Protein is an essential nutrient that supports the body by building and repairing tissues, producing enzymes and hormones, and maintaining immune function. But the question remains: Is there a limit to protein absorption? While some claim the body can only handle 25 grams of protein per meal, others argue there's no true cap. The idea of a strict protein absorption limit is more myth than fact. While 25 grams of protein per meal is a good starting point, it shouldn’t be viewed as a maximum.
Understanding Protein Needs and Absorption
To understand protein absorption, we must first recognize our body's needs. Studies show that consuming at least 25 grams of protein per meal is essential for muscle growth stimulation. However, the optimal protein intake is often higher. For individuals aiming to build muscle, a common approach is to consume 1 gram of protein per pound of body weight daily. This protein should be divided across meals to provide a steady supply of amino acids-the building blocks of protein.
From a nutritional standpoint, the term “absorption” describes the passage of nutrients from the gut into systemic circulation. Based on this definition, the amount of protein that can be absorbed is virtually unlimited. Following digestion of a protein source, the constituent amino acids (AA) are transported through the enterocytes at the intestinal wall, enter the hepatic portal circulation, and the AA that are not utilized directly by the liver, then enter the bloodstream, after which almost all the AA ingested become available for use by tissues. While absorption is not a limiting factor with respect to whole proteins, there may be issues with consumption of individual free-form AA in this regard.
Factors Influencing Protein Absorption
Your body’s ability to absorb protein is significantly influenced by digestive health. Proper protein digestion begins in the stomach, where enzymes and stomach acid break down proteins into smaller peptides. The small intestine is the major site of protein digestion by proteases, digestive enzymes that divide protein particles. While these are naturally produced in the body to aid with overall digestion in most individuals, diseases of the stomach and small intestine can reduce the number of enzymes produced, causing malabsorption of amino acids. There are a number of supplements on the market that can aid digestion if needed, so you can properly absorb nutrients like protein. Digestive enzyme supplements may include one or a combination of multiple digestive enzymes, and some are sold in conjunction with probiotics. If you take a digestive enzyme with your protein, you’ll get higher amino acid levels in your blood. But if you’re vegan or have dietary restrictions, it’s important to note that some supplemental digestive enzymes are made from a pig or cow source.
The source of protein also affects absorption rates. Whole food sources like lean meats, legumes, or eggs slow down absorption when paired with other nutrients, leading to a more gradual release of amino acids. The digestion of a meal, depending on its composition (carbohydrates, proteins, fats) and the protein source, can take more than five hours to be completely digested. Your body does this to ensure that all nutrients are actually absorbed.
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The "Muscle Full" Concept and Protein Synthesis
It has been proposed that muscle protein synthesis (MPS) is maximized in young adults with an intake of ~ 20-25 g of a high-quality protein, consistent with the “muscle full” concept; anything above this amount is believed to be oxidized for energy or transaminated to form alternative bodily compounds. However, these findings are specific to the provision of fast-digesting proteins without the addition of other macronutrients. The preponderance of data indicate that while consumption of higher protein doses (> 20 g) results in greater AA oxidation, this is not the fate for all the additional ingested AAs as some are utilized for tissue-building purposes.
In a study often cited as support for the hypothesis that MPS is maximized at a protein dose of ~ 20-25 g, Areta et al. provided differing amounts of protein to resistance-trained subjects over a 12-h recovery period following performance of a multi-set, moderate repetition leg-extension exercise protocol. A total of 80 g of whey protein was ingested in one of the following three conditions: 8 servings of 10 g every 1.5 h; 4 servings of 20 g every 3 h; or 2 servings of 40 g every 6 h. Results showed that MPS was greatest in those who consumed 4 servings of 20 g of protein, suggesting no additional benefit, and actually a lower rise in MPS when consuming the higher dosage (40 g) under the conditions imposed in the study.
However, it is important to note that a number of factors influence dietary protein metabolism including the composition of the given protein source, the composition of the meal, the amount of protein ingested, and the specifics of the exercise routine. In addition, individual variables such as age, training status, and the amount of lean body mass also impact muscle-building outcomes. A major limitation in the study by Areta et al. is that total protein intake over the 12-h study period was only 80 g, corresponding to less than 1 g/kg of body mass. This is far below the amount necessary to maximize muscle protein balance in resistance-trained individuals who served as participants in the study. It also should be noted that subjects in Areta et al. ingested nothing but whey protein throughout the post-exercise period.
