mouth with steak and eggs. If you missed that window, your training was wasted.
It's one of those rules where its origins cannot be fully traced. When it was finally tested properly, it didn't hold up. What was found was far more interesting than the myth ever was.
The anabolic window was mostly a measurement problem
The idea came from real physiology as resistance training does sensitise muscle to amino acids and muscle protein synthesis does (MPS) rise sharply after a session. However, it's clearer now that the mistake was assuming that door is only open for a short period of time.
In 2013, Schoenfeld, Aragon and Krieger combined the available trials on protein timing and found that once total daily protein intake was accounted for, the timing effect disappeared [1]. The apparent benefit of the post-workout shake was mostly derived from the benefit of consuming more protein overall. Twenty of the twenty-three studies compared a protein supplement against a placebo without matching daily protein between groups, so those trials could never separate timing from quantity [1,2].
The authors were honest about this. They published the meta-analysis and then argued that the evidence base was too weak to settle the question either way [2]. That's rare in the scientific world. What they were pointing to is that the anabolic window was never disproven, it was never properly tested in the first place.
What we started to know more about was that the sensitised state lasted longer than thirty minutes. Estimates pointed at 24 hours or more after a training session [3]. If you train at 7am in the morning and eat protein at 9am, you have not missed your prime time window.
And the doors didn't close after 12 hours either
The most striking recent finding came in 2023. Trommelen and colleagues gave trained men either 25g or 100g of protein after resistance exercise and tracked it with a quadruple isotope tracer method [4] (this method is super interesting as the cows that produced the milk were fed tagged amino acids, so the protein was traceable before anyone drank it, which let the researchers follow a single meal all the way from ingestion to muscle for twelve hours).
The textbook position had always been that between 20 to 25g maximally stimulates MPS and anything beyond that is oxidised. This didn't hold up in the experiment. The 100g dose produced a larger anabolic response that was still running at twelve hours, with amino acid incorporation into muscle rising almost linearly across the whole period [4].
Empowering Nutrition side bar please: There is a wider lesson here, and it applies to far more than protein. Every study is a decision about where to point the light. Whatever sits outside the spotlight will not turn up in the results, and absence of measurement gets read as absence of effect. Set your window at four hours, you will find a four hour response, and that four hours then risks being applied to the whole picture. The 20g to 25g ceiling never came from muscle. It came from the way the studies were built, and repeated often enough, it runs the risk of becoming a "fact".
This does not mean you should eat 100g of protein in one go! It means the ceiling we were all told about was an artefact of how long a period researchers had been measuring for, not a real limit of the tissue. Muscle can use far more than we thought, over far longer period than we thought.
So why does spreading protein through the day help?
Two well designed studies found that spreading protein evenly beats piling most of it into one or two meals. Areta et al gave trained men (yes, unfortunately men again) 80g of protein over twelve hours as either:
8 x 10g
4 x 20g
2 x 40g
The 4 x 20g pattern produced 31% to 48% more myofibrillar protein synthesis than the other two [5]. Mamerow et al fed healthy adults the same daily protein either evenly across three meals or skewed towards dinner, and found 24 hour MPS was 25% higher on the even pattern [6].
That looks decisive right? But these are acute studies measuring synthesis over hours or a day. When you look at training studies measuring actual muscle gained over months, distribution matters much less, and in several trials not at all [7].
From this we can deduce that distribution matters most when total protein is marginal. If you are eating 1.6g to 2.2g per kg of body weight, spread across three or four meals containing something like 0.4g per kg each meal, you are already in the range where the pattern stops being the limiting factor [7]. If you are eating less than 70g a day and most of it at dinner, distribution should be the least of your problems.
There is also a practical argument that has nothing to do with physiology. People who aim for a protein source at every meal tend to hit their daily target. People who aim only for a daily number tend to miss it. Distribution is a useful behavioural habit even where its metabolic advantage may only be small.
Protein before bed is one of the few timing strategies that holds up
Sleep is the longest fasting period most of us take, and it happens during a period when muscle is repairing. For years nobody checked whether that mattered. Res et al gave men 40g of casein thirty minutes before sleep after an evening training session. Overnight MPS increased by around 22% compared with placebo, and whole body protein balance improved [8]. The protein was digested and absorbed normally during sleep, which was not obvious in advance.
Snijders and colleagues ran a longer version of the study: twelve weeks of resistance training with 27.5g of casein before bed. The supplemented group gained more muscle and more strength than the placebo group [9]. More recent work has extended the finding to endurance athletes, where pre-sleep protein raised both mitochondrial and myofibrillar synthesis rates during overnight recovery [10].
Casein was used in these studies because it digests slowly and drips amino acids through the night, but the effect is not exclusive to casein powder. Greek yoghurt, cottage cheese and milk all deliver the same result. The sweet-spot appears to be around 30g to 40g, taken close to sleep, and the benefit is clearest when your evening meal was several hours earlier [11]. So If you train in the evening and eat dinner around 7pm, a protein source at 10pm could be a great idea.
