Peptides are a protein component composed of two or more amino acids bonded in a chain and encrypted into the protein molecule. Most dietary peptides remain inactive while contained in sequence within their respective parent proteins and are only activated when digested by the appropriate enzymes in the gastrointestinal tract or via food processing or fermentation. Many of the health-promoting properties associated with proteins, including nutrient absorption and cardiovascular, endocrine, immune and nervous system benefits, may actually be attributed to the peptides encrypted into those protein molecules. All dietary proteins contain peptides, but milk, eggs, grains and soybeans are among the richest sources.
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Dairy products are among the richest dietary sources of peptides, with several identified in the milk protein casein alone. In a 2005 article in "Archivos Latinoamericano De Nutricion," or the "Latin American Archives of Nutrition," peptides in casein and whey, another milk protein, were found to display an array of therapeutic activities, including antimicrobial, antithrombotic, mineral-carrying and immunomodulatory effects. According to a 2003 report in "Current Pharmaceutical Design," bioactive peptides effective against hypertension have been found in whey, ripened cheese and fermented milk products, such as yogurt.
Next to milk, eggs are one of the richest dietary sources of peptides. In a study published in 2011 in "Menopause," the journal of the North American Menopause Society, a peptide contained in egg yolk called egg yolk water-soluble peptide, or YPEP, proved to have a protective effect on bone metabolism, leading the researchers to deem YPEP a promising alternative to current osteoporosis therapies.
Corn, rice and wheat all contain peptides. In a 2002 report in "Science," researchers identified the autoimmune trigger in wheat gluten that induces celiac disease as a peptide. A 2011 report in the "International Journal of Biological Sciences" found that an effective "edible vaccine" against Alzheimer's disease may be derived from a peptide contained in rice. As of 2011, over 80 different peptides known as plant defensins have been identified with antifungal properties, including several in corn and rice.
Soybeans and other beans and seeds contain peptides as well. Numerous recent studies have identified various peptides in soybeans and associated them with specific health benefits, such as a 2009 study reported in "Experimental and Molecular Medicine" that confirmed the cancer-preventing and tumor-suppressing effects of the soybean peptide known as isoflavone-deprived soy peptide.