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Introduction

Protein is known to have an essential role in muscle and tissue structure and has been shown to promote satiety, supporting weight loss and appetite suppression. An often overlooked, yet equally important protein function is its role as the building blocks of hormones and enzymes vital for maintaining normal health.

To obtain greatest nutritional value and decrease time-to-goal, it is helpful to understand a few things about the factors affecting protein function:

  • Absorption Rate
  • Protein Quality
  • Dietary Intake

In this review, we briefly cover protein quality and absorption, and then narrow our focus on the effects of dietary intake, namely carbohydrates and fats, on protein digestibility and absorption. Importantly, we will review how these dietary habits affect physiology.

Protein Quality and Absorption

A high quality protein will be a rich source of all essential amino acids, which include the branched chain amino acids (BCAA’s) known to stimulate protein synthesis, and allow for efficient digestion and absorption. This is a best-case “quality” scenario for promoting protein synthesis.

Different proteins will vary in their rates of digestion and absorption, resulting in varying physiological effects. Quickly absorbed proteins will lead to a fast spike in blood amino acid levels, promoting protein synthesis for a short time, but eventually leading to increased amino acid oxidation and protein breakdown. Slowly absorbed proteins will generally provide a slow, but steady supply of amino acids for absorption, resulting in only a moderate increase in protein synthesis but a significant effect on long-term protein breakdown prevention, resulting in greater net gain than fast-digesting protein sources.

Related: Fast vs. Slow Protein - Understanding the Effects of Absorption

The Effect of Overall Diet on Protein Digestion

High protein diets have long been associated with greater weight loss benefits vs. high carbohydrate diets, largely due to the satiety benefits and high metabolic costs of protein consumption. This means that protein consumption significantly alters carbohydrate absorption and mitigates blood glucose level increases. Similarly, a high-fiber diet can decrease overall fat absorption. But does high carbohydrate or fat consumption affect protein absorption?

Three key factors could cause a deviation from ideal protein absorption rates. First, the inhibition of proteolytic enzymes, which break dietary proteins into digestible amino acids and peptides, would significantly decrease absorption. Excessive carbohydrate and/or fat consumption could decrease protein absorption due to competitive inhibition effects. Third, some research has indicated that the quality and quantity of microflora moderately affects protein absorption.

Existing clinical research studies have predicted mild decreases in protein absorption when consumed concurrently with high carbohydrate or fat content. However, since most experimental methods for measuring protein absorption rely on indirect measurements of nitrogen excretion, it is difficult to conclusively determine whether these increased nitrogen levels are caused by protein absorption issues or outside factors. More research must be performed in this area to achieve conclusive results.

Overall, current scientific research indicates that the quality and absorption properties of dietary protein sources have a more significant effect on overall protein absorption vs. variations in macronutrient consumption.

Sources

  1. Header Image: Barbara Eckstein (Flickr)
  2. Co-author: Neil Thanedar
  3. Protein Digestibility and Absorption – Food and Agriculture Organization of the United Nations
  4. Factors Affecting the Digestibility of Nutrients – Proceedings of the Nutrition Society
  5. Metabolic Effects of High-Protein, Low-Carbohydrate Diets – The American Journal of Cardiology
     

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