In my last blog, I wrote about the often talked about "anabolic window" and how overall protein consumption is more important than timing of protein consumption.
I'm sure all of us have heard someone say at some point in time, "You can only digest 'X' grams of protein at a time!" The common sentiment here seems to be around 30g. This is, of course, is not only a total misunderstanding of human physiology, but a huge generalization as well.
To discuss this and understand the process behind nutrient digestion/absorption, let's first differentiate exactly what we are discussing here, as well as give a sort of TL;DR for anyone who doesn't wish to proceed any further:
How much protein can someone eat in a meal?
Well, that is dependent on the individual and how much they generally eat at any given time.
How much protein can we absorb at any given time?
It turns out, our small intestines can absorb about 5-10g of protein per hour. This is a rate-limiting step (explained later), but is also dependent on what you eat along with your protein.
How much protein can the body utilize?
Our bodies are like construction sites. Tissues are being broken down and replaced constantly. Exercise and resistance training can exacerbate this issue, causing muscle protein synthesis (MPS) to last anywhere from 24-36 hours. MPS will actually double in rate at the 24-hour mark, post-exercise, and then slowly return to normal levels at the 36 hour mark.
So, if you were to resistance train even just 3-4 days per week, your body would be at an elevated level of MPS for much of your time through a week. This is why it is ideal to try and consume protein at all times of the day, although, as we will discover, the amount of protein per meal should not be limited. Food takes an average of 6-8 hours to pass through your small intestines, so there is plenty of time for protein to be absorbed, even at a rate of 5-10g/hour.
How our bodies digest food
Humans eat food. It is required to live. We eat foods that consist of fats, proteins, carbohydrates, as well as a wide range of micronutrients such as minerals and vitamins. We put food in our mouths, chew them up (or drink them). Food reaches our stomach, where it bathes in acid and becomes an indistinguishable ball of stuff called "chyme."
At this point (in your stomach), your body doesn't care what you ate, the stomach has one job, to break everything down into a liquid paste to push out to the intestines where it is carried along (through a process called "peristalsis"), absorbing nutrients through the intestinal walls via microvilli.
As proteins are transported through the intestines, they must be absorbed and transported by, well, transporters. The transporters are the rate-limiting step in the absorption of protein.
To make a simple analogy, there are only so many taxi cabs in the city of New York. If you have 50 people waiting for a taxi, but only 10 taxi cabs, only 10 people will get a taxi right away. The other people will eventually get a taxi, it will just take longer to transport them. The taxis (protein transports) need to go deliver their passengers and then return to pick up more customers.
The rate of uptake fluctuates between 5-10g of protein per hour depending on the source. So, if you ingest 50g of protein in a meal, only 5-10g of that protein will be absorbed within the first hour, but that doesn't mean the 40-45g of remaining protein will be disposed of as waste.
How long does protein stay in the intestines?
Well, based on what we know so far, protein is absorbed through the intestinal walls through a rate-limiting step of 5-10g/hour and the transit time through the small intestine is roughly 6-8 hours. So, assuming you don't eat and then immediately poop, then your food will take several hours to progress through your digestive system, absorbing as many nutrients as possible along the way.
Although it is not an ideal way to calculate the absorption rate, we can get a general range of how much protein the body can absorb from a single meal. We know that our bodies absorb 5-10g of protein per hour, and we know that food takes 6-8 hours to travel through the small intestine. That gives us a range anywhere from 30-80g of protein absorption from one meal.
What's fascinating is that the body releases a hormone called cholecystokinin (CCK), when dietary protein is present, that actually slows down peristalsis (the movement of food through your digestive system). This allows a greater amount of digestive time for the body to absorb as much protein as it can from the ingested food (90-95% yield). (Sources: here, here, here, and here)
There are other factors that slow or quicken the transit time of food from your mouth to your toilet bowl, such as what you ate, how much you ate, how hydrated you are, etc. etc. So, if you eat a really greasy meal that upsets your stomach, you may not maximize your protein absorption by the time the food transits to the toilet bowl.
So how much protein can I eat in a single sitting/meal?
So now we know that the body not only self-regulates protein digestion (through CCK), but it also self-regulates protein absorption (available transports). So how much protein can someone safely eat in one meal? Well, it should be noted that no study has ever been done to observe the maximum amount of protein someone could safely eat at any one time (pfft, ethics!).
There was a study done where women (average lean mass: 90 lbs.) consumed 54g of protein either in one meal, or spread out over 4 meals, with no observable differences in protein retention. (source) You could then speculate that even more protein could be consumed in one sitting for someone of a larger size.
If you skipped a lot of the reading above, or maybe misunderstood some of the science talk, let's summarize what we know quickly and efficiently:
The food we eat is digested and broken down into smaller "parts" in our stomach so that it can be absorbed and utilized by the body via the intestines. The speed at which the food passes through the intestines is slowed by the body itself so that as many of the macro and micro nutrients can be utilized as possible. The amount of protein you eat in one meal should not be a point of concern. Your body will not take anything over 30g and expel it as waste.
Eating food is enjoyable. If you wish to spread your meals out throughout the day, go for it! If, however, you lead a busy lifestyle and only have a small amount of time (eating window) to get your calories/macronutrients in, don't sweat it. Just because you eat a big dinner with lots of protein does not mean your body is going to miss out on a lot of those valuable nutrients.
Common questions or misconceptions:
Too much protein will be filtered by the liver and excreted
Wrong. The liver does not filter protein out of your blood. The liver may convert amino acids (digested proteins) into other usable forms of energy (such as glycogen) if more protein is available than needed for tissue repair. Amino acids have 3 potential uses:
Used for tissue repair.
Modified (via the liver) for glycogen synthesis.
Modified (via the liver) for fat (adipose) synthesis.
Unused protein will be peed out
The kidneys take in blood solutes through a network of nephron tubules and capillaries called the glomerulus. The glomerulus has a glomerular capillary filtration barrier that allows less than 150 mg/L of protein to pass through over each 24 hour span. That may sound like a lot, but both kidneys combined produce an average of only 1.5L of urine per day; that means that non-pathological kidneys will only allow ~225mg (two tenths of a gram!) of protein to pass into the urine.
Moreover, typically the only proteins
that are even small enough to pass through are albumins (blood plasma proteins), which are some of the smallest blood protein solutes. Proteinuria is almost a guarantee of either glomerular damage or renal tubule defects; in other words, kidney damage.
Protein is digested and either absorbed through the intestines for use or passed on to the colon to be fermented and excreted as waste (poop). The fermentation of protein stinks, leading to either smelly stools or those lovely "protein farts."
Tyler Robbins B.Sc. CSCS
Director of Fitness
Head of CrossFit