Bone and cartilage are living tissue. They rebuild in response to the loads you put on them. Load is the signal, and protein, collagen and minerals are the materials. Most bone loss and most joint pain come from too little of the right load and too much body weight, not from a missing pill.
Bone is a collagen frame hardened with calcium-phosphate crystals. The collagen stops it shattering and the mineral stops it bending. Cells inside it sense strain and rebuild where loads are high. Cartilage, which caps the ends of bones in a joint, has no blood supply. It's fed by joint fluid pumped in and out when you move, and wears down faster under too much weight and inflammation. Tendons and ligaments are mostly collagen too, and are built in response to loading.
Bone cells sense bending and impact and lay down new bone along the lines of stress. Without load, the body takes bone back.
The collagen frame needs protein, glycine and vitamin C. Vitamin D absorbs calcium, K2 activates the protein that locks it in, and magnesium runs both.
Nutrients are the materials, but without load the body has no reason to use them.
Bone responds to big, fast strains, not to gentle repetition. In the LIFTMOR trial, 101 postmenopausal women with low bone mass were assigned either supervised heavy lifting and jumping or a gentle home program, twice a week for 8 months. The heavy group did 5 sets of 5 at over 85% of max: deadlifts, squats, overhead presses and jumping chin-ups with drop landings. Their spine bone density rose 2.9%, while the gentle group's fell 1.2%. The heavy group also gained height and function, and no one broke a vertebra.
Walking keeps you healthy, but it barely loads bone. Bone is built mostly in youth, so lifting and jumping as a teenager or young adult banks more bone for later.
Read more: Exercise: lifting
↑ back to the short listA hip fracture is usually a fall onto a weak bone, and in older adults a hip fracture often costs independence. In the Cochrane review of over 100 trials, exercise programs for older people cut the rate of falls by about 23%. The programs that worked best combined balance work with strength training. Tai chi worked too. Leg strength and balance matter as much as bone density.
Read more: Longevity: muscle
↑ back to the short listEvery step puts several times your body weight through the knee, so a little weight loss takes a lot of load off. In the 18-month IDEA trial, 454 overweight adults with knee arthritis were assigned diet plus exercise, diet alone or exercise alone. The diet groups lost about 10% of body weight. Their knee compression forces and IL-6, an inflammation marker, came down, and the diet-plus-exercise group had the least pain and the best function. Fat tissue also releases inflammatory signals that reach the joint, which is part of why arthritis is more common in the hands of heavier people as well.
"Running wrecks your knees" isn't what the data show. A meta-analysis of 17 studies covering over 114,000 people found hip or knee arthritis in 3.5% of recreational runners, compared with 10.2% of sedentary people. Elite and competitive runners came in at 13.3%. Moving cartilage gets fed, and still cartilage starves. The same goes for tendons, which strengthen with loading and weaken with rest.
Read more: Exercise: running
↑ back to the short listAbout half of bone's volume is protein, mostly collagen. The old idea that protein leaches calcium from bone didn't hold up: in trials and pooled studies, higher protein intake is neutral to slightly helpful for bone density, especially with enough calcium. Older adults who eat little protein lose more muscle and fall more. Muscle loads bone and cushions falls.
Read more: Wild Fish · Longevity: protein
↑ back to the short listThese four work as a system. Vitamin D raises calcium absorption from about 10–15% to 30–40%. K2 activates osteocalcin, the protein that binds calcium into bone. In a 3-year trial in 244 postmenopausal women, 180 mcg a day of K2 (MK-7) slowed bone loss at the spine and hip. Magnesium is needed to activate vitamin D. Calcium is the raw mineral, but in a BMJ review, calcium intake from food wasn't linked to fractures, and supplements didn't prevent fractures in the best-run trials. In the VITAL trial, vitamin D pills didn't prevent fractures in healthy adults who weren't deficient. Correcting a real shortfall matters. Stacking pills on top of enough doesn't help.
Read more: Calcium · Vitamin D · Vitamin K2 · Magnesium
↑ back to the short listTendons and ligaments have little blood flow, but loading them briefly opens a window when they take up amino acids. In a crossover trial, 8 men took gelatin with vitamin C an hour before 6 minutes of skipping. With 15 g, a blood marker of new collagen made in the body doubled. Their blood, added to lab-grown ligaments, also made those ligaments stronger. Vitamin C is required to cross-link collagen. The trial was small, but the approach is cheap and safe.
Read more: Bone Broth · Vitamin C: collagen
↑ back to the short listPieces of collagen survive digestion and reach the blood, where they signal the cells that build cartilage and tendon. Trials of collagen peptides, and of undenatured type II collagen, report modest drops in joint pain and stiffness, including in osteoarthritis. In one 12-month trial, 5 g a day raised bone density in postmenopausal women. The catch is the same as for skin: most of these trials are industry-funded, and independent trials find smaller effects. It's a reasonable thing to try for 3 months, but don't count on it.
Read more: Glycine / Collagen · Skin guide
↑ back to the short listThis is a quick guide. The evidence and references live on the linked pages. Not medical advice.