5.3: Food Proteins
- Page ID
- 156294
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Meat, poultry, fish, eggs, dairy products and nuts provide more protein (per gram) than grains, fruits, and vegetables (Figure 5.3). The exception to this is legumes since leguminous plants have roots that are associated with bacteria that can take nitrogen from the air and incorporate it into amino acids. Those amino acids are used by the plant, so it then has higher protein content than other plants.
| Essential | Nonessential | Conditionally Essential |
|---|---|---|
| Histidine | Alanine | Arginine |
| Isoleucine | Asparagine | Cysteine |
| Leucine | Aspartic acid | Glutamine |
| Lysine | Glutamic acid | Glycine |
| Methionine | Serine | Proline |
| Phenylalanine | Tyrosine | |
| Threonine | ||
| Tryptophan | ||
| Valine |
It is also important to remember that not all protein sources are created equal in terms of the types of amino acids they offer. Animal-derived foods tend to have greater amounts of essential amino acids. Dietary proteins come from both animal and plant sources, and the balance of amino acids they provide can vary significantly. Animal-based proteins —such as meat, poultry, fish, eggs, and dairy—are considered complete proteins because they contain all nine essential amino acids in proportions that closely match human needs. These sources are typically high in quality and bioavailability. In contrast, plant-based proteins—like beans, lentils, nuts, seeds, and grains—are often incomplete proteins, meaning they may lack one or more essential amino acids or have them in lower amounts. The amino acid(s) that are missing or in low amounts are referred to as limiting amino acids. If a limiting amino acid is missing from one’s diet, it is not possible to make the proteins the body needs that contain it. For example, grains are generally low in lysine, while legumes may be low in methionine.
However, by consuming a variety of plantbased proteins throughout the day, such as rice and beans (Figure 5.4), individuals can achieve a complementary balance of amino acids. Protein complementation allows a combination of two or more incomplete protein sources to be combined to supply all nine essential amino acids (Figure 5.5). Understanding the differences in amino acid profiles helps in planning balanced diets, especially for vegetarians and vegans.
Protein Quality
The quality of a protein indicates how well a protein source can meet the body’s amino acid requirements. Protein quality depends upon whether the protein is complete versus incomplete as well as the body’s ability to absorb the amino acids, or bioavailability. A high-quality protein is one that is a complete protein source and provides easily absorbed amino acids, therefore having high bioavailability. In contrast, a low-quality protein is one that is an incomplete protein source and has low bioavailability. Similar to completeness, animal-based foods are typically sources of high quality proteins, whereas plant-based proteins are typically lower quality.
To improve global nutrition, especially in regions where diets are primarily plantbased, scientists have turned to genetic modification (GM) to enhance protein quality in staple crops. This process involves altering the DNA of plants to increase the levels or balance of essential amino acids. Examples of genetic modification to improve protein quality include the following:
- High-lysine corn: Corn is naturally low in lysine and tryptophan, which limits its protein quality. Through genetic engineering, varieties have been developed to express higher levels of these amino acids, making corn a more complete protein source.
- Genetically modified rice and legumes: Rice has been engineered to express additional proteins rich in essential amino acids (like methionine), addressing its natural deficiencies. Similarly, efforts are underway to genetically modify soybeans and other legumes to boost their amino acid profiles.
Improving protein quality through genetic modification holds promise for things such as combating malnutrition in developing countries, supporting plant-based diets with more complete protein sources, and reducing reliance on animal proteins, potentially benefiting sustainability and food security. That said, concerns about GM crops, such as biodiversity loss, regulatory oversight, and public acceptance, must be considered alongside these innovations.