Protein contains amino acids, the building blocks of muscle tissue. Our muscles need amino acids from protein to repair muscle fibers after a workout and to build new muscle. How much do you need if you’re physically active? The protein requirement for an athlete or physically active person is higher than for a sedentary person. Dietitians recommend that sedentary people consume around 0.8 grams of protein per kilogram of body weight. However, The Academy of Nutrition and Dietetics and the American College of Sports Medicine recommend more for athletes. They recommend that physically active people consume 1.2 to 2 grams of protein per kilogram of body weight. The added protein is to help muscles repair after exercise.
People who eat an omnivorous diet and aren’t physically active usually get enough protein in their diet without making an added effort to get more. However, highly active people, especially women, dieters, and vegetarians, must monitor their protein intake closely to avoid a shortfall. If you’re restricting calories, you may fall short in the protein department because you aren’t consuming enough food to meet your body’s protein requirements.
One of the best ways to ensure you’re meeting your protein needs is to eat protein at every meal. Another benefit of helping yourself to a serving of protein at a meal is a protein-rich meal has satiety benefits. But some sources suggest consuming no more than 30 grams of protein at a given meal, as this is the maximum you can absorb at one time. Is there truth to this idea?
What Happens When You Consume a Protein Meal?
When you consume protein with a meal, the acidic environment of your stomach starts the digestion process, the initial step of protein breakdown. But the protein you take in through diet won’t be completely broken down and absorbed until it reaches the small intestines. In the small intestines are transporters that take up amino acids, so they can be transferred to the bloodstream. Studies show that these receptors can absorb around 10 grams of amino acids from protein per hour.
What if you eat a meal containing 30 grams of protein? The transporters can only take up about 10 grams of protein per hour. Not very much if you’re eating a high-protein meal! The context with which you consume protein matters too. For example, you’ll absorb the protein in a liquid form faster than protein from solid food. Plant-based protein will be absorbed more slowly than animal-based due to the fiber in plant-based foods. The type of protein matters too. Of the proteins you find in protein shakes and supplements, whey is absorbed at a faster rate than casein.
What happens to the rest of the amino acids that the transporters can’t take up right away? Cells in the gut use some of the amino acids from a protein meal to support their own nutritional needs. But intestinal cells can actually hold on to amino acids and store them until they’re needed.
Your digestive tract also has the power to slow down and speed up the rate food moves through the intestinal tract to aid in nutrient absorption. If you overload your digestive tract with a high-protein meal, it will delay the rate with which food moves out of the stomach and into the small intestines, so the small intestines have more time to take up amino acids. The body and digestive tract is a master at self-regulation!
Protein Content of a Meal and Muscle Protein Synthesis
Absorption is one thing, but what really matters is the impact protein has on muscle protein synthesis. If you eat a meal containing 30 grams of protein will it boost muscle protein synthesis more if you double or triple the amount of protein? In one study, subjects who took in 30 grams of protein at one meal enjoyed a 50% boost in muscle protein synthesis. When they consumed triple that amount, 90 grams, muscle protein synthesis was the same. So, consuming more didn’t make a difference.
Another study published in the American Journal of Clinical Nutrition found that only 20 grams of protein was enough to maximally stimulate muscle protein synthesis. More than this didn’t boost muscle protein synthesis further. However, this is a generalization based on subjects who took part in a small study. We’re all a little different. The amount that maximally stimulates muscle protein synthesis may vary with how old a person is and how much muscle tissue they have.
However, another study compared amino acid absorption and the impact on muscle protein synthesis of varying amounts of whey protein (10, 20, or 35 grams) in elderly people. The 35-gram dose of protein led to greater absorption of amino acids and higher rates of muscle protein synthesis than did the 10 or 20-gram dose. So, at least in older adults, a slightly higher protein intake, beyond the 20 to 30-gram dose, may be beneficial.
Also interesting is a study showing consuming a large protein load at a meal suppresses the breakdown of muscle protein. We know that the muscle anabolic response is a product of muscle protein synthesis and protein breakdown. Therefore, if a large intake of protein at a meal prevents muscle protein breakdown, it has an anabolic effect independent of its effect on muscle protein synthesis. Many of the studies looking at the anabolic impact of protein meals didn’t take this into account.
Regardless, it makes more sense to spread your protein intake across the day rather than dining on a super-high protein meal and consuming little protein for the rest of the day. Spacing your protein helps keep you feel fuller between meals too.
The Bottom Line
Spread your protein intake out across the day rather than consuming large quantities at a single meal. It makes more sense from a satiety standpoint and some studies suggest 20 to 35 grams is the most your body can use right away for muscle protein synthesis.
References:
· Ten Have GA, et al. Absorption kinetics of amino acids, peptides, and intact proteins. Int J Sport Nutr Exerc Metab. (2007)
· Quick and Dirty Tips. “How Much Protein Can the Body Absorb?”
· Am J Clin Nutr. 2009 Jan;89(1):161-8. doi: 10.3945/ajcn.2008.26401. Epub 2008 Dec 3.
· American Journal of Physiology. Volume 302. Issue 8. April 2012. Pages E992-E999.
· Clin Nutr. 2013 Apr; 32(2): 309–313.
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