I think 5-10 g glycine (or 10-30 g gelatin for omnivores) is a good idea: I take 1 tsp glycine in the evening as sweetener in hibiscus tea.
It's cheap and harmless even at high doses:
Inagawa K et al. 2009. Assessment of acute adverse events of glycine ingestion at a high dose in human volunteers Journal of Urban Living and Health Association Vol. 50 No. 1 P 27-32
Glycine may be a semi-essential amino acid in humans:
Meléndez-Hevia E & de Paz-Lugo P. 2008. Branch-point stoichiometry can generate weak links in metabolism: the case of glycine biosynthesis. Journal of biosciences, 33(5), 771-780.
Meléndez-Hevia E et al. 2009. A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis. Journal of biosciences, 34(6), 853-872.
The best evidence for common glycine deficiencies is pyroglutamic acid in urine among populations consuming little connective tissue / collagen / gelatin:
Jackson AA et al. 1996. Urinary excretion of 5-L-oxoproline (pyroglutamic acid) is increased in normal adults consuming vegetarian or low protein diets. The Journal of nutrition, 126(11), 2813-2822.
Lenton C et al. 1998. Infants in Trinidad excrete more 5-L-oxoproline (L-pyroglutamic acid) in urine than infants in England: an environmental not ethnic difference. British Journal of Nutrition,80(01), 51-55.
Indeed, glycine insufficiency may be the most immediately harmful aspect of low protein diets:
Jackson AA et al 2002. Increased systolic blood pressure in rats induced by a maternal low-protein diet is reversed by dietary supplementation with glycine. Clinical Science, 103(6), 633-639.
Brawley L et al. 2004. Glycine rectifies vascular dysfunction induced by dietary protein imbalance during pregnancy. The Journal of physiology, 554(2), 497-504.
There's substantial evidence supplemental glycine improves sleep in humans:
Inagawa K. et al. 2006. Subjective effects of glycine ingestion before bedtime on sleep quality. Sleep and Biological Rhythms, 4(1), 75-77.
Yamadera W et al. 2007. Glycine ingestion improves subjective sleep quality in human volunteers, correlating with polysomnographic changes. Sleep and Biological rhythms, 5(2), 126-131.
Bannai M & Kawai, N. 2012. New therapeutic strategy for amino acid medicine: glycine improves the quality of sleep. Journal of pharmacological sciences, 118(2), 145-148.
Bannai M et al. 2012. The effects of glycine on subjective daytime performance in partially sleep-restricted healthy volunteers. Frontiers in neurology, 3.
In animal studies, glycine is broadly cytoprotective:
Weinberg JM et al. 1987. Cytoprotective effects of glycine and glutathione against hypoxic injury to renal tubules.Journal of Clinical Investigation, 80(5), 1446.
Yin M et al. 1998. Glycine accelerates recovery from alcohol-induced liver injury. Journal of Pharmacology and Experimental Therapeutics, 286(2), 1014-1019.
Gusev EI.et al. 2000. Neuroprotective effects of glycine for therapy of acute ischaemic stroke. Cerebrovascular Diseases, 10(1), 49-60.
Senthilkumar R et al. 2003. Glycine modulates hepatic lipid accumulation in alcohol-induced liver injury. Pol J Pharmacol,55(4), 603-611.
Qi RB et al. 2007. Glycine receptors contribute to cytoprotection of glycine in myocardial cells.Chinese Medical Journal, 120(10), 915.
Mikalauskas S et al. 2011. Dietary glycine protects from chemotherapy-induced hepatotoxicity. Amino acids, 40(4), 1139-1150.
Petrat F et al. 2011. Protection from glycine at low doses in ischemia-reperfusion injury of the rat small intestine. European Surgical Research, 46(4), 180-187.
Petrat F et al. 2012. Glycine, a simple physiological compound protecting by yet puzzling mechanism (s) against ischaemia–reperfusion injury: current knowledge. British journal of pharmacology, 165(7), 2059-2072.
Barakat HA & Hamza AH 2012. Glycine alleviates liver injury induced by deficiency in methionine and or choline in rats. Eur Rev Med Pharmacol Sci,16(6), 728-736.
Ruiz-Ramirez A et al 2014. Glycine restores glutathione and protects against oxidative stress in vascular tissue from sucrose-fed rats. Clinical Science, 126(1), 19-29.
This is partly mediated by glycine's anti-inflammatory effects:
Wheeler MD & Thurman RG 1999. Production of superoxide and TNF-α from alveolar macrophages is blunted by glycine. American Journal of Physiology-Lung Cellular and Molecular Physiology, 277(5), L952-L959.
Wheeler MD et al. 2000. Dietary glycine blunts lung inflammatory cell influx following acute endotoxin. American Journal of Physiology-Lung Cellular and Molecular Physiology, 279(2), L390-L398.
Wheeler MD et al. 2000. Glycine-gated chloride channels in neutrophils attenuate calcium influx and superoxide production. The FASEB Journal, 14(3), 476-484.
Mauriz JL et al. 2001. Dietary glycine inhibits activation of nuclear factor kappa B and prevents liver injury in hemorrhagic shock in the rat. Free Radical Biology and Medicine, 31(10), 1236-1244.
Li X et al. 2001. Dietary glycine prevents peptidoglycan polysaccharide-induced reactive arthritis in the rat: role for glycine-gated chloride channel. Infection and immunity, 69(9), 5883-5891.
