Dietary Sources of Glycine and Proline for Adults Over 50
For a healthy adult over 50 already meeting calcium and vitamin D recommendations, the best dietary sources of glycine and proline are animal-derived collagen-rich foods—particularly bone broth, skin-on poultry, pork skin, and gelatin—while supplementation is generally unnecessary if consuming adequate protein from varied sources, though emerging evidence suggests endogenous synthesis may be insufficient for optimal collagen production in aging adults. 1, 2
Why Glycine and Proline Matter for Bone Health
Glycine, proline, and hydroxyproline together constitute 57% of all amino acids in collagen, which accounts for one-third of total body protein and is essential for maintaining bone structure, cartilage integrity, and connective tissue strength. 1
Collagen synthesis is severely diminished in osteoarthritis and age-related bone loss, and enhancing collagen production requires large amounts of glycine, proline, and lysine. 2
Recent evidence indicates that endogenous synthesis of glycine and proline is inadequate for maximal collagen production, even in healthy adults, making dietary intake particularly important. 1, 3
Endogenous Synthesis Limitations
Mammals synthesize glycine from threonine, serine, choline, and hydroxyproline, and proline from arginine and (in some species) from glutamine and glutamate in the small intestine, but this synthesis is insufficient under normal feeding conditions. 1, 3
Glycine is now recognized as a "conditionally essential" amino acid because de novo synthesis cannot meet the demands for collagen synthesis, particularly in aging, disease states, or periods of increased metabolic demand. 4, 3
High concentrations of glycine (above 1.0 mM in tissue culture) increase type II collagen synthesis by 60-75% in articular chondrocytes, suggesting that physiological glycine availability is severely deficient for optimal collagen production. 2
Best Dietary Sources of Glycine and Proline
Animal-Derived Collagen Sources (Highest Content)
Gelatin contains the highest percentages of glycine (10-35% of total protein), proline plus hydroxyproline (9.6-35% of total protein), making it the single richest dietary source. 5
Bone broth, made by simmering bones and connective tissue for extended periods, provides substantial amounts of glycine, proline, and hydroxyproline in bioavailable form. 1, 5
Skin-on poultry (chicken, turkey), pork skin, and fish skin are excellent sources because skin is predominantly collagen. 5
Meat and bone meal, fish meal, and poultry byproducts contain 10-35% glycine and 9.6-35% proline plus hydroxyproline of total protein. 5
Other Animal Protein Sources
Blood meal and casein are rich in branched-chain amino acids and glutamine (13% of total protein) but contain moderate amounts of glycine and proline. 5
Except for gelatin and feather meal, most animal protein sources contain 10-32% glutamate plus glutamine, 15-25% branched-chain amino acids, and 8-14% aspartate plus asparagine. 5
Plant-Based Sources (Lower Content)
Peanut meal and cottonseed meal contain high arginine (14-16% of total protein), which serves as a precursor for proline synthesis, but provide lower direct glycine and proline content compared to animal sources. 5
Corn and sorghum are poor sources of glycine, proline, and several essential amino acids including cysteine, lysine, methionine, and tryptophan (0.9-3% of total protein). 5
Soybean products provide moderate amounts of amino acids but are not optimal sources of glycine or proline compared to animal-derived collagen. 5
Practical Dietary Recommendations
Daily Protein Intake Strategy
Consume 25-30 grams of high-quality protein per meal, distributed across three meals daily, to optimize muscle protein synthesis and provide substrate for collagen production. 6
Include at least one serving daily of collagen-rich foods: 1 cup of bone broth, 3-4 ounces of skin-on poultry or fish, or 1-2 tablespoons of unflavored gelatin powder dissolved in liquid. 1, 5
Prioritize animal protein sources (except gelatin) as they provide excellent balance of both nutritionally essential and nonessential amino acids needed for overall health. 5
Specific Food Examples
Homemade or high-quality commercial bone broth (chicken, beef, or fish) simmered for 12-24 hours to maximize collagen extraction. 1, 5
Chicken thighs or drumsticks with skin, roasted or braised. 5
Pork shoulder or pork belly dishes that include skin and connective tissue. 5
Whole fish preparations (such as sardines) or fish head soup common in Asian cuisines. 5
Unflavored gelatin powder (derived from animal collagen) added to smoothies, oatmeal, or used to make sugar-free gelatin desserts. 5
Is Supplementation Necessary?
Evidence Against Routine Supplementation
If consuming adequate total protein (1.0-1.2 g/kg body weight daily) from varied sources including some collagen-rich foods, supplementation is generally unnecessary for healthy adults. 6
The body can synthesize glycine from serine, threonine, and choline, and proline from arginine, provided overall protein and micronutrient intake is adequate. 1, 3
Evidence Supporting Supplementation Consideration
Emerging research suggests that even under normal feeding conditions, glycine synthesis is insufficient for maximal collagen production, growth, and metabolic health. 1, 4, 3
Glycine supplementation has demonstrated benefits for metabolic regulation, anti-oxidative capacity, wound healing, immunity, and treatment of metabolic disorders in obesity, diabetes, cardiovascular disease, and inflammatory conditions. 4, 3
Dietary supplementation of glycine is effective in treating metabolic disorders in patients with cardiovascular diseases, inflammatory diseases, obesity, cancers, and diabetes, suggesting potential preventive benefits. 4
Practical Supplementation Guidance (If Chosen)
Glycine supplementation doses studied range from 3-15 grams daily, typically divided into 2-3 doses, with 3-5 grams before bedtime showing benefits for sleep quality and neurological function. 4
Collagen peptide supplements (hydrolyzed collagen) provide 8-10 grams of glycine and proline per 20-gram serving and may be more palatable than pure glycine powder. 1
Gelatin powder (10-20 grams daily) provides a food-based alternative to isolated amino acid supplements, supplying glycine, proline, and hydroxyproline in ratios similar to endogenous collagen. 5
Important Caveats and Considerations
Amino Acid Balance
Gelatin and collagen supplements are deficient in several essential amino acids (particularly tryptophan, methionine, and cysteine), so they should complement—not replace—complete protein sources. 1, 5
Maintain adequate intake of all essential amino acids through varied protein sources to support overall protein synthesis and metabolic function. 5
Individual Variation
Glycine requirements may be higher in individuals with inflammatory conditions, metabolic disorders, poor wound healing, or increased oxidative stress. 4, 3
Older adults may have reduced efficiency of endogenous glycine synthesis due to age-related decline in kidney and liver function, potentially increasing dietary requirements. 3
Integration with Bone Health Strategy
Continue optimizing calcium (1,200 mg/day) and vitamin D (800-1,000 IU/day) intake as these remain the cornerstone of bone health in adults over 50. 6, 7
Combine adequate glycine and proline intake with weight-bearing or resistance training exercise, which stimulates both collagen synthesis and bone remodeling. 6, 7
Smoking cessation and limiting alcohol to 1-2 drinks daily are essential, as both tobacco and excessive alcohol impair collagen synthesis and bone health. 6, 7
Safety Profile
Glycine supplementation up to 15 grams daily has an excellent safety profile with minimal adverse effects, primarily limited to mild gastrointestinal symptoms in some individuals. 4
No upper tolerable limit has been established for glycine or proline from food sources, and toxicity is extremely rare. 4, 3