Glycine is another nonessential amino acid, vital in the body’s construction of proteins. First isolated in 1820, glycine is the simplest of all 22 naturally-occurring amino acids. In comparison to the other amino acids, glycine remains the second most common amino acid found in both enzymes and proteins. This sweet tasting amino is significant in the synthesis of nucleic acids, bile acids, and other amino acids in the human body.
Physiologically, glycine (like GABA and glutamic acid) acts as an inhibitory neurotransmitter in the brain and spinal cord. It also plays important roles in the process of prevention for muscular degeneration, improvement of glycogen storage, and for the reparation of damaged tissues located throughout the body. An important amino acid, glycine will be synthesized and converted from serine and threonine to fulfill its physiological processes, if needed.
Natural food sources that contain high amounts of glycine include; fish, meats, beans, and dairy products. Serving sizes below are based upon 100 grams of a given food source and are expressed in milligrams, representing the amount of glycine contained. 
|Vegetables and Vegetable Products|
|3100||Seaweed, spirulina, dried|
|Nut and Seed Products|
|3430||Seeds, sesame flour, low-fat|
|3080||Seeds, sunflower seed flour, partially defatted|
|2410||Seeds, pumpkin and squash seed kernels, roasted, without salt [pepitas]|
|2380||Seeds, cottonseed meal, partially defatted (glandless)|
|Legumes and Legume Products|
|3600||Soy protein isolate|
|3600||Soy protein isolate, potassium type|
|3150||Peanut flour, defatted|
|2690||Soy protein concentrate, produced by alcohol extraction|
|2690||Soy protein concentrate, produced by acid wash|
|Finfish and Shellfish Products|
|3010||Fish, cod, Atlantic, dried and salted|
|3250||Chicken, broilers or fryers, skin only, cooked, roasted|
|3210||Turkey, young tom, skin only, cooked, roasted|
|3140||Turkey, all classes, skin only, cooked, roasted|
|2610||Beef, cured, breakfast strips, cooked|
|4400||Pork, fresh, variety meats and by-products, ears, frozen, raw|
|3140||Pork, fresh, variety meats and by-products, ears, frozen, cooked, simmered|
|Lamb, Veal, and Game Products|
|2290||Veal, variety meats and by-products, liver, cooked, braised [calf liver]|
|Sausages and Luncheon Meats|
|2260||Beef, cured, corned beef, canned|
|Dairy and Egg Products|
|2840||Egg, white, dried|
|11900||Snacks, pork skins, plain|
|19100||Gelatins, dry powder, unsweetened|
|9670||Gelatin, drinking, orange-flavor, powder|
Glycine is a primary component in many of the body’s intricate internal processes. Among its most important duties include; the biosynthesis of nucleic acids, bile acids, and porphyrins. Porphyrins are pigments found in both plants and animals, and are important ingredients in hemoglobin (carries oxygen in the blood). Glycine is also directly responsible for the development of DNA and RNA, both of which represent genetic coding on various biological levels.
Skin proteins, collagen, and phospholipids (which are responsible for the creation of cell membranes) are made by this amino acid. Glycine is also directly responsible for specific biochemical reactions within our skeletal muscles that assist in the production of usable energy. “Having too much of this amino acid in your body can cause fatigue, but having the proper amount produces more energy.” 
In association with muscular activities, glycine prevents muscular degeneration by supplying additional creatine throughout the body. This may prove especially important in one’s ability to use glycine for the reparation of damaged tissues throughout the body, and promoting sound, overall health.
Clinically, glycine has shown promise in maintaining the overall health of the prostate gland in men. Like glutamic acid, glycine is found abundantly in prostate fluid.. In combination with the amino acids alanine and glutamic acid, glycine has been used used in various clincial applications to reduce the symptoms associated with benign prostatic hyperplasia, or BPH. [3, 4]
Neurological benefits may also be derived from proper dietary supplementation of glycine. This particular amino acid may prevent hyperactivity in the brain, while also being effective at limiting (perhaps even preventing) epileptic seizures. Scientists speculate that this is the result of glycine’s ability to enhance the activity of certain chemical messengers in the brain (i.e. neurotransmitters). Glycine may prove effective in conditions associated with memory and cognitive behavior as well. 
Because glycine is considered a nonessential amino acid, a Recommended Daily Allowance has not been established.  However, the U.S. National Academy of Sciences recommends that healthy people achieve .36 grams of highly bioavailable protein for each pound of bodyweight - equaling 0.8 grams of protein, per kilogram of bodyweight.
Very few people are glycine deficient. Because the body may produce its own supply of glycine in times of need or stress, deficiency is often seen in persons neglecting proteins in their diet.
There has been no clear scientific data supporting a definite toxicity from excessive consumption of glycine. However, animal models suggest that there is potential for the onset of toxicity when taking mega doses of this amino; which may result in neurotoxicity and glycine-induced hyponatremia.  It is also recommended that persons suffering form kidney or liver impairments not consume high levels on amino acids.
1. Nutrition Data. “999 Foods; Highest in Glycine.” (2004) http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml
2. Balch, Phyllis A., James F. “Amino Acids.” Prescription for Nutritional Healing. Ed. Amy C. Tecklenberg. New York, NY: Penguin Putnam Inc., 3rd Ed. 2000. 42-53.
3. Damrau F. Benign prostatic hypertrophy: amino acid therapy for symptomatic relief. J Am Geriatr Soc 1962; 10:426-30.
4. Feinblatt HM, Gant JC. Palliative treatment of benign prostatic hypertrophy. Value of glycine-alanine-glutamic acid combination. J Maine Med Assoc 1958; 49:99-101, 124.
5. File SE, Fluck E, Fernandes C. Beneficial effects of glycine (Bioglycin) on memory and attention in yound and middle-aged adults. J Clin Psychopharmacol 1999; 19:506-12.
6. Zest for life information page. “RDA of amino acids.” (1999-2003) http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml (14 Sept. 2004).
7. Panel on Micronutrients, Subcommittees on Upper Reference Level of Nutrients and of Interpretation and Use of Dietary Reference Intakes, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) (5 Sept. 2002)