Fast vs. Slow-Digesting Proteins
Whey is a “fast-acting” protein; its absorption rate has been estimated at ~ 10 g per hour. At this rate, it would take just 2 h to fully absorb a 20-g dose of whey. While the rapid availability of AA will tend to spike MPS, earlier research examining whole body protein kinetics showed that concomitant oxidation of some of the AA may result in a lower net protein balance when compared to a protein source that is absorbed at a slower rate. For example, cooked egg protein has an absorption rate of ~ 3 g per hour, meaning complete absorption of an omelet containing the same 20 g of protein would take approximately 7 h, which may help attenuate oxidation of AA and thus promote greater whole-body net positive protein balance.
Although some studies have shown similar effects of fast and slow proteins on net muscle protein balance and fractional synthetic rate, other studies have demonstrated a greater anabolic effect of whey compared to more slowly digested sources both at rest, and after resistance exercise. However, the majority of these findings were during shorter testing periods (4 h or less), whereas longer testing periods (5 h or more) tend to show no differences between whey and casein on MPS or nitrogen balance. The presence versus absence of milk fat coingested with micellar casein did not delay the rate of protein-derived circulating amino acid availability or myofibrillar protein synthesis. Furthermore, the coingestion of carbohydrate with casein delayed digestion and absorption, but still did not impact muscle protein accretion compared to a protein-only condition.
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On the other end of the spectrum is casein protein. Casein is a slow digesting protein. During cheesemaking, special enzymes are added to heated milk, which cause the casein in the milk to coagulate, or change to a solid state. The curds of casein are then able to be used in food products or dietary supplements. Although casein and whey protein are both derived from milk, the body absorbs them very differently. When a person consumes casein, levels of these amino acids stay elevated in the blood for about 4-5 hours (whereas in whey, these levels are elevated in the blood for about 90 mins). That’s because casein forms curds once exposed to the acids in your stomach, just as it does in cheesemaking. These curds actually lengthen your body’s digestion and absorption processes.
Impact of Meal Composition
Consumption of slower-acting protein sources, particularly when consumed in combination with other macronutrients, would delay absorption and thus conceivably enhance the utilization of the constituent amino acids. The implication is that accompanying macronutrients’ potential to alter digestion rates does not necessarily translate to alterations in the anabolic effect of the protein feeding - at least in the case of slow-digesting protein such as casein.
Kim et al. endeavored to investigate this topic by provision of either 40 or 70 g of beef protein consumed as part of a mixed meal on two distinct occasions separated by a ~ 1 week washout period. Results showed that the higher protein intake promoted a significantly greater whole-body anabolic response, which was primarily attributed to a greater attenuation of protein breakdown. Given that participants ate large, mixed meals as whole foods containing not only protein, but carbohydrates and dietary fats as well, it is logical to speculate that this delayed digestion and absorption of AAs compared to liquid consumption of isolated protein sources. This, in turn, would have caused a slower release of AA into circulation and hence may have contributed to dose-dependent differences in the anabolic response to protein intake.
Maximizing Anabolism: Protein Intake Recommendations
Based on the current evidence, we conclude that to maximize anabolism one should consume protein at a target intake of 0.4 g/kg/meal across a minimum of four meals in order to reach a minimum of 1.6 g/kg/day. As for the maximum protein dose in one sitting, a 2018 study published in the Journal of the International Society of Sports Nutrition concluded that to maximize anabolism - aka the process by which proteins are formed from amino acids - one should consume protein at a max intake of 0.55 grams per kilogram per meal across four meals. This aligns with a daily intake of 2.2 grams per kilogram per day, which is on the upper end of what’s generally recommended.
Recent research employed a randomized, double-blind, within-subject design whereby resistance-trained men participated in two trials separated by ~ 2 weeks. During one trial subjects received 20 g of whey protein immediately after performing a total body resistance training bout; during the other trial the same protocol was instituted but subjects received a 40-g whey bolus following training. Results showed that the myofibrillar fractional synthetic rate was ~ 20% higher from consumption of the 40 g compared to the 20 g condition. The researchers speculated that the large amount of muscle mass activated from the total body RT bout necessitated a greater demand for AA that was met by a higher exogenous protein consumption.