Women, the first meal, and a finding that deserves more attention
A lot of you will know where I stand on eating something, anything, before an early morning session - particularly us women. So this was the part I really wanted to dig into.
Most of what we know about protein and muscle came from studies on young men. Where women have been tested directly, the headline is reassuring, MPS after resistance exercise in the fed state is not meaningfully different between young men and young women, despite the enormous difference in testosterone [12,13]. Women build muscle through the same machinery, at broadly the same rate relative to their starting point. Simply put - if women train and eat protein, their muscle responds the same way a man's does. She starts from a smaller base, so the end result is smaller in absolute terms, but the process itself is not slower and it is not weaker. Testosterone is not the limiting factor once thought.
However, the picture changes with age. Older women show a blunted synthetic response to both exercise and protein feeding compared with older men [14]. And that for me, points to higher protein intake and more attention to distribution in women during and after menopause, not less.
I came across a 2021 paper that, in my opinion, has not had the attention it deserves. Aoyama et al showed in mice that the muscle's own circadian clock governs how well it responds to protein, and that protein eaten in the early active phase produced more hypertrophy than the same protein eaten later. Breed mice without Bmal1, the gene that keeps the clock running inside the muscle itself, and the timing effect vanished entirely [15].
It's worth noting that the human arm of that study was small and observational, 60 older Japanese women, of whom 18 ate more of their protein at breakfast and 42 ate their protein skewed more towards dinner. The breakfast group had greater skeletal muscle volume and higher grip strength [15].
I want to be very careful here. This was only sixty women, an observational study in one population and the mechanistic work was done in mice. This is not grounds for telling women they must eat protein at breakfast. But it is a plausible mechanism with supporting human data, pointing at something most protein research has ignored completely - the same nutrient may not do the same thing at different times of day. Given how many women skip protein at breakfast entirely, it is worth taking seriously while we wait for better trials.
Protein does not arrive to the party alone
Van Vliet et al gave trained men either whole eggs or the equivalent protein as egg whites after resistance exercise. Whole eggs produced greater muscle protein synthesis, despite the egg whites delivering more leucine into the bloodstream in the early phase [16].
Elliot et al found something similar with milk: whole milk drove greater amino acid uptake across the leg of study participants than skimmed milk matched either for protein or for calories [17].
I've mentioned this many times before in my blogs, that the way whole-foods are delivered makes a difference - it's often referred to as the "food matrix". Protein eaten inside a whole-food comes packaged with fat, micronutrients, phospholipids (outer layer of a cell) and other components, and that package appears to do something an isolated protein does not. We do not fully understand the mechanism. It may involve digestion rate, insulin response, or membrane phospholipids supporting the synthetic machinery itself. The takeaway is isolated protein powders are convenient and they work, but they are not superior to food and the evidence shows they may be slightly inferior. So stop throwing the yolk away!
Sweeteners - the constant conversation starter
This is where I want to be precise as I get many, many questions about sugar versus sweetener - which is good, bad or neutral? It's not straight forward and my answer is always "it depends and in what context?".
Most flavoured protein powders contain non-nutritive sweeteners: sucralose, acesulfame potassium, aspartame, or sugar alcohols such as sorbitol, xylitol and maltitol. There are three separate questions and they have different answers.
Do sweeteners disturb the gut?
There is reasonable evidence that some of them alter the gut microbiome. Suez and colleagues showed that saccharin induced glucose intolerance in mice and in a subset of human volunteers, mediated by changes in gut bacteria [18]. Sucralose and acesulfame potassium have both been shown to shift microbial composition in animal studies, but with more variable results in humans [19]. In cell models, sucralose and aspartame increased intestinal permeability (sometimes referred to as "leaky gut") by disrupting tight junction proteins [20]. The human evidence is generally weak and responses vary a lot between individuals, but it's worth keeping an eye on.
Do they trigger IBS type symptoms?
Here the evidence is stronger and the mechanism is simpler. Sugar alcohols are polyols, (the "P" in FODMAP). They ferment rapidly in the colon and draw water into the bowel, which produces gas, bloating and loose stools in susceptible people [21]. Whey protein concentrate also retains lactose, which can be a problem for anyone who is lactose intolerant. Whey isolate contains far less.
This matters more for athletes than for the general population. Exercise induced gastrointestinal symptoms affect somewhere between 30% and 90% of endurance athletes, driven by reduced blood flow to the gut, impaired barrier function (a leakier gut wall) and altered motility (changing transit speed) during hard training [22,23]. Prevalence is higher in younger athletes and in women [23]. If your gut is already under stress from training, adding a large dose of polyols may be asking for trouble.
Do they impair protein absorption?
I could not find evidence that sweeteners interfere with amino acid transport or protein digestion. The theoretical route would be indirect, through barrier disruption and inflammation, but that remains unproven in humans [20]. If your protein shake makes you bloated and sends you to the bathroom, don't worry about the lost protein. The issue you would want to uderstand better is how your gut handles the protein or the other ingredients.