Tsune I et al. 2003. Dietary glycine prevents chemical-induced experimental colitis in the rat. Gastroenterology, 125(3), 775-785.
Bruck R et al. 2003. Glycine modulates cytokine secretion, inhibits hepatic damage and improves survival in a model of endotoxemia in mice. Liver international, 23(4), 276-282.
Meza DL et al 2006. Effect of glycine in the immune response of the experimentally diabetic rats. Revista Alergia México, 53(6), 212.
Yamanouchi K et al. 2007. Glycine reduces hepatic warm ischaemia–reperfusion injury by suppressing inflammatory reactions in rats. Liver International, 27(9), 1249-1254.
Garcia-Macedo R et al. 2008. Glycine increases mRNA adiponectin and diminishes pro-inflammatory adipokines expression in 3T3-L1 cells. European journal of pharmacology, 587(1), 317-321.
Alarcon-Aguilar FJ et al 2008. Glycine regulates the production of pro-inflammatory cytokines in lean and monosodium glutamate-obese mice.European journal of pharmacology, 599(1), 152-158.
Wang HD et al 2009. Glycine inhibits the LPS-induced increase in cytosolic Ca2+ concentration and TNFα production in cardiomyocytes by activating a glycine receptor. Acta Pharmacologica Sinica, 30(8), 1107-1114.
Almanza-Perez JC 2010. Glycine regulates inflammatory markers modifying the energetic balance through PPAR and UCP-2. Biomedicine & Pharmacotherapy, 64(8), 534-540.
Bruns H. et al. 2011. Glycine and taurine equally prevent fatty livers from Kupffer cell-dependent injury: an in vivo microscopy study. Microcirculation (New York, NY: 1994), 18(3), 205.
Ceyhan G O et al. 2011. Prophylactic glycine administration attenuates pancreatic damage and inflammation in experimental acute pancreatitis. Pancreatology, 11(1), 57-67.
Hasegawa S et al. 2012. Cysteine, histidine and glycine exhibit anti‐inflammatory effects in human coronary arterial endothelial cells. Clinical & Experimental Immunology, 167(2), 269-274.
Wu HW et al 2012. Effects of glycine on phagocytosis and secretion by Kupffer cells in vitro. World journal of gastroenterology: WJG, 18(20), 2576.
Vieira CP et al. 2015. Glycine improves biochemical and biomechanical properties following inflammation of the Achilles tendon. The Anatomical Record, 298(3), 538-545.
The anti-inflammatory effect of glycine may also account for tumor growth retardation:
Rose ML et al. 1999. Dietary glycine inhibits the growth of B16 melanoma tumors in mice. Carcinogenesis,20(5), 793-798.
Rose, ML et al. 1999. Dietary glycine prevents the development of liver tumors caused by the peroxisome proliferator WY-14,643. Carcinogenesis, 20(11), 2075-2081.
Amin K et al. 2003. Dietary glycine inhibits angiogenesis during wound healing and tumor growth. Cancer biology & therapy, 2(2), 173-178.
Yamashina S et al. 2007. Glycine as a potent anti‐angiogenic nutrient for tumor growth. Journal of gastroenterology and hepatology, 22(s1), S62-S64.
Glycine prevents glycation driven cataracts:
Ramakrishnan S & Sulochana KN. 1993. Decrease in glycation of lens proteins by lysine and glycine by scavenging of glucose and possible mitigation of cataractogenesis. Experimental eye research, 57(5), 623-628.
Alvarado-Vásquez N. et al 2006. Oral glycine administration attenuates diabetic complications in streptozotocin-induced diabetic rats. Life sciences,79(3), 225-232.
Bahmani F et al. 2012. Glycine therapy inhibits the progression of cataract in streptozotocin-induced diabetic rats. Molecular vision, 18, 439.
And perhaps of most interest on this board, glycine supplementation appears to mimic methionine restriction, with clearance of excess methionine via glycine N-methyltransferase.
Benevenga NJ & Harper AE. 1967. Alleviation of methionine and homocystine toxicity in the rat. The Journal of nutrition 93.1: 44-52.
Peng YS et al. 1981. Alleviation of methionine toxicity by glycine and serine in rats pretreated with excess retinol. Nutrition Reports International.
Sugiyama K et al. 1987. Effect of dietary glycine on methionine metabolism in rats fed a high-methionine diet. Journal of nutritional science and vitaminology, 33(3), 195-205.
Rowling MJ et al. 2002. Hepatic glycine N-methyltransferase is up-regulated by excess dietary methionine in rats. The Journal of nutrition, 132(9), 2545-2550.
Luka Z et al 2002). Mutations in human glycine N-methyltransferase give insights into its role in methionine metabolism. Human genetics, 110(1), 68-74.
Fukada SI et al 2006. Suppression of methionine-induced hyperhomocysteinemia by glycine and serine in rats.Bioscience, biotechnology, and biochemistry, 70(10), 2403-2409.
Fukada SI et al 2008. Effects of various amino acids on methionine-induced hyperhomocysteinemia in rats. Bioscience, biotechnology, and biochemistry, 72(7), 1940-1943.
Brind J et al. 2011. Dietary glycine supplementation mimics lifespan extension by dietary methionine restriction in Fisher 344 rats. The FASEB Journal, 25(1_MeetingAbstracts), 528-2.