Early last week, I received an email from a reader. The subject was protein absorption, and it referred to something I’d written several weeks past in this post  on Leptin Reset among other things. I had suggested that your body can only deal with about thirty grams of protein in one sitting. I immediately realized that this statement wasn’t nuanced enough and might give the wrong impression, so I explained what I meant in a bit more detail in the comment board . After receiving the following email (thanks for keeping me honest, Brandon!) I figured I would revisit this topic and further articulate what we know about the human body and dietary protein utilization.
I had a question about protein intake. In an older post, you mentioned something about anything beyond 30 grams of protein in a single meal likely being turned into glucose or fat, but I’ve heard different things from others. So, how much protein can you really use in one meal?
First off, “meal” is the key word, one that I omitted in my original statement. Eating 50 grams of straight protein – and protein alone, whether it’s whey protein  or 99% fat-free lean sirloin – is probably going to have a different effect than eating a mixed meal which contains 50-some odd grams of protein in addition to fat and digestible, fermentable , and insoluble carbohydrates. In other words, protein is rarely eaten alone. It’s eaten atop salads, alongside veggies, with starches, and it often comes imbued with animal fat  (as meat). It’s eaten as a meal.
Digestion takes a long time, and it’s not a segmented procession of different meals through the gastrointestinal tract. Food isn’t separated into “meals” in your stomach. It’s just all food, all mashed together. If you still have breakfast in your stomach when your lunch enters the picture, lunch and breakfast will meet and mingle. Stomach acid breaks the food down into a big semifluid mass of partially digested food components, water, digestive enzymes, and hydrochloric acid – all referred to as chyme . When the chyme is “ready,” it’s pushed through the duodenum (where nutrient extraction begins) and into the intestines.
Once the protein part of the chyme has been broken down by enzymes into amino acids, amino acid absorption by the intestines can begin. Amino acid transporters  grab amino acids and transport them through the cells lining the intestinal wall and from there into the bloodstream to be delivered to other parts of the body. But because the pool of amino acid transporters is limited, amino acids are typically absorbed by the small intestine at a rate of 5-10 grams per hour. This would suggest, at first glance, that the original “30 grams of protein per sitting” figure is close to correct. If you can only absorb, say, 7 grams of protein per hour, and the bolus of digested protein and other foods takes 4 to 5 hours  to wind its way through the intestine, you’re gonna absorb maybe 28-35 grams. Right?
Not exactly. It turns out that our digestive process is fairly fine-tuned and regulated by the composition of whatever’s being digested. The presence of protein in chyme actually causes the secretion of CCK , a hormone that slows down the intestinal contractions  that move food along, thus giving the protein more time to be transported by amino acid transporters. So, while it’s technically true that we aren’t absorbing 30 grams (let alone more) of protein all at once, it will eventually be absorbed.
Clearly, a fair amount is directed to normal tissue growth and repair. If it’s much more than that, a few possibilities enter the picture. Some will be directed towards a short term storage option referred to as the “labile protein reserve.” In this case, skeletal muscle tissue will take up some of the excess amino acids and simply store then for possible near future use (like within a day or so or for emergencies). Beyond that, your body will want to oxidize excess amino acids directly for fuel or convert them to glucose in the liver via gluconeogenesis. It will convert the nitrogen from the amino acids into ammonia and you will excrete that in your urine. If levels get too high in the bloodstream, your pH will shift (more acidic) and calcium will be called upon to balance pH out again. As we have seen with some people going full Primal, larger intakes of protein can offset lowered carb intake by generating significant rises in glucose through gluconeogenesis. Indeed, there are a lot of options the body has when you eat a high protein meal.
I still wouldn’t “stuff” myself on protein. If you have to force feed that lean chicken breast, perhaps your body’s trying to tell you something. Upon digestion, excess carbs and fat can be limitlessly and easily absorbed and assimilated (as body fat or glycogen) or burned off, but excess protein still requires extra work. Protein digestion produces toxic metabolites that we can usually get rid of, even on higher protein intakes, but there’s clearly a limit. Our bodies finding protein to be extremely filling ? Our intestines actually slowing down the digestive process when protein is present? This is just how our bodies absorb protein at their own pace. They take their sweet time, but they get it all.
Okay, so we do absorb most of the protein we eat, whether it’s a 30-gram whey shake that’s absorbed in a couple hours or a large porterhouse whose protein is absorbed in ten hours. We’re not pooping amino acids. But are we using all that protein? Is it doing uniquely proteiny stuff, like building muscle? Or will everything above 30 grams get converted into glucose?
That depends. Many factors affect how your body utilizes a given amount of protein:
If you’re a bigger person (longer limbs, more potential spots for amino acids to be utilized), you can handle more absolute protein, and that protein will be more likely to go toward muscle protein synthesis.
If you’re smaller, the reverse is true. You simply don’t need as much absolute protein for structural demands, and you’ll have a lower threshold before protein becomes an energy substrate. If you eat the same steak as the bigger guy (all else being equal), you won’t “use” as many of the amino acids as he will.
Are you sedentary? You require less protein. Your muscles aren’t getting the signal to build and grow stronger and adapt to an imposed demand. The RDA of 0.36g/lb bodyweight should suffice.
Are you moderately active, jogging here and there, maybe riding the bike to work once or twice a week, and doing some basic lifting in the gym? You can use a bit more, as physical activity increases protein demand .
Are you actively trying to pack on muscle mass, and working out accordingly? You can use a lot more protein, up to and perhaps even over 1g/lb bodyweight.
Muscle maintenance in the elderly seems to require larger relative amounts of dietary protein. In one study , the RDA of 0.36g/lb bodyweight wasn’t enough to prevent some muscle catabolism.
Chronic stress, as indicated by chronically elevated cortisol levels, reduces muscle protein synthesis  (protein doing proteiny stuff) and increases gluconeogenesis  (conversion of protein into glucose).
It’s also worth noting that amino acids do other stuff, too, beyond building muscle and providing energy substrates when in excess. It’s not as if amino acids that don’t go toward rippling pecs are immediately converted into glucose. No, they’re also:
- Taken up by the intestines, both as an energy source for the small intestine and to form a reserve pool of amino acids  that the body can draw upon during fasting  or starvation (when there’s little to no dietary amino acids input) instead of breaking down skeletal muscle to turn into energy, at least in rats.
- Precursors for various neurotransmitters (like serotonin and dopamine).
So, to answer the question – yes, we’re probably absorbing all the protein we eat, but, depending on what we’re doing with our time, how old we are, how large or small we are, how active we are, how much we’re lifting, and how much stress we’re under, we’re all using the protein we absorb in different ways and proportions. As is often the case, the answer brings up even more questions. But that’s okay, because that’s just something else to explore.
Until next time, Grok on!