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Protein Needs During Pregnancy
Protein plays an essential role during pregnancy, supporting both the mother’s and baby’s health. Experts recommend spreading protein intake evenly across meals to ensure the body absorbs and utilizes it effectively.
High-Protein Diets: Safety and Considerations
The International Society of Sports Nutrition states that high-protein diets-even exceeding 3 grams of protein per kilogram of body weight per day-are safe for healthy individuals. Concerns about the impact of high-protein diets on health are largely unfounded for healthy individuals.
Acute vs. Chronic Studies
An important distinction needs to be made between acute meal challenges comparing different protein amounts (including serial feedings in the acute phase following resistance training) and chronic meal feedings comparing different protein distributions through the day, over the course of several weeks or months. Longitudinal studies examining body composition have not consistently corroborated the results of acute studies examining muscle protein flux.
Lifestyle Factors and Gene Expression
You might have just had the most growth-stimulating resistance training workout ever, which indeed would activate genes associated with muscle growth and adaptation. If, after that “perfect” workout, you consumed alcohol to the point of intoxication every day for the next week, failed to get adequate sleep, and layered on high stress levels, no amount of protein or protein timing can save you. Your muscles would not recover, let alone adapt and grow, and if you continued to train hard with that lifestyle, there’s a good chance that you’d develop an injury. Your gene expression would be tuned more toward survival, which means that you’d be breaking down more muscle protein than you added, regardless of your protein intake at any level. This is why the ultimate takehome from this article is that protein intake is necessary, but not sufficient for muscle growth. If lifestyle or other factors (injury, illness, etc) have tilted your gene expression towards muscle protein breakdown (through upregulating genes associated with protein turnover), no amount of protein supplementation can fix that.
Practical Considerations and Protein Timing
Avoiding social events or career opportunities to avoid missing one of your 8 scheduled 30 grams of protein meals falls into the “who cares” territory. Eat protein consistently, don’t sweat missing a meal, and live your life. The reduced levels of stress hormones alone from not fretting over a meal schedule will be more anabolic than getting that 30 g shot of protein right at 3:15 pm.
On the other hand, let’s say that you’re wondering how much whey protein isolate (a rapidly absorbed protein) to include in your post-workout protein shake. Knowing that there’s an acute limit to stimulating MPS might steer you into consuming 25-40 g protein, rather than 300.
The best way to get maximum protein absorption is to spread out your protein consumption throughout the day. If you’re searching for the fastest absorbing protein, the fastest digesting ones are the proteins that have been separated from their whole food source and turned into protein powder. Protein powders are usually consumed as a liquid without much fiber or fat, and generally speaking, liquids digest faster than whole foods. So that means protein powder supplements poured into protein shakes or smoothies will be absorbed more quickly than most dietary protein. Whey protein hydrolysates are usually the fastest digesting of all supplements because hydrolysates are processed further than whey protein isolate or whey protein concentrate.
Plant-Based Protein Options
Of those most common vegan or plant proteins (pea protein, soy protein, brown rice protein, hemp protein and other seed, nut, or legume proteins), pea protein has the fastest absorption rate. Although it’s not quite as fast absorbing as whey, pea protein has proven it can deliver similar results.
In a 2015 study published in the Journal of the International Society of Sports Nutrition, male participants between the ages of 18 and 35 followed the same 12-week upper-body lifting program. One group supplemented with 25 grams of pea protein twice a day, and another took the exact same amount of whey protein. Each 25 grams of protein from pea protein delivered around two grams of leucine, one of the 3 branched chain amino acids (BCAAs) that stimulates protein synthesis. Whey delivered 2.65 grams of leucine, but both the pea protein and the whey protein fell right in the middle of the optimal leucine range. Researchers recorded biceps muscle thickness at the beginning and end of the study, and at the end of the 12 weeks, both groups showed identical increases in biceps muscle thickness and both pea protein and whey protein types showed greater results than placebo.
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