{"id":1705,"date":"2026-09-16T19:22:58","date_gmt":"2026-09-16T19:22:58","guid":{"rendered":"https:\/\/epinutrition.org\/?page_id=1705"},"modified":"2026-09-16T19:22:58","modified_gmt":"2026-09-16T19:22:58","slug":"proteins-journey-chapter-7-incorporation-becoming-muscle-protein","status":"publish","type":"page","link":"https:\/\/epinutrition.org\/?page_id=1705","title":{"rendered":"Protein\u2019s Journey \u2014 Chapter 7 \u2014 Incorporation: Becoming Muscle Protein"},"content":{"rendered":"\n<style>.pj{max-width:1080px;margin:auto;color:#203b32;font:18px\/1.7 Arial,sans-serif}.pj *{box-sizing:border-box}.pj a{color:#185c42}.pj .hero{display:grid;grid-template-columns:230px 1fr;gap:36px;align-items:center;background:#f4f3e9;padding:32px;border-radius:14px}.pj .cover{width:100%;height:auto}.pj h2{font:32px\/1.25 Georgia,serif}.pj h3{font:25px\/1.3 Georgia,serif}.pj .actions{display:flex;gap:12px;flex-wrap:wrap}.pj .button{display:inline-block;padding:12px 22px;border:2px solid #185c42;border-radius:7px;text-decoration:none;color:#185c42!important;font-weight:bold}.pj .primary{background:#185c42;color:white!important}.pj section{margin-top:44px;scroll-margin-top:25px}.pj li{margin:9px 0}.pj video{width:100%;aspect-ratio:16\/9;background:#102b26}.pj details{border:1px solid #d2dcd4;border-radius:8px;margin:12px 0;overflow:hidden}.pj summary{cursor:pointer;padding:18px;background:#f6f8f4}.pj .episode-body{padding:20px}.pj .time{display:inline-block;margin-left:12px;font-size:14px}.pj .table-wrap{overflow-x:auto}.pj table{border-collapse:collapse;width:100%}.pj th,.pj td{padding:10px;border:1px solid #ccd8ce;text-align:left}.pj .reference{font-size:16px;overflow-wrap:anywhere}.pj a:focus-visible,.pj summary:focus-visible{outline:3px solid #c3922e;outline-offset:3px}@media(max-width:640px){.pj .hero{grid-template-columns:1fr;padding:22px}.pj .cover{max-width:210px}.pj h2{font-size:28px}.pj .episode-body{padding:12px}}<\/style>\n<div class=\"pj\"><p><a href=\"https:\/\/epinutrition.org\/?page_id=1648#read-book\">\u2190 Book contents<\/a> \u00b7 <a href=\"https:\/\/epinutrition.org\/?page_id=1703\">Previous<\/a> \u00b7 <a href=\"https:\/\/epinutrition.org\/?page_id=1707\">Next<\/a><\/p>\n<p>An amino acid inside a muscle cell has reached the site where new muscle proteins can be made. It now belongs to a pool of available building materials, alongside amino acids supplied by previous meals and the recycling of existing proteins.<\/p>\n<p>Its presence makes construction possible. Incorporation occurs when the cell joins that amino acid to others in a growing protein chain.<\/p>\n<p>This is the central event we have been following from the plate: <strong>the transformation of available amino acids into the proteins that maintain and renew muscle.<\/strong><\/p>\n<p>Understanding this event also helps us distinguish the supply of materials, the signals regulating their use, and the work actually completed.<\/p>\n<h3>The cell follows instructions<\/h3>\n<p>Muscle cells manufacture many kinds of proteins. Some contribute to contraction, others support energy production, and others maintain cellular structures or regulate chemical reactions.<\/p>\n<p>Each protein requires a particular sequence of amino acids.<\/p>\n<p>The instructions originate in DNA. A working copy, called messenger RNA, carries the sequence information to a ribosome\u2014the cellular machinery that assembles proteins.<\/p>\n<p>Small molecules called transfer RNAs deliver amino acids to the ribosome. Each has been loaded with its appropriate amino acid. As the ribosome reads the message, it joins the amino acids in the specified order.<\/p>\n<p>This process is called <strong>translation<\/strong>.<\/p>\n<p>The resulting chain must fold into an appropriate shape and may undergo further processing or assembly before it can perform its function. Making a protein therefore involves both producing the chain and preparing it for useful work.<\/p>\n<h3>Construction requires a complete supply<\/h3>\n<p>A protein chain needs the amino acids specified by its instructions. An abundance of one amino acid cannot substitute for another that is required at a particular position.<\/p>\n<p>Leucine illustrates this distinction. It contributes to nutrient signaling and is also a building material. Yet leucine alone cannot supply everything needed to construct muscle proteins.<\/p>\n<p>The cell needs access to all the required amino acids, energy to support assembly, and functioning protein-making machinery.<\/p>\n<p>The supply need not come entirely from the meal just eaten. Amino acids released by normal protein breakdown also contribute. Dietary intake replenishes this shared supply and provides essential amino acids that the body cannot manufacture.<\/p>\n<p>This is why the contribution of a meal and the total rate of muscle protein synthesis are related but different questions.<\/p>\n<h3>Measuring what becomes protein<\/h3>\n<p>To study incorporation, researchers use amino acids carrying stable isotope labels. These nonradioactive labels make the amino acids distinguishable in laboratory measurements.<\/p>\n<p>When labeled dietary protein is consumed, scientists can follow its amino acids into the blood and look for the label in proteins collected through muscle biopsies.<\/p>\n<p>Finding the dietary label within muscle protein demonstrates that material from the meal has entered newly synthesized protein.<\/p>\n<p>In a study of older men recovering from resistance exercise, increasing milk-protein intake across the tested doses increased the incorporation of dietary amino acids into myofibrillar proteins\u2014the protein fraction associated with the muscle\u2019s contractile machinery. [1]<\/p>\n<p>This provides evidence of dietary materials being used. It does not mean that every additional gram consumed produces an equivalent amount of lasting muscle gain, or that the dose response continues indefinitely.