If you tolerate your protein powder, carry on. If you get bloating, gas or urgency after shakes, try an unflavoured or minimally sweetened isolate before concluding you cannot tolerate whey. The protein is rarely the issue, it could be whats added to make it taste like cookies and cream!
OK Natalie, what does this mean?
Total daily protein is the thing that matters most and aim for 1.6g to 2.2 g per kg of body weight if you are training seriously. Try and spread it across three or four meals of roughly 0.3 to 0.4 g per kg of your body weight. Not because missing a window costs you anything, but because the daily total is what counts, and three or four solid servings is the simplest way to reach it. And that matters most if your intake is on the low side.
Have a protein source before bed if you train in the evening or if your dinner is eaten early. This is the timing strategy with the best supporting evidence. AND eat protein at breakfast, particularly if you are a woman. Get most of it from whole foods. Use powder to fill gaps, not as the foundation.
So you can stop running out the gym to consume your protein, turns out the window is not thirty minutes, it's most of the day!
Want to find out more?
At Empowering Nutrition I look at the whole picture. Macronutrients and micronutrients, yes, but also what is happening in your gut, including microbiome testing where it is likely to tell us something useful. That is often where the answer is hiding when your training is going well, your diet looks right on paper, and you still feel bloated, uncomfortable or flat.
If that sounds like you, get in touch.
References:
Schoenfeld BJ, Aragon AA, Krieger JW. The effect of protein timing on muscle strength and hypertrophy: a meta-analysis. Journal of the International Society of Sports Nutrition. 2013;10:53.
Aragon AA, Schoenfeld BJ. Evidence inconclusive: comment and response on protein timing. JISSN. 2016.
Aragon AA, Schoenfeld BJ. Nutrient timing revisited: is there a post-exercise anabolic window? JISSN. 2013;10:5.
Trommelen J, van Lieshout GAA, Nyakayiru J, et al. The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Reports Medicine. 2023;4(12).
Areta JL, Burke LM, Ross ML, et al. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. The Journal of Physiology. 2013;591(9):2319-2331.
Mamerow MM, Mettler JA, English KL, et al. Dietary protein distribution positively influences 24-h muscle protein synthesis in healthy adults. The Journal of Nutrition. 2014;144(6):876-880.
Witard OC, Mettler S. Protein intake distribution: beneficial, detrimental, or inconsequential for muscle anabolism? IJSNEM. 2024;34(5).
Res PT, Groen B, Pennings B, et al. Protein ingestion before sleep improves postexercise overnight recovery. Medicine and Science in Sports and Exercise. 2012;44(8):1560-1569.
Snijders T, Res PT, Smeets JSJ, et al. Protein ingestion before sleep increases muscle mass and strength gains during prolonged resistance-type exercise training in healthy young men. The Journal of Nutrition. 2015;145(6):1178-1184.
Pre-sleep protein ingestion increases mitochondrial protein synthesis rates during overnight recovery from endurance exercise: a randomized controlled trial. 2023.
Trommelen J, van Loon LJC. Pre-sleep protein ingestion to improve the skeletal muscle adaptive response to exercise training. Nutrients. 2016;8(12):763.
West DWD, Burd NA, Churchward-Venne TA, et al. Sex-based comparisons of myofibrillar protein synthesis after resistance exercise in the fed state. Journal of Applied Physiology. 2012;112(11):1805-1813.
Landen S, Jacques M, Hiam D, et al. Physiological and molecular sex differences in human skeletal muscle in response to exercise training. The Journal of Physiology. 2023.
Sex-based differences in muscle protein turnover and metabolism in aging. Sports Science Exchange, GSSI.
Aoyama S, Kim HK, Tanaka M, et al. Distribution of dietary protein intake in daily meals influences skeletal muscle hypertrophy via the muscle clock. Cell Reports. 2021;36(1):109336.
van Vliet S, Shy EL, Abou Sawan S, et al. Consumption of whole eggs promotes greater stimulation of postexercise muscle protein synthesis than consumption of isonitrogenous amounts of egg whites in young men. AJCN. 2017;106(6):1401-1412.
Elliot TA, Cree MG, Sanford AP, et al. Milk ingestion stimulates net muscle protein synthesis following resistance exercise. MSSE. 2006;38(4):667-674.
Suez J, Korem T, Zeevi D, et al. Artificial sweeteners induce glucose intolerance by altering the gut microbiota. Nature. 2014;514(7521):181-186.
Effects of sweeteners on the gut microbiota: a review of experimental studies and clinical trials. Advances in Nutrition.
Shil A, Chichger H. Artificial sweeteners disrupt tight junctions and barrier function in the intestinal epithelium through activation of the sweet taste receptor, T1R3.
Monash University FODMAP team. Protein powders and IBS.
Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome. Alimentary Pharmacology and Therapeutics. 2017;46(3):246-265.
Lis DM, Kings D, Larson-Meyer DE. Dietary restrictions in endurance runners to mitigate exercise-induced gastrointestinal symptoms. JISSN. 2020.
Comments