<\/p>\n<h3>Two measurements answer different questions<\/h3>\n<p>Researchers may measure the contribution of labeled dietary amino acids or estimate the overall rate of protein synthesis using tracer methods.<\/p>\n<p><strong>Dietary amino acid incorporation<\/strong> asks how much material from a particular feeding enters muscle protein.<\/p>\n<p><strong>Muscle protein synthesis<\/strong> asks how rapidly the measured muscle protein pool is being made, using amino acids from the available supply.<\/p>\n<p>A common measure is the <strong>fractional synthetic rate<\/strong>: the percentage of a specified protein pool synthesized per unit of time. Calculating it requires accounting for the labeling of the available precursor amino acids, as well as the increase in label within protein.<\/p>\n<p>It is not the percentage of the meal converted into muscle, and it is not the percentage by which muscle size increased.<\/p>\n<p>The protein fraction also matters. A measurement of mixed muscle proteins combines several types. A myofibrillar measurement focuses more closely on the contractile apparatus. Other measurements can examine mitochondrial proteins involved in energy production.<\/p>\n<p>These details tell us what kind of renewal an experiment has captured.<\/p>\n<h3>From assembly to regulation<\/h3>\n<p>The assembly process is regulated in response to nutrients, activity, and other conditions. Chapter 8 examines mTORC1, an important part of that control system.<\/p>\n<h3>New protein serves renewal as well as growth<\/h3>\n<p>Muscle continually replaces proteins. Increased synthesis can therefore contribute to maintenance, repair, remodeling, or enlargement.<\/p>\n<p>A resistance-training study in ten young men helps illustrate this. The large early increase in myofibrillar protein synthesis occurred alongside greater muscle damage and was not associated with subsequent hypertrophy. Later in training, after damage had diminished, synthesis was associated with muscle growth. [2]<\/p>\n<p>Moreover, synthesis is only one side of protein balance. Existing proteins are also being broken down. Lasting increases in muscle protein require synthesis to exceed breakdown over time.<\/p>\n<h3>Why incorporation belongs at the center<\/h3>\n<p>Following incorporation gives this book a concrete biological focus. We can ask whether dietary amino acids reach muscle proteins and what influences that process.<\/p>\n<p>We then continue the investigation: which proteins were made, how much was retained, and whether repeated responses support useful muscle function.<\/p>\n<p>This approach gives digestion, meal composition, blood delivery, and cellular signaling their proper places in a connected process.<\/p>\n<p><strong>Incorporation is the point at which available amino acids become new protein. Its lasting value depends on the renewal, retention, and function that follow.<\/strong><\/p>\n<p>The next chapter examines mTOR more closely\u2014how it helps regulate this process, and what its activity can and cannot tell us.<\/p>\n<h3>References<\/h3>\n<p class=\"reference\">1. Holwerda AM, Paulussen KJM, Overkamp M, et al. Dose-dependent increases in whole-body net protein balance and dietary protein-derived amino acid incorporation into myofibrillar protein during recovery from resistance exercise in older men. <em>Journal of Nutrition<\/em>. 2019;149(2):221\u2013230. <a href=\"https:\/\/pubmed.ncbi.nlm.nih.gov\/30722014\/\">doi:10.1093\/jn\/nxy263<\/a><\/p>\n<p class=\"reference\">2. Damas F, Phillips SM, Libardi CA, et al. Resistance training-induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage. <em>Journal of Physiology<\/em>. 2016;594(18):5209\u20135222. <a href=\"https:\/\/pubmed.ncbi.nlm.nih.gov\/27219125\/\">doi:10.1113\/JP272472<\/a><\/p><p><a href=\"https:\/\/epinutrition.org\/?page_id=1648#read-book\">\u2190 Book contents<\/a> \u00b7 <a href=\"https:\/\/epinutrition.org\/?page_id=1703\">Previous<\/a> \u00b7 <a href=\"https:\/\/epinutrition.org\/?page_id=1707\">Next<\/a><\/p><\/div>\n","protected":false},"excerpt":{"rendered":"<p>\u2190 Book contents \u00b7 Previous \u00b7 Next An amino acid inside a muscle cell has reached the site where new muscle proteins can be made. It now belongs to a pool of available building materials, alongside amino acids supplied by previous meals and the recycling of existing proteins. Its presence makes construction possible. Incorporation occurs&#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"pmpro_default_level":"","_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false,"_kad_post_classname":"","footnotes":""},"class_list":["post-1705","page","type-page","status-publish","hentry","pmpro-has-access"],"brizy_media":[],"_links":{"self":[{"href":"https:\/\/epinutrition.org\/index.php?rest_route=\/wp\/v2\/pages\/1705","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/epinutrition.org\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/epinutrition.org\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/epinutrition.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/epinutrition.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1705"}],"version-history":[{"count":2,"href":"https:\/\/epinutrition.org\/index.php?rest_route=\/wp\/v2\/pages\/1705\/revisions"}],"predecessor-version":[{"id":1735,"href":"https:\/\/epinutrition.org\/index.php?rest_route=\/wp\/v2\/pages\/1705\/revisions\/1735"}],"wp:attachment":[{"href":"https:\/\/epinutrition.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1705"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}